imagecolorresolve() behavior difference in 24-bit and 8-bit images

When using PHP for image processing, the imagecolorresolve() function often appears in scenes that process palette images. However, when many developers use this function, they often wonder: **What is the difference in their behavior in 24-bit (TrueColor) images and 8-bit (palette) images? **Today we will talk about this issue in depth.

What is imagecolorresolve() ?

First, let’s briefly introduce imagecolorresolve() . The function of this function is:
Find if the specified color already exists in a palette image. If it exists, return the color index; if it does not exist, it tries to add this color to the palette and returns a new index.

Basic usage examples:

 <?php
// Create a picture 8 Bit palette image
$image = imagecreate(100, 100);

// Definition of red
$red = imagecolorresolve($image, 255, 0, 0);

// Draw a rectangle with this color
imagefilledrectangle($image, 10, 10, 90, 90, $red);

// Output image
header('Content-Type: image/png');
imagepng($image);

// Destroy resources
imagedestroy($image);
?>

Here, if red already exists in the palette, imagecolorresolve() returns the existing color index; otherwise, it will automatically add red to the palette.

24-bit (TrueColor) Behavior in an image

When you create a 24-bit TrueColor image with imagecreatetruecolor() , the image itself does not have a palette . Each pixel can independently store RGB color values.

So the question is: What happens if you call imagecolorresolve() on TrueColor image?

The answer is:
Even if you call imagecolorresolve() , PHP does not really look up or add color indexes, but returns an integer calculated from RGB values ​​(i.e. color identification).

For example:

 <?php
// Create a picture 24 Bit TrueColor image
$image = imagecreatetruecolor(100, 100);

// Define blue
$blue = imagecolorresolve($image, 0, 0, 255);

// Draw a rectangle with this color
imagefilledrectangle($image, 10, 10, 90, 90, $blue);

// Output image
header('Content-Type: image/png');
imagepng($image);

imagedestroy($image);
?>

Here $blue is an integer (say 255 ), not a color index. In this mode, you don't even have to worry about the existence of the color, as each pixel can save the complete RGB information separately.

8-bit (color palette) behavior in images

In an 8-bit image (created by imagecreate() ), the image's colors are limited, with only 256 colors at most.

Therefore, when imagecolorresolve() is called, PHP will:

1. Iterate through the entire color palette to see if the specified color already exists.

2. If present, return its index.

3. If not present, add to the palette (if space is sufficient).

4. If the palette is full, return the index of the existing color closest to the specified color.

This is also why sometimes 8-bit images are prone to color distortion when processing complex images - because the colors are "approximate".

Example:

 <?php
$image = imagecreate(100, 100);

// Try adding colors that exceed the palette limit
for ($i = 0; $i < 300; $i++) {
    $color = imagecolorresolve($image, rand(0,255), rand(0,255), rand(0,255));
}

// Generate a picture，For viewing
header('Content-Type: image/png');
imagepng($image);

// Resource release
imagedestroy($image);
?>

In the above code, although 300 different colors were tried to add, there will be only up to 256 colors in the image, and the excess will be "approximately matched".

summary

Therefore, to summarize:

• In 24-bit images, imagecolorresolve() is not so necessary. You can use imagecolorallocate() directly or construct the color yourself.

• In 8-bit images, it is very important, especially when you need to control the number and distribution of the image color.

Tips

If you frequently encounter color deficiency when processing 8-bit images, you can consider calling imagecreatetruecolor() before generating the image, and then converting the TrueColor image into a palette image using imagetruecolortopalette() as needed. PHP will automatically optimize the color.

Sample code reference: