cairo_t

## cairo_t cairo_t — The cairo drawing context |

## Synopsis

    typedef     cairo_t;
    cairo_t*    cairo_create                    (cairo_surface_t *target);
    cairo_t*    cairo_reference                 (cairo_t *cr);
    void        cairo_destroy                   (cairo_t *cr);
    cairo_status_t cairo_status                 (cairo_t *cr);
    void        cairo_save                      (cairo_t *cr);
    void        cairo_restore                   (cairo_t *cr);
    cairo_surface_t* cairo_get_target           (cairo_t *cr);
    void        cairo_push_group                (cairo_t *cr);
    void        cairo_push_group_with_content   (cairo_t *cr,
                                                 cairo_content_t content);
    cairo_pattern_t* cairo_pop_group            (cairo_t *cr);
    void        cairo_pop_group_to_source       (cairo_t *cr);
    cairo_surface_t* cairo_get_group_target     (cairo_t *cr);
    void        cairo_set_source_rgb            (cairo_t *cr,
                                                 double red,
                                                 double green,
                                                 double blue);
    void        cairo_set_source_rgba           (cairo_t *cr,
                                                 double red,
                                                 double green,
                                                 double blue,
                                                 double alpha);
    void        cairo_set_source                (cairo_t *cr,
                                                 cairo_pattern_t *source);
    void        cairo_set_source_surface        (cairo_t *cr,
                                                 cairo_surface_t *surface,
                                                 double x,
                                                 double y);
    cairo_pattern_t* cairo_get_source           (cairo_t *cr);
    enum        cairo_antialias_t;
    void        cairo_set_antialias             (cairo_t *cr,
                                                 cairo_antialias_t antialias);
    cairo_antialias_t cairo_get_antialias       (cairo_t *cr);
    void        cairo_set_dash                  (cairo_t *cr,
                                                 const double *dashes,
                                                 int num_dashes,
                                                 double offset);
    enum        cairo_fill_rule_t;
    void        cairo_set_fill_rule             (cairo_t *cr,
                                                 cairo_fill_rule_t fill_rule);
    cairo_fill_rule_t cairo_get_fill_rule       (cairo_t *cr);
    enum        cairo_line_cap_t;
    void        cairo_set_line_cap              (cairo_t *cr,
                                                 cairo_line_cap_t line_cap);
    cairo_line_cap_t cairo_get_line_cap         (cairo_t *cr);
    enum        cairo_line_join_t;
    void        cairo_set_line_join             (cairo_t *cr,
                                                 cairo_line_join_t line_join);
    cairo_line_join_t cairo_get_line_join       (cairo_t *cr);
    void        cairo_set_line_width            (cairo_t *cr,
                                                 double width);
    double      cairo_get_line_width            (cairo_t *cr);
    void        cairo_set_miter_limit           (cairo_t *cr,
                                                 double limit);
    double      cairo_get_miter_limit           (cairo_t *cr);
    enum        cairo_operator_t;
    void        cairo_set_operator              (cairo_t *cr,
                                                 cairo_operator_t op);
    cairo_operator_t cairo_get_operator         (cairo_t *cr);
    void        cairo_set_tolerance             (cairo_t *cr,
                                                 double tolerance);
    double      cairo_get_tolerance             (cairo_t *cr);
    void        cairo_clip                      (cairo_t *cr);
    void        cairo_clip_preserve             (cairo_t *cr);
    void        cairo_reset_clip                (cairo_t *cr);
    void        cairo_fill                      (cairo_t *cr);
    void        cairo_fill_preserve             (cairo_t *cr);
    void        cairo_fill_extents              (cairo_t *cr,
                                                 double *x1,
                                                 double *y1,
                                                 double *x2,
                                                 double *y2);
    cairo_bool_t cairo_in_fill                  (cairo_t *cr,
                                                 double x,
                                                 double y);
    void        cairo_mask                      (cairo_t *cr,
                                                 cairo_pattern_t *pattern);
    void        cairo_mask_surface              (cairo_t *cr,
                                                 cairo_surface_t *surface,
                                                 double surface_x,
                                                 double surface_y);
    void        cairo_paint                     (cairo_t *cr);
    void        cairo_paint_with_alpha          (cairo_t *cr,
                                                 double alpha);
    void        cairo_stroke                    (cairo_t *cr);
    void        cairo_stroke_preserve           (cairo_t *cr);
    void        cairo_stroke_extents            (cairo_t *cr,
                                                 double *x1,
                                                 double *y1,
                                                 double *x2,
                                                 double *y2);
    cairo_bool_t cairo_in_stroke                (cairo_t *cr,
                                                 double x,
                                                 double y);
    void        cairo_copy_page                 (cairo_t *cr);
    void        cairo_show_page                 (cairo_t *cr);

