PHP Image manipulation moire effect - php

Question

I am making a site where people can upload their own background images. One of the things users really having a problem understanding is image sizes and resolution. If I say 'Make sure your background is at least 1080x1920 pixels (Full HiDef)' I am guessing a good 70% of people will not know what this means or how to do it.
So what I would like to do is enlarge small images in a nicer way than just making them blurry. What I would like to do is something like this:-
http://nanotux.com/plugins/fullscreenr/index.html
where basically every other dot is a black pixel, that way small images will be twice the size when they are stretched to 1080x1920 and generally look better.
Does anyone know of a way to do this with PHP's image functions?
(as an aside does anyone know what this type of effect should be called? Moire? would that be accurate?)
Thanks in advance

Answer 1

If I understood you well, you want to set every second pixel in x and y dimensions to black on the stretched image?
This should do the trick (not tested, I relied on PHP documentation).
$initialImage = ... // handle to the image
$srcWidth = imagesx($initialImage);
$srcHeight = imagesy($initialImage);
$maxX = 1920;
$maxY = 1080;
$newImage = imagecreatetruecolor($max_x, $max_y);
imagecopyresampled ($newImage, $initialImage, 0,0,0,0, $maxX, $maxY, $srcWidth, $srcHeight);
$BLACK = imagecolorallocate($newImage, 0, 0, 0);
for($x=0; $x+=2; $x<$max_x){
for($y=0; $y+=2; $y<$max_y){
imagesetpixel($newImage, $x, $y, $BLACK);
}
}
Documentation: imagesetpixel, imagecolorallocate, imagecopyresampled,
imagecreatetruecolor.
Read PHP documentation and examples there. Remember to use imagecopyresampled instead of imagecopyresized to have better quality.

Answer 2

The most optimal way would be to create a transparent raster (.png) "by hand" (i.e. create it programmatically as in jakub.gieryluk's solution), and overlay that, multiple times if needed, via imagecopy().
Drawing pixel by pixel is painfully slow.