User`s guide

7 Working with Acquired Image Data

7-16

clear vid

Specifying the Color Space

For most image acquisition devices, the video format of the video stream determines the

color space of the acquired image data, that is, the way color information is represented

numerically.

For example, many devices represent colors as RGB values. In this color space, colors

are represented as a combination of various intensities of red, green, and blue. Another

color space, widely used for digital video, is the YCbCr color space. In this color space,

luminance (brightness or intensity) information is stored as a single component (Y).

Chrominance (color) information is stored as two color-difference components (Cb and

Cr). Cb represents the difference between the blue component and a reference value. Cr

represents the difference between the red component and a reference value.

The toolbox can return image data in grayscale, RGB, and YCbCr. To specify the color

representation of the image data, set the value of the ReturnedColorSpace property.

To display image frames using the image, imagesc, or imshow functions, the data must

use the RGB color space. Another MathWorks product, the Image Processing Toolbox

software, includes functions that convert YCbCr data to RGB data, and vice versa.

Note Some devices that claim to support the YUV color space actually support the YCbCr

color space. YUV is similar to YCbCr but not identical. The difference between YUV and

YCbCr is the scaling factor applied to the result. YUV refers to a particular scaling factor

used in composite NTSC and PAL formats. In most cases, you can specify the YCbCr

color space for devices that support YUV.

You can determine your device’s default color space using this code:

vid.ReturnedColorSpace, where vid is the name of the video object. An example of

this is shown in step 2 in the example below. There may be situations when you wish to

change the color space. The example below shows a case where the default color space is

rgb, and you change it to grayscale (step 3).

The following example illustrates how to specify the color space of the returned image

data.

Create an image acquisition object — This example creates a video input object

for a generic Windows image acquisition device. To run this example on your system,