## Description

cairo_t is the main object used when drawing with cairo. To draw with cairo, you create a cairo_t, set the target surface, and drawing options for the cairo_t, create shapes with functions like `cairo_move_to()` and `cairo_line_to()`, and then draw shapes with `cairo_stroke()` or `cairo_fill()`.

cairo_t's can be pushed to a stack via `cairo_save()`. They may then safely be changed, without loosing the current state. Use `cairo_restore()` to restore to the saved state.

## Details

### cairo_t

    typedef struct _cairo cairo_t;

A cairo_t contains the current state of the rendering device, including coordinates of yet to be drawn shapes.

### cairo_create ()

    cairo_t*    cairo_create                    (cairo_surface_t *target);

Creates a new cairo_t with all graphics state parameters set to default values and with `target` as a target surface. The target surface should be constructed with a backend-specific function such as `cairo_image_surface_create()` (or any other `cairo_<backend>_surface_create` variant).

This function references `target`, so you can immediately call `cairo_surface_destroy()` on it if you don't need to maintain a separate reference to it.

`target` : | target surface for the context
Returns : | a newly allocated cairo_t with a reference count of 1. The initial reference count should be released with `cairo_destroy()` when you are done using the cairo_t. This function never returns `NULL`. If memory cannot be allocated, a special cairo_t object will be returned on which `cairo_status()` returns `CAIRO_STATUS_NO_MEMORY`. You can use this object normally, but no drawing will be done.

### cairo_reference ()

    cairo_t*    cairo_reference                 (cairo_t *cr);

Increases the reference count on `cr` by one. This prevents `cr` from being destroyed until a matching call to `cairo_destroy()` is made.

`cr` : | a cairo_t
Returns : | the referenced cairo_t.

### cairo_destroy ()

    void        cairo_destroy                   (cairo_t *cr);

Decreases the reference count on `cr` by one. If the result is zero, then `cr` and all associated resources are freed. See `cairo_reference()`.

`cr` : | a cairo_t

### cairo_status ()

    cairo_status_t cairo_status                 (cairo_t *cr);

Checks whether an error has previously occurred for this context.

`cr` : | a cairo context
Returns : | the current status of this context, see cairo_status_t

### cairo_save ()

    void        cairo_save                      (cairo_t *cr);

Makes a copy of the current state of `cr` and saves it on an internal stack of saved states for `cr`. When `cairo_restore()` is called, `cr` will be restored to the saved state. Multiple calls to `cairo_save()` and `cairo_restore()` can be nested; each call to `cairo_restore()` restores the state from the matching paired `cairo_save()`.

It isn't necessary to clear all saved states before a cairo_t is freed. If the reference count of a cairo_t drops to zero in response to a call to `cairo_destroy()`, any saved states will be freed along with the cairo_t.

`cr` : | a cairo_t

### cairo_restore ()

    void        cairo_restore                   (cairo_t *cr);

Restores `cr` to the state saved by a preceding call to `cairo_save()` and removes that state from the stack of saved states.

`cr` : | a cairo_t

### cairo_get_target ()

    cairo_surface_t* cairo_get_target           (cairo_t *cr);

Gets the target surface for the cairo context as passed to `cairo_create()`.

This function will always return a valid pointer, but the result can be a "nil" surface if `cr` is already in an error state, (ie. `cairo_status()` `!=` `CAIRO_STATUS_SUCCESS`). A nil surface is indicated by `cairo_surface_status()` `!=` `CAIRO_STATUS_SUCCESS`.

`cr` : | a cairo context
Returns : | the target surface. This object is owned by cairo. To keep a reference to it, you must call `cairo_surface_reference()`.

### cairo_push_group ()

    void        cairo_push_group                (cairo_t *cr);

Temporarily redirects drawing to an intermediate surface known as a group. The redirection lasts until the group is completed by a call to `cairo_pop_group()` or `cairo_pop_group_to_source()`. These calls provide the result of any drawing to the group as a pattern, (either as an explicit object, or set as the source pattern).

This group functionality can be convenient for performing intermediate compositing. One common use of a group is to render objects as opaque within the group, (so that they occlude each other), and then blend the result with translucence onto the destination.

