PHP - Replace colour within image - php

Question

I hope someone can help,
I have made a script that masks images... however it is reliant on a colour to mask with ( 'green screen' style). The trouble is if the image that I am masking contains that colour it is ruined.
What I am looking to do is prior to masking the image replace any occurance of my keying colour (0,0,255) with a similar colour such as 0,0,254.
I have found a few solutions based around gif's or 256 colour PNG as they are indexed..
So my question is also will it be more efficient to convert it to a gif or 256 png then look through the index and replace the colour or search through every pixel and replace the colours.
Thanks,

Answer 1

You need to open the input file and scan each pixel to check for your chromokey value.
Something like this:
// Open input and output image
$src = imagecreatefromJPEG('input.jpg') or die('Problem with source');
$out = ImageCreateTrueColor(imagesx($src),imagesy($src)) or die('Problem In Creating image');
// scan image pixels
for ($x = 0; $x < imagesx($src); $x++) {
for ($y = 0; $y < imagesy($src); $y++) {
$src_pix = imagecolorat($src,$x,$y);
$src_pix_array = rgb_to_array($src_pix);
// check for chromakey color
if ($src_pix_array[0] == 0 && $src_pix_array[1] == 0 && $src_pix_array[2] == 255) {
$src_pix_array[2] = 254;
}
imagesetpixel($out, $x, $y, imagecolorallocate($out, $src_pix_array[0], $src_pix_array[1], $src_pix_array[2]));
}
}
// write $out to disc
imagejpeg($out, 'output.jpg',100) or die('Problem saving output image');
imagedestroy($out);
// split rgb to components
function rgb_to_array($rgb) {
$a[0] = ($rgb >> 16) & 0xFF;
$a[1] = ($rgb >> 8) & 0xFF;
$a[2] = $rgb & 0xFF;
return $a;
}

Answer 2

Here is the replace colour solution that first converts to 256 pallet:
//Open Image
$Image = imagecreatefromJPEG('input.jpg') or die('Problem with source');
//set the image to 256 colours
imagetruecolortopalette($Image,0,256);
//Find the Chroma colour
$RemChroma = imagecolorexact( $Image, 0,0,255 );
//Replace Chroma Colour
imagecolorset($Image,$RemChroma,0,0,254);
//Use function to convert back to true colour
imagepalettetotruecolor($Image);
function imagepalettetotruecolor(&$img)
{
if (!imageistruecolor($img))
{
$w = imagesx($img);
$h = imagesy($img);
$img1 = imagecreatetruecolor($w,$h);
imagecopy($img1,$img,0,0,0,0,$w,$h);
$img = $img1;
}
}
I personally prefer radio4fans solution as it is lossless, but if speed is your goal this is superior.