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Convert a true color image to a palette image
(PHP 4 >= 4.0.6, PHP 5)
imagetruecolortopalette() converts a truecolor image to a palette image. The code for this function was originally drawn from the Independent JPEG Group library code, which is excellent. The code has been modified to preserve as much alpha channel information as possible in the resulting palette, in addition to preserving colors as well as possible. This does not work as well as might be hoped. It is usually best to simply produce a truecolor output image instead, which guarantees the highest output quality. Parameters
Code Examples / Notes » imagetruecolortopalettedarkelder
TrueColor images should be converted to Palette images with this function. So, if you want to use imagecolorstotal() function [ http://php.net/manual/en/function.imagecolorstotal.php ] , you should first convert the image to a palette image with imagetruecolortopalette();
php
The palette created by this function often looks quite awful (at least it did on all of my test images). A better way to convert your true-colour images is by first making a resized copy of them with imagecopyresampled() to a 16x16 pixel destination. The resized image then contains only 256 pixels, which is exactly the number of colours you need. These colours usually look a lot better than the ones generated by imagetruecolortopalette(). The only disadvantage to this method I have found is that different-coloured details in the original image are lost in the conversion. zmorris
Sometimes this function gives ugly/dull colors (especially when ncolors < 256). Here is a replacement that uses a temporary image and ImageColorMatch() to match the colors more accurately. It might be a hair slower, but the file size ends up the same: <?php function ImageTrueColorToPalette2( $image, $dither, $ncolors ) { $width = imagesx( $image ); $height = imagesy( $image ); $colors_handle = ImageCreateTrueColor( $width, $height ); ImageCopyMerge( $colors_handle, $image, 0, 0, 0, 0, $width, $height, 100 ); ImageTrueColorToPalette( $image, $dither, $ncolors ); ImageColorMatch( $colors_handle, $image ); ImageDestroy( $colors_handle ); } ?> jemore
If you open a truecolor image (with imageCreateFromPng for example), and you save it directly to GIF format with imagegif, you can have a 500 internal server error. You must use imageTrueColorToPalette to reduce to 256 colors before saving the image in GIF format.
burninleo
If You know which palette to use (e.g. 255 colors greyscale) You may achieve better results using the following way: 1. Create new image 2. Apply palette 3. imagecopy() the content This is especially helpful if you created a greyscale picture in trucolor-mode (to use antialiasing for example) but need to send it as palette (to use transparency in Internet Explorer). The following example will *not* create great results from "real" truecolor images but works well on grey truecolor images: <?PHP function imageTruecolorToGrayscale(&$image) { $copy = $image; $dx = imagesx($image); $dy = imagesy($image); $image = imagecreate($dx, $dy); // 254 Colors + 1 reserved for transparency $transparency = imagecolorallocate($image, 0, 255, 0); $max = 255; $dd = 254; for ($i=0; $i<$dd; $i++) { $val = round($max * $i / ($dd-1)); imagecolorallocate($image, $val, $val, $val); } imagecopy($image, $copy, 0, 0, 0, 0, $dx, $dy); imagedestroy($copy); return $transparency; } ?> will
a basic palette to true color function <?php function imagepalettetotruecolor(&$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; } } ?> |
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