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When using PHP to process images, imagecolorresolve() is a commonly used function that returns the index value of a given color. If the color already exists in the palette, the existing index is returned, otherwise the color will be added to the palette. But sometimes, we will encounter such an error message:
Warning: imagecolorresolve(): supplied resource is not a valid gd image resource
This error means that the first parameter passed to imagecolorresolve() is not a valid GD image resource. So how should we investigate and solve it? This article will explain in detail.
The most common problem is that the image resources are not created correctly at all, such as calling functions such as imagecreatefromjpeg() and imagecreatefrommpng() that fail.
Example error code:
<?php
$img = imagecreatefromjpeg('https://m66.net/uploads/sample.jpg');
$color = imagecolorresolve($img, 255, 0, 0);
?>
If the path 'https://m66.net/uploads/sample.jpg' is not accessible or is not a valid JPEG image, imagecreatefromjpeg() will return false instead of an image resource.
Solution: Add checks to make sure $img is a valid resource.
Correct writing:
<?php
$img = imagecreatefromjpeg('https://m66.net/uploads/sample.jpg');
if (!$img) {
die('Image loading failed，Please check whether the image path or format is correct。');
}
$color = imagecolorresolve($img, 255, 0, 0);
?>
If the path is written incorrectly, or the file does not exist, it will also lead to the inability to create image resources.
suggestion:
Use file_exists() to check the local file first.
If it is a remote image, use get_headers() to check whether the URL is valid.
Example:
<?php
$url = 'https://m66.net/uploads/sample.jpg';
$headers = get_headers($url, 1);
if (strpos($headers[0], '200') === false) {
die('The remote image does not exist or is not accessible。');
}
$img = imagecreatefromjpeg($url);
if (!$img) {
die('Unable to create image resources。');
}
$color = imagecolorresolve($img, 255, 0, 0);
?>
imagecolorresolve() belongs to PHP's GD library function. If your PHP environment does not install or enable GD extension, even if the code is written without problems, an error will occur.
Solution:
Check phpinfo() to confirm whether the GD module is enabled.
If not enabled, the Linux system can execute the following command to install:
sudo apt-get install php-gd
sudo service apache2 restart
Windows system needs to be found in the php.ini file ; extension=gd , remove the previous semicolon and restart the server.
Sometimes due to logic problems, image resources are destroyed in advance. For example, imagedestroy() is called incorrectly, which will also lead to incoming invalid resources.
Example error:
<?php
$img = imagecreate(100, 100);
imagedestroy($img); // Destroyed！
$color = imagecolorresolve($img, 255, 0, 0); // Report an error
?>
Make sure that the image resource is still valid until imagecolorresolve() is called.
When an error is reported by supplyed resource is not a valid gd image resource , the order of troubleshooting can be:
Confirm that the image resource is successfully created.
Check that the file path or remote URL is correct.
Make sure that the PHP environment is installed and enabled for GD extensions.
Check the code logic to avoid resource destruction in advance.
As long as you consciously add checks to each step, most of these errors can be easily avoided.
