PHP convert true color image to palette image

php Xiaobian Yuzai brings you a tutorial on converting true color images into palette images. In image processing, it is a common requirement to convert true color images into palette images, which can reduce the file size of the image, improve the loading speed, and is suitable for some scenes that do not have high color requirements. This article will introduce how to use the GD library in PHP to implement this conversion process, helping you better process images and optimize web page performance.

PHP Convert true color image to palette image

In computer graphics, a truecolor image stores the complete color information of each pixel, using 24-bit or 32-bit representation. Palette images use a palette in which a limited number of colors are stored, with each pixel's index corresponding to the color in the palette.

Converting a true color image to a palette image requires the following steps:

1. Create a color palette

First, you need to create a palette that contains the collection of colors that you want to use for the palette image. The number of colors should be less than or equal to 256 (8-bit mode).

2. Quantify true color images

Next, the true color image needs to be quantized to reduce its color space to the colors in the palette. The following algorithms can be used:

• Average Slicing: Calculate the average for each color channel and round it to the nearest color in the palette.
• Middle value slicing: Sort the pixels in order and select the color of the middle pixel as the palette color.
• Octree Quantization: Use Octree Data structure Subdivide the color space into subspaces and select the color with the highest frequency in each subspace.

3. Create palette index image

After quantization, a palette-indexed image needs to be created, where each pixel value represents a color index in the palette.

4. Generate palette file

Finally, a palette file needs to be generated, which contains the RGB values ​​of the colors in the palette. Palette files typically use formats such as ACT, PAL, or BMP.

PHP code example

The following

php code example uses the GD library to convert a true color image to a palette image:

<?php

//Load true color image
$image = imagecreatefrompng("image.png");

//Create color table
$palette = array(
"white" => array(255, 255, 255),
"black" => array(0, 0, 0),
"red" => array(255, 0, 0),
"green" => array(0, 255, 0),
"blue" => array(0, 0, 255)
);

//Quantize the image through the intermediate value algorithm
imagepalettetotruecolor($image);
imagequantize($image, 256, 0, GD_TRUE_COLOR_FIXED);

//Create a color index map
$indexedImage = imagecreatetruecolor(imagesx($image), imagesy($image));
imagecopy($indexedImage, $image, 0, 0, 0, 0, imagesx($image), imagesy($image));

//Generate palette file
$paletteFile = "palette.act";
file_put_contents($paletteFile, pack("CCCCCCCCCCCCCCCCC", $palette["white"][0], $palette["white"][1], $palette["white"][2], $palette["black" ][0], $palette["black"][1], $palette["black"][2], $palette["red"][0], $palette["red"][1], $ palette["red"][2], $palette["green"][0], $palette["green"][1], $palette["green"][2], $palette["blue"] [0], $palette["blue"][1], $palette["blue"][2]));

//Save palette image
imagepng($indexedImage, "palette.png");

?>

Advantage

Converting true color images to paletted images has the following advantages:

• File Size Reduction: Palette images use indexing instead of color information, thus reducing file size.
• Faster loading times: Palette images load faster than truecolor images because there is no need to decode the color information of each pixel.
• Less memory footprint: Palette images take up less space in memory because the palettes are stored in separate files.

shortcoming

Converting true color images to paletted images also has some disadvantages:

• Reduced color fidelity: The number of colors in a palette image is limited, so the color fidelity is reduced compared to true color images.
• Not suitable for gradients: Palette images have difficulty representing gradients because they use discrete colors.
• Complex textures are lost: Palette images can have difficulty representing complex textures because they can only use a limited number of colors.

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