Groups can be nested arbitrarily deep by making balanced calls to `cairo_push_group()`/`cairo_pop_group()`. Each call pushes/pops the new target group onto/from a stack.

The `cairo_push_group()` function calls `cairo_save()` so that any changes to the graphics state will not be visible outside the group, (the pop_group functions call `cairo_restore()`).

By default the intermediate group will have a content type of CAIRO_CONTENT_COLOR_ALPHA. Other content types can be chosen for the group by using `cairo_push_group_with_content()` instead.

As an example, here is how one might fill and stroke a path with translucence, but without any portion of the fill being visible under the stroke:

    cairo_push_group (cr);
    cairo_set_source (cr, fill_pattern);
    cairo_fill_preserve (cr);
    cairo_set_source (cr, stroke_pattern);
    cairo_stroke (cr);
    cairo_pop_group_to_source (cr);
    cairo_paint_with_alpha (cr, alpha);

`cr` : | a cairo context

Since 1.2

### cairo_push_group_with_content ()

    void        cairo_push_group_with_content   (cairo_t *cr,
                                                 cairo_content_t content);

Temporarily redirects drawing to an intermediate surface known as a group. The redirection lasts until the group is completed by a call to `cairo_pop_group()` or `cairo_pop_group_to_source()`. These calls provide the result of any drawing to the group as a pattern, (either as an explicit object, or set as the source pattern).

The group will have a content type of `content`. The ability to control this content type is the only distinction between this function and `cairo_push_group()` which you should see for a more detailed description of group rendering.

`cr` : | a cairo context
`content` : | a `cairo_content_t` indicating the type of group that will be created

Since 1.2

### cairo_pop_group ()

    cairo_pattern_t* cairo_pop_group            (cairo_t *cr);

Terminates the redirection begun by a call to `cairo_push_group()` or `cairo_push_group_with_content()` and returns a new pattern containing the results of all drawing operations performed to the group.

The `cairo_pop_group()` function calls `cairo_restore()`, (balancing a call to `cairo_save()` by the push_group function), so that any changes to the graphics state will not be visible outside the group.

`cr` : | a cairo context
Returns : | a newly created (surface) pattern containing the results of all drawing operations performed to the group. The caller owns the returned object and should call `cairo_pattern_destroy()` when finished with it.

Since 1.2

### cairo_pop_group_to_source ()

    void        cairo_pop_group_to_source       (cairo_t *cr);

Terminates the redirection begun by a call to `cairo_push_group()` or `cairo_push_group_with_content()` and installs the resulting pattern as the source pattern in the given cairo context.

The behavior of this function is equivalent to the sequence of operations:

    cairo_pattern_t *group = cairo_pop_group (cr);
    cairo_set_source (cr, group);
    cairo_pattern_destroy (group);

but is more convenient as their is no need for a variable to store the short-lived pointer to the pattern.

The `cairo_pop_group()` function calls `cairo_restore()`, (balancing a call to `cairo_save()` by the push_group function), so that any changes to the graphics state will not be visible outside the group.

`cr` : | a cairo context

Since 1.2

### cairo_get_group_target ()

    cairo_surface_t* cairo_get_group_target     (cairo_t *cr);

Gets the target surface for the current group as started by the most recent call to `cairo_push_group()` or `cairo_push_group_with_content()`.

This function will return NULL if called "outside" of any group rendering blocks, (that is, after the last balancing call to `cairo_pop_group()` or `cairo_pop_group_to_source()`).

`cr` : | a cairo context
Returns : | the target group surface, or NULL if none. This object is owned by cairo. To keep a reference to it, you must call `cairo_surface_reference()`.

Since 1.2

### cairo_set_source_rgb ()

    void        cairo_set_source_rgb            (cairo_t *cr,
                                                 double red,
                                                 double green,
                                                 double blue);

Sets the source pattern within `cr` to an opaque color. This opaque color will then be used for any subsequent drawing operation until a new source pattern is set.

The color components are floating point numbers in the range 0 to 1. If the values passed in are outside that range, they will be clamped.

`cr` : | a cairo context
`red` : | red component of color
`green` : | green component of color
`blue` : | blue component of color

### cairo_set_source_rgba ()

    void        cairo_set_source_rgba           (cairo_t *cr,
                                                 double red,
                                                 double green,
                                                 double blue,
                                                 double alpha);

Sets the source pattern within `cr` to a translucent color. This color will then be used for any subsequent drawing operation until a new source pattern is set.

