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# Data Representation

[Laboratory]

A digital image is a two dimensional matrix where its elements are called pixels (abbreviation of picture elements). The process of displaying an image creates a graphical representation of this matrix where the pixel values are assigned a particular grey-level (monochromatic image) or a particular color.

Below is a graphical representation of an image of a gull using the Display operator available in Khoros. Pixels with low values are assigned dark grey-levels, and high pixel values are assigned bright grey-levels. When you point the cursor at a particular pixel, the small position window below the image gives the actual pixel value and its coordinates. The top-left pixel is located at coordinate (0,0). The size of the image give the number of pixels in each direction: x or width and y or height directions. In the image shown below, there are 256 pixels in the width direction and 256 pixels in the height direction.

Original image

Click on the image to invoke the Khoros display operator. On the Khoros display, move the mouse around the image window and observe the pixel values at the specified location indicated in the position window. To exit the display operator, click on the "Quit" button, press the "Q" key or perform a double-click on the display window.

If it does not work, click here An image can have pixel values that can be of several data types: a binary image has just two possible values often assigned to black and white; a grey-level image has often positive integer values from 0 to a maximum. It is possible to have pixels with negative values, pixels with real numbers, and even pixels with complex values. An example of an image with negative pixel values are thermal images with negative temperatures. Images with pixels that are real numbers can be found in images that represent a sinusoid wave with values varying from -1 to +1. Images with complex pixel values can be found in some image transforms like the Discrete Fourier Transform. Pixels can also represent a color using three values: Red, Green and Blue, in the RGB color model.

Digital Images can be stored as files in the computer. The file contains not only the pixel values associated with each coordinate, but also an image header which provides additional information such as its size, the data type of the pixel elements, the color model, and other information.

The header associated with the `gull` image provides the following information:

    Dimensions: Width=256, Height=256
    Pixel type: unsigned byte
    Color Model: Greyscale

Examples of Images with Distinct Data Types

Below are images with different size and pixel data types. Click on them so that the display operator will be called. Observe the pixel values under the cursor.

- Image with pixel type BIT

  Size: Width=162, Height=117, Pixel type: bit, Color Model: None

- Image with pixel type FLOAT

  Size: Width=128, Height=128, Pixel type: float, Color Model: None

- Image with pixel type COMPLEX

  Size: Width=128, Height=128, Pixel type: complex, Color Model: None

- Color image, 3 elements (RGB) per pixel, pixel type Unsigned BYTE

  Size: Width=256, Height=256, Elements=3, Pixel type: unsigned byte, Color Model: RGB

- 1D Signal

  Size: Width=701, Height=1, Pixel type: double, Color Model: None

- Volume Data

  Size: Width=40, Height=32, Depth=32, Pixel type: float, Color Model: None

### Spatial Resolution

Spatial resolution is the size a pixel represents in the real world. The spatial resolution of an image is related to its height (rows) and width (columns) per world dimension.

- Lab: Data Representation