Programming PHP

Example 9-12. A gray rectangle with a 50% alpha channel overlaid
Figure 9-12 shows the output of Example 9-12 (alpha blending is still turned off).
Figure 9-12. A gray rectangle over the orange ellipse
If we change Example 9-12 to enable alpha blending just before the call to
ImageFilledRectangle( ), we get the image shown in Figure 9-13.
Figure 9-13. Image with alpha blending enabled
Identifying Colors
To check the color index for a specific pixel in an image, use ImageColorAt( ):
$color = ImageColorAt(image, x, y);
For images with an 8-bit color palette, the function returns a color index that you
then pass to ImageColorsForIndex( ) to get the actual RGB values:
$values = ImageColorsForIndex(image, index);
The array returned by ImageColorsForIndex( ) has keys "red", "green", and "blue". If
you call ImageColorsForIndex( ) on a color from a true color image, the returned
array has an extra key, "alpha".
230 | Chapter 9: Graphics
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