Better way to get map of all pixels of an image with GD

Question

The following code uses the SimpleImage library

I have a function that gets a pixel from an image:

function getPixel($x, $y) {

    $colors = imagecolors         


        

Answer 1

There is a faster way, but it's very tricky. What it involves is exporting the image to the GD2 format, then accessing the bytes directly. GD stores the pixels in chunks, so looping through the pixels is not straight forward. When done properly, it does yield much better performance since you're no longer incurring the overhead of calling a function on every pixel.

Here's some code that I have that I had written which capture the alpha channel from an image. As you can see, the loop isn't so terribly obvious because of chunking:

    imagesavealpha($image, true);
    ob_start();
    imagegd2($image);
    $gdRawData = ob_get_clean();

    $array = unpack("Nheader/nversion/nimageWidth/nimageHeight/nchunkSize/ndataFormat/ncolumnCount/nrowCount", $gdRawData);
    $imageWidth = $array['imageWidth'];
    $imageHeight = $array['imageHeight'];

    $chunkWidth = $chunkHeight = $array['chunkSize'];
    $columnCount = $array['columnCount'];
    $rowCount = $array['rowCount'];
    $lastColumnWidth = $imageWidth - ($columnCount - 1) * $chunkWidth;
    $lastRowHeight = $imageHeight - ($rowCount - 1) * $chunkHeight;

