Original Full-Size AVIF/WEBP Conversion Fails on Palette-Based PNGs When GD is Used

[b1ink0]

Bug Description

When uploading a PNG that was compressed using https://imageresizer.com/image-compressor, I encountered following PHP warnings during AVIF and WebP conversion.

AVIF conversion warning:

Warning: imageavif(): avif error - avif doesn't support palette images in /var/www/html/wp-includes/class-wp-image-editor.php on line 585

WebP conversion warning:

 Warning: imagewebp(): Palette image not supported by webp in /var/www/html/wp-includes/class-wp-image-editor.php on line 585

Based on inspection of the PHP GD source:

• gdImageAvifCtx() (imageavif) source
• gdImageWebp() (imagewebp) source

Both imageavif() and imagewebp() functions fail when passed a palette-based image, because GD requires truecolor images for these conversions.

It appears these compression tools convert standard PNGs into palette-based (indexed color) PNGs. Due to this when uploading a palette-based (indexed color) PNG to WordPress media library, the original full-size AVIF/WebP file fails to convert and ends up as a 0-byte file.

However, resized sub-sizes are unaffected. Upon reviewing the GD editor code in WordPress core, I noticed that the _resize() method in WP_Image_Editor_GD always converts the source image to truecolor before any further processing hence, sub-sizes convert without issue.

References:

• libpng – Chapter 8: Indexed color
• Indexed color overview – Drububu

Steps to reproduce

1. Force GD as the default image editor:

add_filter( 'wp_image_editors', function () {
  return [ 'WP_Image_Editor_GD' ];
} );

2. Ensure the Modern Image Formats plugin is installed and activated.
3. Upload a palette-based PNG image (created or optimized by an online compressor like imageresizer.com).

Sample palette PNG image:

Sample transparent palette PNG image:

Screenshots

AVIF conversion warning:

WebP conversion warning:

Additional Context

• GD Version: bundled (2.1.0 compatible)

[b1ink0]

A potential solution to avoid conversion failure for original uploads is to hook into wp_handle_upload_prefilter and convert palette PNGs to truecolor before they are passed to GD’s conversion functions.

Example code snippet that fixes this issue:

add_filter( 'wp_handle_upload_prefilter', 'convert_palette_png_to_truecolor' );

function convert_palette_png_to_truecolor( $file ) {
	if ( ! isset( $file['tmp_name'], $file['name'] ) ) {
		return $file;
	}

	$ext = strtolower( pathinfo( $file['name'], PATHINFO_EXTENSION ) );

	if ( 'png' !== $ext ) {
		return $file;
	}

	$editor = wp_get_image_editor( $file['tmp_name'] );

	if ( is_wp_error( $editor ) || ! $editor instanceof WP_Image_Editor_GD ) {
		return $file; // Skip conversion if not using GD.
	}

	$image = imagecreatefrompng( $file['tmp_name'] );
	if ( ! $image || imageistruecolor( $image ) ) {
		return $file;
	}

	// Preserve transparency.
	imagealphablending( $image, false );
	imagesavealpha( $image, true );

	// Convert the palette to truecolor.
	imagepalettetotruecolor( $image );

	// Overwrite the upload with the new truecolor PNG.
	imagepng( $image, $file['tmp_name'] );
	imagedestroy( $image );

	return $file;
}

cc: @adamsilverstein

[mukeshpanchal27]

Actually, looks like a possible duplicate of #1561