The color and alpha components are floating point numbers in the range 0 to 1. If the values passed in are outside that range, they will be clamped.

`cr` : | a cairo context
`red` : | red component of color
`green` : | green component of color
`blue` : | blue component of color
`alpha` : | alpha component of color

### cairo_set_source ()

    void        cairo_set_source                (cairo_t *cr,
                                                 cairo_pattern_t *source);

Sets the source pattern within `cr` to `source`. This pattern will then be used for any subsequent drawing operation until a new source pattern is set.

Note: The pattern's transformation matrix will be locked to the user space in effect at the time of `cairo_set_source()`. This means that further modifications of the current transformation matrix will not affect the source pattern. See `cairo_pattern_set_matrix()`.

XXX: I'd also like to direct the reader's attention to some (not-yet-written) section on cairo's imaging model. How would I do that if such a section existed? (cworth).

`cr` : | a cairo context
`source` : | a cairo_pattern_t to be used as the source for subsequent drawing operations.

### cairo_set_source_surface ()

    void        cairo_set_source_surface        (cairo_t *cr,
                                                 cairo_surface_t *surface,
                                                 double x,
                                                 double y);

This is a convenience function for creating a pattern from `surface` and setting it as the source in `cr` with `cairo_set_source()`.

The `x` and `y` parameters give the user-space coordinate at which the surface origin should appear. (The surface origin is its upper-left corner before any transformation has been applied.) The `x` and `y` patterns are negated and then set as translation values in the pattern matrix.

Other than the initial translation pattern matrix, as described above, all other pattern attributes, (such as its extend mode), are set to the default values as in `cairo_pattern_create_for_surface()`. The resulting pattern can be queried with `cairo_get_source()` so that these attributes can be modified if desired, (eg. to create a repeating pattern with `cairo_pattern_set_extend()`).

`cr` : | a cairo context
`surface` : | a surface to be used to set the source pattern
`x` : | User-space X coordinate for surface origin
`y` : | User-space Y coordinate for surface origin

### cairo_get_source ()

    cairo_pattern_t* cairo_get_source           (cairo_t *cr);

Gets the current source pattern for `cr`.

`cr` : | a cairo context
Returns : | the current source pattern. This object is owned by cairo. To keep a reference to it, you must call `cairo_pattern_reference()`.

### enum cairo_antialias_t

    typedef enum _cairo_antialias {
        CAIRO_ANTIALIAS_DEFAULT,
        CAIRO_ANTIALIAS_NONE,
        CAIRO_ANTIALIAS_GRAY,
        CAIRO_ANTIALIAS_SUBPIXEL
    } cairo_antialias_t;

Specifies the type of antialiasing to do when rendering text or shapes.

`CAIRO_ANTIALIAS_DEFAULT` | Use the default antialiasing for the subsystem and target device
`CAIRO_ANTIALIAS_NONE` | Use a bilevel alpha mask
`CAIRO_ANTIALIAS_GRAY` | Perform single-color antialiasing (using shades of gray for black text on a white background, for example).
`CAIRO_ANTIALIAS_SUBPIXEL` | Perform antialiasing by taking advantage of the order of subpixel elements on devices such as LCD panels

### cairo_set_antialias ()

    void        cairo_set_antialias             (cairo_t *cr,
                                                 cairo_antialias_t antialias);

Set the antialiasing mode of the rasterizer used for drawing shapes. This value is a hint, and a particular backend may or may not support a particular value. At the current time, no backend supports `CAIRO_ANTIALIAS_SUBPIXEL` when drawing shapes.

Note that this option does not affect text rendering, instead see `cairo_font_options_set_antialias()`.

`cr` : | a cairo_t
`antialias` : | the new antialiasing mode

### cairo_get_antialias ()

    cairo_antialias_t cairo_get_antialias       (cairo_t *cr);

Gets the current shape antialiasing mode, as set by `cairo_set_shape_antialias()`.

`cr` : | a cairo context
Returns : | the current shape antialiasing mode.

### cairo_set_dash ()

    void        cairo_set_dash                  (cairo_t *cr,
                                                 const double *dashes,
                                                 int num_dashes,
                                                 double offset);

Sets the dash pattern to be used by `cairo_stroke()`. A dash pattern is specified by `dashes`, an array of positive values. Each value provides the length of alternate "on" and "off" portions of the stroke. The `offset` specifies an offset into the pattern at which the stroke begins.

