Page 281 - Open Soource Technologies 304.indd

Unit 11: Graphics

You can use the ImageTrueColorToPalette( ) function to convert a true colour image to one Notes
with a colour index (also known as a paletted image).

Develop a PHP program to show a white circle on a black background.
11.8.1 Using the Alpha Channel

In above Example, we turned off alpha blending before drawing our background and our ellipse.
Alpha blending is a toggle that determines whether the alpha channel, if present, should be
applied when drawing. If alpha blending is off, the old pixel is replaced with the new pixel.
If an alpha channel exists for the new pixel, it is maintained, but all pixel information for the
original pixel being overwritten is lost.
Example illustrates alpha blending by drawing a gray rectangle with a 50% alpha channel over
an orange ellipse.
A gray rectangle with a 50% alpha channel overlaid.

<?php
$im = ImageCreateTrueColor(150,150);

$white = ImageColorAllocate($im,255,255,255);
ImageAlphaBlending($im, false);
ImageFilledRectangle($im,0,0,150,150,$white);

$red = ImageColorResolveAlpha($im,255,50,0,63);
ImageFilledEllipse($im,75,75,80,50,$red);
$gray = ImageColorResolveAlpha($im,70,70,70,63);
ImageAlphaBlending($im, false);

ImageFilledRectangle($im,60,60,120,120,$gray);
header(‘Content-Type: image/png’);

ImagePNG($im);
?>
Figure 11.12 shows the output of example (alpha blending is still turned off).

Figure 11.12: A Gray Rectangle over the Orange Ellipse

If we change Example to enable alpha blending just before the call to Image-FilledRectangle(
), we get the image shown in Figure 11.13.

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