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Fix image quality degradation caused by quantizeImage on resized PNG assets (Trac #63448) #8810

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Fix image quality degradation caused by quantizeImage on resized PNG assets (Trac #63448) #8810

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e003a91
Fix: Avoid image quality degradation caused by quantizeImage() on res…
elvismdev May 16, 2025
f6ed800
Refactor: Improve image quantization logic to avoid quality degradati…
elvismdev May 16, 2025
4be91f3
Fix: Apply PNG quantization only to true indexed images with ≤ max co…
elvismdev May 17, 2025
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59 changes: 29 additions & 30 deletions src/wp-includes/class-wp-image-editor-imagick.php
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Original file line number Diff line number Diff line change
Expand Up @@ -484,38 +484,37 @@ protected function thumbnail_image( $dst_w, $dst_h, $filter_name = 'FILTER_TRIAN
$this->image->setOption( 'png:compression-filter', '5' );
$this->image->setOption( 'png:compression-level', '9' );
$this->image->setOption( 'png:compression-strategy', '1' );
// Check to see if a PNG is indexed, and find the pixel depth.
if ( is_callable( array( $this->image, 'getImageDepth' ) ) ) {
$indexed_pixel_depth = $this->image->getImageDepth();

// Indexed PNG files get some additional handling.
if ( 0 < $indexed_pixel_depth && 8 >= $indexed_pixel_depth ) {
// Check for an alpha channel.
if (
is_callable( array( $this->image, 'getImageAlphaChannel' ) )
&& $this->image->getImageAlphaChannel()
) {
$this->image->setOption( 'png:include-chunk', 'tRNS' );
} else {
$this->image->setOption( 'png:exclude-chunk', 'all' );
}

// Reduce colors in the images to maximum needed, using the global colorspace.
$max_colors = pow( 2, $indexed_pixel_depth );
if ( is_callable( array( $this->image, 'getImageColors' ) ) ) {
$current_colors = $this->image->getImageColors();
$max_colors = min( $max_colors, $current_colors );
}
$this->image->quantizeImage( $max_colors, $this->image->getColorspace(), 0, false, false );

/**
* If the colorspace is 'gray', use the png8 format to ensure it stays indexed.
*/
if ( Imagick::COLORSPACE_GRAY === $this->image->getImageColorspace() ) {
$this->image->setOption( 'png:format', 'png8' );
}

// Handle alpha chunk inclusion or exclusion.
if (
is_callable( array( $this->image, 'getImageAlphaChannel' ) ) &&
$this->image->getImageAlphaChannel()
) {
$this->image->setOption( 'png:include-chunk', 'tRNS' );
} else {
$this->image->setOption( 'png:exclude-chunk', 'all' );
}

// Only apply quantization to actual indexed PNGs.
if (
is_callable( array( $this->image, 'getImageColors' ) ) &&
is_callable( array( $this->image, 'getImageDepth' ) ) &&
is_callable( array( $this->image, 'getImageProperty' ) )
) {
$current_colors = $this->image->getImageColors();
$bit_depth = $this->image->getImageDepth();
$max_colors = pow( 2, $bit_depth );
$color_type = $this->image->getImageProperty( 'png:IHDR.color-type-orig' );

if ( $current_colors <= $max_colors && '3' === $color_type ) {
$this->image->quantizeImage( $current_colors, $this->image->getColorspace(), 0, false, false );
}
}

// If grayscale, ensure format is png8 to retain index palette.
if ( Imagick::COLORSPACE_GRAY === $this->image->getImageColorspace() ) {
$this->image->setOption( 'png:format', 'png8' );
}
}

/*
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