Each "on" segment will have caps applied as if the segment were a separate sub-path. In particular, it is valid to use an "on" length of 0.0 with CAIRO_LINE_CAP_ROUND or CAIRO_LINE_CAP_SQUARE in order to distributed dots or squares along a path.

Note: The length values are in user-space units as evaluated at the time of stroking. This is not necessarily the same as the user space at the time of `cairo_set_dash()`.

If `num_dashes` is 0 dashing is disabled.

If `num_dashes` is 1 a symmetric pattern is assumed with alternating on and off portions of the size specified by the single value in `dashes`.

If any value in `dashes` is negative, or if all values are 0, then `cairo_t` will be put into an error state with a status of CAIRO_STATUS_INVALID_DASH.

`cr` : | a cairo context
`dashes` : | an array specifying alternate lengths of on and off stroke portions
`num_dashes` : | the length of the dashes array
`offset` : | an offset into the dash pattern at which the stroke should start

### enum cairo_fill_rule_t

    typedef enum _cairo_fill_rule {
        CAIRO_FILL_RULE_WINDING,
        CAIRO_FILL_RULE_EVEN_ODD
    } cairo_fill_rule_t;

cairo_fill_rule_t is used to select how paths are filled. For both fill rules, whether or not a point is included in the fill is determined by taking a ray from that point to infinity and looking at intersections with the path. The ray can be in any direction, as long as it doesn't pass through the end point of a segment or have a tricky intersection such as intersecting tangent to the path. (Note that filling is not actually implemented in this way. This is just a description of the rule that is applied.)

New entries may be added in future versions.

`CAIRO_FILL_RULE_WINDING` | If the path crosses the ray from left-to-right, counts +1. If the path crosses the ray from right to left, counts -1. (Left and right are determined from the perspective of looking along the ray from the starting point.) If the total count is non-zero, the point will be filled.
`CAIRO_FILL_RULE_EVEN_ODD` | Counts the total number of intersections, without regard to the orientation of the contour. If the total number of intersections is odd, the point will be filled.

### cairo_set_fill_rule ()

    void        cairo_set_fill_rule             (cairo_t *cr,
                                                 cairo_fill_rule_t fill_rule);

Set the current fill rule within the cairo context. The fill rule is used to determine which regions are inside or outside a complex (potentially self-intersecting) path. The current fill rule affects both cairo_fill and cairo_clip. See cairo_fill_rule_t for details on the semantics of each available fill rule.

`cr` : | a cairo_t
`fill_rule` : | a fill rule, specified as a cairo_fill_rule_t

### cairo_get_fill_rule ()

    cairo_fill_rule_t cairo_get_fill_rule       (cairo_t *cr);

Gets the current fill rule, as set by `cairo_set_fill_rule()`.

`cr` : | a cairo context
Returns : | the current fill rule.

### enum cairo_line_cap_t

    typedef enum _cairo_line_cap {
        CAIRO_LINE_CAP_BUTT,
        CAIRO_LINE_CAP_ROUND,
        CAIRO_LINE_CAP_SQUARE
    } cairo_line_cap_t;

enumeration for style of line-endings

`CAIRO_LINE_CAP_BUTT` | start(stop) the line exactly at the start(end) point
`CAIRO_LINE_CAP_ROUND` | use a round ending, the center of the circle is the end point
`CAIRO_LINE_CAP_SQUARE` | use squared ending, the center of the square is the end point

### cairo_set_line_cap ()

    void        cairo_set_line_cap              (cairo_t *cr,
                                                 cairo_line_cap_t line_cap);

Sets the current line cap style within the cairo context. See cairo_line_cap_t for details about how the available line cap styles are drawn.

As with the other stroke parameters, the current line cap style is examined by `cairo_stroke()`, `cairo_stroke_extents()`, and `cairo_stroke_to_path()`, but does not have any effect during path construction.

`cr` : | a cairo context, as a cairo_t
`line_cap` : | a line cap style, as a cairo_line_cap_t

### cairo_get_line_cap ()

    cairo_line_cap_t cairo_get_line_cap         (cairo_t *cr);

Gets the current line cap style, as set by `cairo_set_line_cap()`.

`cr` : | a cairo context
Returns : | the current line cap style.

