Check if image is white; if so make transparent - php

Question

I have an image, and I want to make it transparent if it's completely white. I have the following code already for GD for getting a part of the image:
$srcp = imagecreatefrompng("../images/".$_GET['b'].".png");
$destp = imagecreate(150, 150);
imagecopyresampled($destp, $srcp, 0, 0, 40, 8, 150, 150, 8, 8);
header('Content-type: image/png');
imagepng($destp);
But how can I have it first check if the image is completely white, if so change it to transparent, and apply that to $destp?

Answer 1

EDIT:
Based on re-reading the question, and the discussion below, I believe this is what you're looking for:
$width = 150;
$height = 150;
$srcp = imagecreatefrompng("../images/".$_GET['b'].".png");
$destp = imagecreatetruecolor(150, 150);
$white = 0;
for ($y = 0; $y < $height; $y++)
{
for ($x = 0; $x < $width; $x++)
{
$currentColor = imagecolorat($srcp, $x, $y);
$colorParts = imagecolorsforindex($srcp, $currentColor);
if (($colorParts['red'] == 255 &&
$colorParts['green'] == 255 &&
$colorParts['blue'] == 255))
{
$white++;
}
}
}
if ($white == ($width * $height))
{
imagecopyresampled($destp, $srcp, 0, 0, 40, 8, 150, 150, 8, 8);
}
else
{
imagesavealpha($destp, true);
imagefill($destp, 0, 0, imagecolorallocatealpha($destp, 0, 0, 0, 127));
}
header('Content-type: image/png');
imagepng($destp);
This produces a blank image if the original image's 8x8 slice is all white, otherwise it resamples the 8x8 slice to 150x150.
Original:
I haven't ever done anything with PHP GD before, and thought it would be a fun challenge. Here's how I ended up making this happen:
$filename = 'test.png'; // input image
$image = imagecreatefrompng($filename);
// grab the input image's size
$size = getimagesize($filename);
$width = $size[0];
$height = $size[1];
$newImage = imagecreatetruecolor($width, $height);
// make sure that the image will retain alpha when saved
imagesavealpha($newImage, true);
// fill with transparent pixels first or else you'll
// get black instead of transparent
imagefill($newImage, 0, 0, imagecolorallocatealpha($newImage, 0, 0, 0, 127));
// loop through all the pixels
for ($y = 0; $y < $height; $y++)
{
for ($x = 0; $x < $width; $x++)
{
// get the current pixel's colour
$currentColor = imagecolorat($image, $x, $y);
// then break it into easily parsed bits
$colorParts = imagecolorsforindex($image, $currentColor);
// if it's NOT white
if (!($colorParts['red'] == 255 &&
$colorParts['green'] == 255 &&
$colorParts['blue'] == 255))
{
// then keep the same colour
imagesetpixel($newImage, $x, $y, $currentColor);
}
}
}
// final output, the second arg is the filename
imagepng($newImage, 'newImage.png');
It allowed me to turn this:
Into this (hard to see the alpha here, but you can open it to see):

Answer 2

A simple strightforward solution would be to use imagecolorat and iterate through all the pixels of the png and if all are white change it to transparent.
Hope this helps.

Answer 3

After a quick look through the GD manual pages, I think that the following should work for you (it did on my test server, anyway):
<?php
// Create image instance
$im = imagecreatefromgif('test.gif');
if (imagecolorstotal($im) == 1) {
$rgb = imagecolorat($im, 1, 1); // feel free to test any pixel, but if they're all
// the same colour, then it shouldn't really matter
$colors = imagecolorsforindex($im, $rgb);
if ($colors['red'] == 255 && $colors['green'] == 255 && $colors['blue'] == 255) {
// I'm ignoring $colors['alpha'] but that's optional.
echo "Yup; it's white...";
}
} else {
// if you've got more than one colour then the image has to be at least
// two colours, so not really worth checking if any pixels are white
}
// Free image
imagedestroy($im);
?>
References:
imagecolorat().
imagecolorstotal().