    static $transparencyToAlpha = array("\x7f" => "\x00", "\x7f" => "\x01", "\x7e" => "\x02", "\x7e" => "\x03", "\x7d" => "\x04", "\x7d" => "\x05", "\x7c" => "\x06", "\x7c" => "\x07", "\x7b" => "\x08", "\x7b" => "\x09", "\x7a" => "\x0a", "\x7a" => "\x0b", "\x79" => "\x0c", "\x79" => "\x0d", "\x78" => "\x0e", "\x78" => "\x0f", "\x77" => "\x10", "\x77" => "\x11", "\x76" => "\x12", "\x76" => "\x13", "\x75" => "\x14", "\x75" => "\x15", "\x74" => "\x16", "\x74" => "\x17", "\x73" => "\x18", "\x73" => "\x19", "\x72" => "\x1a", "\x72" => "\x1b", "\x71" => "\x1c", "\x71" => "\x1d", "\x70" => "\x1e", "\x70" => "\x1f", "\x6f" => "\x20", "\x6f" => "\x21", "\x6e" => "\x22", "\x6e" => "\x23", "\x6d" => "\x24", "\x6d" => "\x25", "\x6c" => "\x26", "\x6c" => "\x27", "\x6b" => "\x28", "\x6b" => "\x29", "\x6a" => "\x2a", "\x6a" => "\x2b", "\x69" => "\x2c", "\x69" => "\x2d", "\x68" => "\x2e", "\x68" => "\x2f", "\x67" => "\x30", "\x67" => "\x31", "\x66" => "\x32", "\x66" => "\x33", "\x65" => "\x34", "\x65" => "\x35", "\x64" => "\x36", "\x64" => "\x37", "\x63" => "\x38", "\x63" => "\x39", "\x62" => "\x3a", "\x62" => "\x3b", "\x61" => "\x3c", "\x61" => "\x3d", "\x60" => "\x3e", "\x60" => "\x3f", "\x5f" => "\x40", "\x5f" => "\x41", "\x5e" => "\x42", "\x5e" => "\x43", "\x5d" => "\x44", "\x5d" => "\x45", "\x5c" => "\x46", "\x5c" => "\x47", "\x5b" => "\x48", "\x5b" => "\x49", "\x5a" => "\x4a", "\x5a" => "\x4b", "\x59" => "\x4c", "\x59" => "\x4d", "\x58" => "\x4e", "\x58" => "\x4f", "\x57" => "\x50", "\x57" => "\x51", "\x56" => "\x52", "\x56" => "\x53", "\x55" => "\x54", "\x55" => "\x55", "\x54" => "\x56", "\x54" => "\x57", "\x53" => "\x58", "\x53" => "\x59", "\x52" => "\x5a", "\x52" => "\x5b", "\x51" => "\x5c", "\x51" => "\x5d", "\x50" => "\x5e", "\x50" => "\x5f", "\x4f" => "\x60", "\x4f" => "\x61", "\x4e" => "\x62", "\x4e" => "\x63", "\x4d" => "\x64", "\x4d" => "\x65", "\x4c" => "\x66", "\x4c" => "\x67", "\x4b" => "\x68", "\x4b" => "\x69", "\x4a" => "\x6a", "\x4a" => "\x6b", "\x49" => "\x6c", "\x49" => "\x6d", "\x48" => "\x6e", "\x48" => "\x6f", "\x47" => "\x70", "\x47" => "\x71", "\x46" => "\x72", "\x46" => "\x73", "\x45" => "\x74", "\x45" => "\x75", "\x44" => "\x76", "\x44" => "\x77", "\x43" => "\x78", "\x43" => "\x79", "\x42" => "\x7a", "\x42" => "\x7b", "\x41" => "\x7c", "\x41" => "\x7d", "\x40" => "\x7e", "\x40" => "\x7f", "\x3f" => "\x80", "\x3f" => "\x81", "\x3e" => "\x82", "\x3e" => "\x83", "\x3d" => "\x84", "\x3d" => "\x85", "\x3c" => "\x86", "\x3c" => "\x87", "\x3b" => "\x88", "\x3b" => "\x89", "\x3a" => "\x8a", "\x3a" => "\x8b", "\x39" => "\x8c", "\x39" => "\x8d", "\x38" => "\x8e", "\x38" => "\x8f", "\x37" => "\x90", "\x37" => "\x91", "\x36" => "\x92", "\x36" => "\x93", "\x35" => "\x94", "\x35" => "\x95", "\x34" => "\x96", "\x34" => "\x97", "\x33" => "\x98", "\x33" => "\x99", "\x32" => "\x9a", "\x32" => "\x9b", "\x31" => "\x9c", "\x31" => "\x9d", "\x30" => "\x9e", "\x30" => "\x9f", "\x2f" => "\xa0", "\x2f" => "\xa1", "\x2e" => "\xa2", "\x2e" => "\xa3", "\x2d" => "\xa4", "\x2d" => "\xa5", "\x2c" => "\xa6", "\x2c" => "\xa7", "\x2b" => "\xa8", "\x2b" => "\xa9", "\x2a" => "\xaa", "\x2a" => "\xab", "\x29" => "\xac", "\x29" => "\xad", "\x28" => "\xae", "\x28" => "\xaf", "\x27" => "\xb0", "\x27" => "\xb1", "\x26" => "\xb2", "\x26" => "\xb3", "\x25" => "\xb4", "\x25" => "\xb5", "\x24" => "\xb6", "\x24" => "\xb7", "\x23" => "\xb8", "\x23" => "\xb9", "\x22" => "\xba", "\x22" => "\xbb", "\x21" => "\xbc", "\x21" => "\xbd", "\x20" => "\xbe", "\x20" => "\xbf", "\x1f" => "\xc0", "\x1f" => "\xc1", "\x1e" => "\xc2", "\x1e" => "\xc3", "\x1d" => "\xc4", "\x1d" => "\xc5", "\x1c" => "\xc6", "\x1c" => "\xc7", "\x1b" => "\xc8", "\x1b" => "\xc9", "\x1a" => "\xca", "\x1a" => "\xcb", "\x19" => "\xcc", "\x19" => "\xcd", "\x18" => "\xce", "\x18" => "\xcf", "\x17" => "\xd0", "\x17" => "\xd1", "\x16" => "\xd2", "\x16" => "\xd3", "\x15" => "\xd4", "\x15" => "\xd5", "\x14" => "\xd6", "\x14" => "\xd7", "\x13" => "\xd8", "\x13" => "\xd9", "\x12" => "\xda", "\x12" => "\xdb", "\x11" => "\xdc", "\x11" => "\xdd", "\x10" => "\xde", "\x10" => "\xdf", "\x0f" => "\xe0", "\x0f" => "\xe1", "\x0e" => "\xe2", "\x0e" => "\xe3", "\x0d" => "\xe4", "\x0d" => "\xe5", "\x0c" => "\xe6", "\x0c" => "\xe7", "\x0b" => "\xe8", "\x0b" => "\xe9", "\x0a" => "\xea", "\x0a" => "\xeb", "\x09" => "\xec", "\x09" => "\xed", "\x08" => "\xee", "\x08" => "\xef", "\x07" => "\xf0", "\x07" => "\xf1", "\x06" => "\xf2", "\x06" => "\xf3", "\x05" => "\xf4", "\x05" => "\xf5", "\x04" => "\xf6", "\x04" => "\xf7", "\x03" => "\xf8", "\x03" => "\xf9", "\x02" => "\xfa", "\x02" => "\xfb", "\x01" => "\xfc", "\x01" => "\xfd", "\x00" => "\xfe", "\x00" => "\xff");

    $alphaDataSize = $imageWidth * $imageHeight;
    $alphaData = str_repeat("\x00", $alphaDataSize);

    // loop through all rows 
    for($r = 0, $j = 23; $r < $rowCount; $r++) {
        $rowHeight = ($r == $rowCount - 1) ? $lastRowHeight : $chunkHeight;
        // loop through chunks in each row
        for($c = 0; $c < $columnCount; $c++) {
            $columnWidth = ($c == $columnCount - 1) ? $lastColumnWidth : $chunkWidth;
            $firstY = $r * $chunkHeight;
            $lastY = $firstY + $rowHeight;
            // loop through scanlines in each chunk
            for($y = $firstY; $y < $lastY; $y++) {
                $firstX = $c * $chunkWidth;
                $firstAlphaPosition = $y * $imageWidth + $firstX;
                $lastAlphaPosition = $firstAlphaPosition + $columnWidth;
                // loop through each pixel in each chunk scanline
                for($i = $firstAlphaPosition; $i < $lastAlphaPosition; $i++, $j+= 4) {
                    $alphaData[$i] = $transparencyToAlpha[$gdRawData[$j]];
                }
            }
        }
    }

Each pixel consists of four bytes: alpha, red, green, and blue. Here, I'm only capturing the first byte.