### enum cairo_line_join_t

    typedef enum _cairo_line_join {
        CAIRO_LINE_JOIN_MITER,
        CAIRO_LINE_JOIN_ROUND,
        CAIRO_LINE_JOIN_BEVEL
    } cairo_line_join_t;

### cairo_set_line_join ()

    void        cairo_set_line_join             (cairo_t *cr,
                                                 cairo_line_join_t line_join);

Sets the current line join style within the cairo context. See cairo_line_join_t for details about how the available line join styles are drawn.

As with the other stroke parameters, the current line join style is examined by `cairo_stroke()`, `cairo_stroke_extents()`, and `cairo_stroke_to_path()`, but does not have any effect during path construction.

`cr` : | a cairo context, as a cairo_t
`line_join` : | a line joint style, as a cairo_line_join_t

### cairo_get_line_join ()

    cairo_line_join_t cairo_get_line_join       (cairo_t *cr);

Gets the current line join style, as set by `cairo_set_line_join()`.

`cr` : | a cairo context
Returns : | the current line join style.

### cairo_set_line_width ()

    void        cairo_set_line_width            (cairo_t *cr,
                                                 double width);

Sets the current line width within the cairo context. The line width value specifies the diameter of a pen that is circular in user space, (though device-space pen may be an ellipse in general due to scaling/shear/rotation of the CTM).

Note: When the description above refers to user space and CTM it refers to the user space and CTM in effect at the time of the stroking operation, not the user space and CTM in effect at the time of the call to `cairo_set_line_width()`. The simplest usage makes both of these spaces identical. That is, if there is no change to the CTM between a call to `cairo_set_line_with()` and the stroking operation, then one can just pass user-space values to `cairo_set_line_width()` and ignore this note.

As with the other stroke parameters, the current line width is examined by `cairo_stroke()`, `cairo_stroke_extents()`, and `cairo_stroke_to_path()`, but does not have any effect during path construction.

The default line width value is 2.0.

`cr` : | a cairo_t
`width` : | a line width

### cairo_get_line_width ()

    double      cairo_get_line_width            (cairo_t *cr);

`cr` : | a cairo context
Returns : | the current line width value exactly as set by `cairo_set_line_width()`. Note that the value is unchanged even if the CTM has changed between the calls to `cairo_set_line_width()` and `cairo_get_line_width()`.

### cairo_set_miter_limit ()

    void        cairo_set_miter_limit           (cairo_t *cr,
                                                 double limit);

`cr` : |
`limit` : |

### cairo_get_miter_limit ()

    double      cairo_get_miter_limit           (cairo_t *cr);

Gets the current miter limit, as set by `cairo_set_miter_limit()`.

`cr` : | a cairo context
Returns : | the current miter limit.

### enum cairo_operator_t

    typedef enum _cairo_operator {
        CAIRO_OPERATOR_CLEAR,

        CAIRO_OPERATOR_SOURCE,
        CAIRO_OPERATOR_OVER,
        CAIRO_OPERATOR_IN,
        CAIRO_OPERATOR_OUT,
        CAIRO_OPERATOR_ATOP,

        CAIRO_OPERATOR_DEST,
        CAIRO_OPERATOR_DEST_OVER,
        CAIRO_OPERATOR_DEST_IN,
        CAIRO_OPERATOR_DEST_OUT,
        CAIRO_OPERATOR_DEST_ATOP,

        CAIRO_OPERATOR_XOR,
        CAIRO_OPERATOR_ADD,
        CAIRO_OPERATOR_SATURATE
    } cairo_operator_t;

### cairo_set_operator ()

    void        cairo_set_operator              (cairo_t *cr,
                                                 cairo_operator_t op);

Sets the compositing operator to be used for all drawing operations. See cairo_operator_t for details on the semantics of each available compositing operator.

XXX: I'd also like to direct the reader's attention to some (not-yet-written) section on cairo's imaging model. How would I do that if such a section existed? (cworth).

`cr` : | a cairo_t
`op` : | a compositing operator, specified as a cairo_operator_t

### cairo_get_operator ()

    cairo_operator_t cairo_get_operator         (cairo_t *cr);

Gets the current compositing operator for a cairo context.

`cr` : | a cairo context
Returns : | the current compositing operator.

### cairo_set_tolerance ()

    void        cairo_set_tolerance             (cairo_t *cr,
                                                 double tolerance);

Sets the tolerance used when converting paths into trapezoids. Curved segments of the path will be subdivided until the maximum deviation between the original path and the polygonal approximation is less than `tolerance`. The default value is 0.1. A larger value will give better performance, a smaller value, better appearance. (Reducing the value from the default value of 0.1 is unlikely to improve appearance significantly.)

`cr` : | a cairo_t
`tolerance` : | the tolerance, in device units (typically pixels)

### cairo_get_tolerance ()

    double      cairo_get_tolerance             (cairo_t *cr);

Gets the current tolerance value, as set by `cairo_set_tolerance()`.

`cr` : | a cairo context
Returns : | the current tolerance value.

### cairo_clip ()

    void        cairo_clip                      (cairo_t *cr);

Establishes a new clip region by intersecting the current clip region with the current path as it would be filled by `cairo_fill()` and according to the current fill rule (see `cairo_set_fill_rule()`).

After cairo_clip, the current path will be cleared from the cairo context.

The current clip region affects all drawing operations by effectively masking out any changes to the surface that are outside the current clip region.

Calling `cairo_clip()` can only make the clip region smaller, never larger. But the current clip is part of the graphics state, so a temporary restriction of the clip region can be achieved by calling `cairo_clip()` within a `cairo_save()`/`cairo_restore()` pair. The only other means of increasing the size of the clip region is `cairo_reset_clip()`.

`cr` : | a cairo context

### cairo_clip_preserve ()

    void        cairo_clip_preserve             (cairo_t *cr);

Establishes a new clip region by intersecting the current clip region with the current path as it would be filled by `cairo_fill()` and according to the current fill rule (see `cairo_set_fill_rule()`).

Unlike `cairo_clip()`, cairo_clip_preserve preserves the path within the cairo context.

The current clip region affects all drawing operations by effectively masking out any changes to the surface that are outside the current clip region.

Calling `cairo_clip()` can only make the clip region smaller, never larger. But the current clip is part of the graphics state, so a temporary restriction of the clip region can be achieved by calling `cairo_clip()` within a `cairo_save()`/`cairo_restore()` pair. The only other means of increasing the size of the clip region is `cairo_reset_clip()`.

`cr` : | a cairo context

### cairo_reset_clip ()

    void        cairo_reset_clip                (cairo_t *cr);

Reset the current clip region to its original, unrestricted state. That is, set the clip region to an infinitely large shape containing the target surface. Equivalently, if infinity is too hard to grasp, one can imagine the clip region being reset to the exact bounds of the target surface.

Note that code meant to be reusable should not call `cairo_reset_clip()` as it will cause results unexpected by higher-level code which calls `cairo_clip()`. Consider using `cairo_save()` and `cairo_restore()` around `cairo_clip()` as a more robust means of temporarily restricting the clip region.

`cr` : | a cairo context

### cairo_fill ()

    void        cairo_fill                      (cairo_t *cr);

A drawing operator that fills the current path according to the current fill rule, (each sub-path is implicitly closed before being filled). After cairo_fill, the current path will be cleared from the cairo context. See `cairo_set_fill_rule()` and `cairo_fill_preserve()`.

`cr` : | a cairo context

### cairo_fill_preserve ()

    void        cairo_fill_preserve             (cairo_t *cr);

A drawing operator that fills the current path according to the current fill rule, (each sub-path is implicitly closed before being filled). Unlike `cairo_fill()`, cairo_fill_preserve preserves the path within the cairo context.

See `cairo_set_fill_rule()` and `cairo_fill()`.

`cr` : | a cairo context

### cairo_fill_extents ()

    void        cairo_fill_extents              (cairo_t *cr,
                                                 double *x1,
                                                 double *y1,
                                                 double *x2,
                                                 double *y2);

`cr` : |
`x1` : |
`y1` : |
`x2` : |
`y2` : |

### cairo_in_fill ()

    cairo_bool_t cairo_in_fill                  (cairo_t *cr,
                                                 double x,
                                                 double y);

Tests whether the given point is on the area filled by doing a `cairo_stroke()` operation on `cr` given the current path and filling parameters.

See `cairo_fill()`, `cairo_set_fill_rule()` and `cairo_fill_preserve()`.

`cr` : | a cairo context
`x` : | X coordinate of the point to test
`y` : | Y coordinate of the point to test
Returns : |

### cairo_mask ()

    void        cairo_mask                      (cairo_t *cr,
                                                 cairo_pattern_t *pattern);

A drawing operator that paints the current source using the alpha channel of `pattern` as a mask. (Opaque areas of `pattern` are painted with the source, transparent areas are not painted.)

