Let's talk about how to set black to transparent in php

In PHP programming, image processing functions are often used, such as scaling and cropping images. One problem often encountered is how to make the black background in the image transparent and realize PNG 24-bit transparent images. .

Today we will discuss how to set black to transparent using PHP.

First, we need to use PHP's GD library to process images. The GD library is a popular graphics processing library that can be used to process various graphics files.

In PHP, you can open the GD library through the following code:

<?php
// 开启GD库
if(!function_exists(&#39;imagecreate&#39;)){
   echo &#39;GD库未安装！&#39;;
}
?>

Next, we need to understand how to use the GD library to operate images.

The basic process of operating images in the PHP-GD library is as follows:

1. Create an image;
2. Read an image;
3. Copy An image;
4. Image scaling, cropping and rotation;
5. Picture output display.

Here, we mainly focus on the fourth step, how to scale, crop and rotate the image.

For scaling and rotation operations, you can directly use the relevant functions. For cropping operations, we need to use the imagecopy() function. The imagecopy() function can copy a part of the original image to the target image to achieve a cropping effect.

When using these functions to operate pictures, we need to pay attention to some details. For example, when saving pictures, we need to pay attention to the saved format and color model. Normally, we need to save the image in PNG format and use the True Color model.

Now, let’s return to our problem, how to make the black background in the picture transparent.

We can achieve this through the following steps:

1. Use the imagecreatefrompng() function to create an image in PNG format;
2. Loop through all the pixels of the image;
3. Use the imagecolorat() function to get the color of each pixel;
4. Judge whether the color is black;
5. If the color is black, set the transparency of the pixel is 0;
6. Finally, use the imagepng() function to save the image in PNG format.

The following is the PHP code for this operation:

$src_image = imagecreatefrompng('source.png');
$width = imagesx($src_image);
$height = imagesy($src_image);
for($x = 0; $x < $width; $x++){
    for($y = 0; $y < $height; $y++){
        $rgb = imagecolorat($src_image, $x, $y);
        $colors = imagecolorsforindex($src_image, $rgb);
        if($colors['red'] == 0 && $colors['green'] == 0 && $colors['blue'] == 0){
            imagesetpixel($src_image, $x, $y, imagecolorallocatealpha($src_image, 0, 0, 0, 127));
        }
    }
}
imagesavealpha($src_image, true);
imagepng($src_image, 'result.png');
imagedestroy($src_image);

In this code, we first create the original image using the imagecreatefrompng() function. Then, use the imagesx() and imagesy() functions to obtain the width and height of the original image. Next, we used a double loop to go through all the pixels, use the imagecolorat() function to get the color of each pixel, and use the imagecolorsforindex() function to get the RGB value of the color. If the color is black, we use the imagesetpixel() function to set the transparency of the pixel to 0 and save the image in PNG format.

Using the above code, we can convert a PNG format image with a black background into a PNG format image with a transparent background.

To sum up, using PHP to set black as a transparent background is very useful when we use image processing functions. Mastering this skill can make us more comfortable when processing pictures.

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