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Web Technologies-I

Notes
Figure 11.13: Image with Alpha Blending Enabled

Develop a PHP program to show an orange circle over the gray ellipse.

11.8.2 Identifying Colours
To check the colour index for a specific pixel in an image, use ImageColorAt( ):

$color = ImageColorAt(image, x, y);

For images with an 8-bit colour palette, the function returns a colour 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 colour from a true colour image, the returned array has an
extra key, “alpha”.
11.8.3 True Colour Indexes

The colour index returned by ImageColorResolveAlpha( ) is really a 32-bit signed long,
with the first three bytes holding the red, green, and blue values, respectively. The next bit
indicates whether antialiasing is enabled for this colour, and the remaining seven bits hold the
transparency value.

$green = ImageColorResolveAlpha($im,0,0,255,127);

This code sets $green to 2130771712, which in hex is 0x7F00FF00 and in binary is 0111111100
0000001111111100000000.
This is equivalent to the following ImageColorResolveAlpha( ) call:
$green = 127<<24 | 0<<16 | 255<<8 | 0;

You can also drop the two 0 entries in this example and just make it:
$green = 127<<24 | 255<<8;

To deconstruct this value, you can use something like this:
$a = ($col & 0x7F000000) >> 24;

$r = ($col & 0x00FF0000) >> 16;

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