imagetruecolortopalette

(PHP 4 >= 4.0.6, PHP 5, PHP 7, PHP 8)

imagetruecolortopalette — Convert a true color image to a palette image

Descrizione

imagetruecolortopalette(GdImage $image, bool $dither, int $num_colors): bool

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.

Elenco dei parametri

image: Una risorsa immagine, restituita da una delle funzioni di creazione immagine, come imagecreatetruecolor().
dither: Indicates if the image should be dithered - if it is true then dithering will be used which will result in a more speckled image but with better color approximation.
num_colors: Sets the maximum number of colors that should be retained in the palette.

Valori restituiti

Restituisce true in caso di successo, false in caso di fallimento.

Log delle modifiche

Versione	Descrizione
8.0.0	`image` expects a GdImage instance now; previously, a valid `gd` resource was expected.

Esempi

Example #1 Converting a true color image to a palette-based image

<?php
// Create a new true color image
$im = imagecreatetruecolor(100, 100);

// Convert to palette-based with no dithering and 255 colors
imagetruecolortopalette($im, false, 255);

// Save the image
imagepng($im, './paletteimage.png');
imagedestroy($im);
?>

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User Contributed Notes 6 notes

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zmorris at zsculpt dot com ¶

20 years ago

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 );
}
?>

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djcassis(a)gmail.com ¶

16 years ago

>> zmorris at zsculpt dot com

I don't have the imageColorMatch() function on my server, but I could slighty improve the quality of the GIF/PNG image by converting it first to 256 colors, then to true colors and finally to the desired number of colors.

<?php

$dither = true;
$colors = 64;

$tmp = imageCreateFromJpeg('example.jpg');
$width = imagesX($tmp);
$height = imagesY($tmp);
imageTrueColorToPalette($tmp, $dither, 256);
$image = imageCreateTrueColor($width, $height);
imageCopy($image, $tmp, 0, 0, 0, 0, $width, $height);
imageDestroy($tmp);
imageTrueColorToPalette($image, $dither, $colors);

?>

Final $image will still have less than 64 colors, but more than if it was directly converted to 64 colors, and they match the JPEG image more.

Dunno why true colors to palette conversions are such a problem...

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php at roelvanmastbergen dot nl ¶

20 years ago

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.

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jemore at nospaM dot m6net dot fr ¶

21 years ago

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.

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darkelder at php dot net ¶

21 years ago

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();

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-4

will at fnatic dot com ¶

18 years ago

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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