11.2.19 Special FX, ScreenThis is NOT the latest copy of this book; click here for the latest version.
Like interlacing, the screen effect is also done by colouring various pixels black so that only part of the image shows through. Also like interlacing, a screened image may only look good until you try it yourself and see the results on your own screen, so do not be surprised if my picture here does not look quite right!
Behind the scenes, the screen effect is essentially just two-way interlacing - the black lines go horizontally and vertically, leaving the squares of the original picture peeking through. As a result of it being so close to the interlacing code, it should be quite easy for you:
function screen(&$image) {
$imagex = imagesx($image);
$imagey = imagesy($image);
GLOBAL $black;
for($x = 1; $x <= $imagex; $x += 2) {
imageline($image, $x, 0, $x, $imagey, $black);
}
for($y = 1; $y <= $imagey; $y += 2) {
imageline($image, 0, $y, $imagex, $y, $black);
}
}
The rest of the script remains the same, you just need to change the function call. As you can see, it does exactly the same as interlacing, except once for columns and once for rows. Easy!
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