`cr` : | a cairo context
`pattern` : | a cairo_pattern_t

### cairo_mask_surface ()

    void        cairo_mask_surface              (cairo_t *cr,
                                                 cairo_surface_t *surface,
                                                 double surface_x,
                                                 double surface_y);

A drawing operator that paints the current source using the alpha channel of `surface` as a mask. (Opaque areas of `surface` are painted with the source, transparent areas are not painted.)

`cr` : | a cairo context
`surface` : | a cairo_surface_t
`surface_x` : | X coordinate at which to place the origin of `surface`
`surface_y` : | Y coordinate at which to place the origin of `surface`

### cairo_paint ()

    void        cairo_paint                     (cairo_t *cr);

A drawing operator that paints the current source everywhere within the current clip region.

`cr` : | a cairo context

### cairo_paint_with_alpha ()

    void        cairo_paint_with_alpha          (cairo_t *cr,
                                                 double alpha);

A drawing operator that paints the current source everywhere within the current clip region using a mask of constant alpha value `alpha`. The effect is similar to `cairo_paint()`, but the drawing is faded out using the alpha value.

`cr` : | a cairo context
`alpha` : | alpha value, between 0 (transparent) and 1 (opaque)

### cairo_stroke ()

    void        cairo_stroke                    (cairo_t *cr);

A drawing operator that strokes the current path according to the current line width, line join, line cap, and dash settings. After cairo_stroke, the current path will be cleared from the cairo context. See `cairo_set_line_width()`, `cairo_set_line_join()`, `cairo_set_line_cap()`, `cairo_set_dash()`, and `cairo_stroke_preserve()`.

Note: Degenerate segments and sub-paths are treated specially and provide a useful result. These can result in two different situations:

1. Zero-length "on" segments set in `cairo_set_dash()`. If the cap style is CAIRO_LINE_CAP_ROUND or CAIRO_LINE_CAP_SQUARE then these segments will be drawn as circular dots or squares respectively. In the case of CAIRO_LINE_CAP_SQUARE, the orientation of the squares is determined by the direction of the underlying path.

2. A sub-path created by `cairo_move_to()` followed by either a `cairo_close_path()` or one or more calls to `cairo_line_to()` to the same coordinate as the `cairo_move_to()`. If the cap style is CAIRO_LINE_CAP_ROUND then these sub-paths will be drawn as circular dots. Note that in the case of CAIRO_LINE_CAP_SQUARE a degenerate sub-path will not be drawn at all, (since the correct orientation is indeterminate).

In no case will a cap style of CAIRO_LINE_CAP_BUTT cause anything to be drawn in the case of either degenerate segments or sub-paths.

`cr` : | a cairo context

### cairo_stroke_preserve ()

    void        cairo_stroke_preserve           (cairo_t *cr);

A drawing operator that strokes the current path according to the current line width, line join, line cap, and dash settings. Unlike `cairo_stroke()`, cairo_stroke_preserve preserves the path within the cairo context.

See `cairo_set_line_width()`, `cairo_set_line_join()`, `cairo_set_line_cap()`, `cairo_set_dash()`, and `cairo_stroke_preserve()`.

`cr` : | a cairo context

### cairo_stroke_extents ()

    void        cairo_stroke_extents            (cairo_t *cr,
                                                 double *x1,
                                                 double *y1,
                                                 double *x2,
                                                 double *y2);

`cr` : |
`x1` : |
`y1` : |
`x2` : |
`y2` : |

### cairo_in_stroke ()

    cairo_bool_t cairo_in_stroke                (cairo_t *cr,
                                                 double x,
                                                 double y);

Tests whether the given point is on the area stroked by doing a `cairo_stroke()` operation on `cr` given the current path and stroking parameters.

See cairo_stroke, `cairo_set_line_width()`, `cairo_set_line_join()`, `cairo_set_line_cap()`, `cairo_set_dash()`, and `cairo_stroke_preserve()`.

`cr` : | a cairo context
`x` : | X coordinate of the point to test
`y` : | Y coordinate of the point to test
Returns : |

### cairo_copy_page ()

    void        cairo_copy_page                 (cairo_t *cr);

Emits the current page for backends that support multiple pages, but doesn't clear it, so, the contents of the current page will be retained for the next page too. Use `cairo_show_page()` if you want to get an empty page after the emission.

`cr` : | a cairo context

### cairo_show_page ()

    void        cairo_show_page                 (cairo_t *cr);

Emits and clears the current page for backends that support multiple pages. Use `cairo_copy_page()` if you don't want to clear the page.

`cr` : | a cairo context