Source

File: wp-includes/class-wp-image-editor-imagick.php

protected function thumbnail_image( $dst_w, $dst_h, $filter_name = 'FILTER_TRIANGLE', $strip_meta = true ) {
		$allowed_filters = array(
			'FILTER_POINT',
			'FILTER_BOX',
			'FILTER_TRIANGLE',
			'FILTER_HERMITE',
			'FILTER_HANNING',
			'FILTER_HAMMING',
			'FILTER_BLACKMAN',
			'FILTER_GAUSSIAN',
			'FILTER_QUADRATIC',
			'FILTER_CUBIC',
			'FILTER_CATROM',
			'FILTER_MITCHELL',
			'FILTER_LANCZOS',
			'FILTER_BESSEL',
			'FILTER_SINC',
		);

		/**
		 * Set the filter value if '$filter_name' name is in the allowed list and the related
		 * Imagick constant is defined or fall back to the default filter.
		 */
		if ( in_array( $filter_name, $allowed_filters, true ) && defined( 'Imagick::' . $filter_name ) ) {
			$filter = constant( 'Imagick::' . $filter_name );
		} else {
			$filter = defined( 'Imagick::FILTER_TRIANGLE' ) ? Imagick::FILTER_TRIANGLE : false;
		}

		/**
		 * Filters whether to strip metadata from images when they're resized.
		 *
		 * This filter only applies when resizing using the Imagick editor since GD
		 * always strips profiles by default.
		 *
		 * @since 4.5.0
		 *
		 * @param bool $strip_meta Whether to strip image metadata during resizing. Default true.
		 */
		if ( apply_filters( 'image_strip_meta', $strip_meta ) ) {
			$this->strip_meta(); // Fail silently if not supported.
		}

		try {
			/*
			 * To be more efficient, resample large images to 5x the destination size before resizing
			 * whenever the output size is less that 1/3 of the original image size (1/3^2 ~= .111),
			 * unless we would be resampling to a scale smaller than 128x128.
			 */
			if ( is_callable( array( $this->image, 'sampleImage' ) ) ) {
				$resize_ratio  = ( $dst_w / $this->size['width'] ) * ( $dst_h / $this->size['height'] );
				$sample_factor = 5;

				if ( $resize_ratio < .111 && ( $dst_w * $sample_factor > 128 && $dst_h * $sample_factor > 128 ) ) {
					$this->image->sampleImage( $dst_w * $sample_factor, $dst_h * $sample_factor );
				}
			}

			/*
			 * Use resizeImage() when it's available and a valid filter value is set.
			 * Otherwise, fall back to the scaleImage() method for resizing, which
			 * results in better image quality over resizeImage() with default filter
			 * settings and retains backward compatibility with pre 4.5 functionality.
			 */
			if ( is_callable( array( $this->image, 'resizeImage' ) ) && $filter ) {
				$this->image->setOption( 'filter:support', '2.0' );
				$this->image->resizeImage( $dst_w, $dst_h, $filter, 1 );
			} else {
				$this->image->scaleImage( $dst_w, $dst_h );
			}

			// Set appropriate quality settings after resizing.
			if ( 'image/jpeg' === $this->mime_type ) {
				if ( is_callable( array( $this->image, 'unsharpMaskImage' ) ) ) {
					$this->image->unsharpMaskImage( 0.25, 0.25, 8, 0.065 );
				}

				$this->image->setOption( 'jpeg:fancy-upsampling', 'off' );
			}

			if ( 'image/png' === $this->mime_type ) {
				$this->image->setOption( 'png:compression-filter', '5' );
				$this->image->setOption( 'png:compression-level', '9' );
				$this->image->setOption( 'png:compression-strategy', '1' );
				$this->image->setOption( 'png:exclude-chunk', 'all' );
			}

			/*
			 * If alpha channel is not defined, set it opaque.
			 *
			 * Note that Imagick::getImageAlphaChannel() is only available if Imagick
			 * has been compiled against ImageMagick version 6.4.0 or newer.
			 */
			if ( is_callable( array( $this->image, 'getImageAlphaChannel' ) )
				&& is_callable( array( $this->image, 'setImageAlphaChannel' ) )
				&& defined( 'Imagick::ALPHACHANNEL_UNDEFINED' )
				&& defined( 'Imagick::ALPHACHANNEL_OPAQUE' )
			) {
				if ( $this->image->getImageAlphaChannel() === Imagick::ALPHACHANNEL_UNDEFINED ) {
					$this->image->setImageAlphaChannel( Imagick::ALPHACHANNEL_OPAQUE );
				}
			}

			// Limit the bit depth of resized images to 8 bits per channel.
			if ( is_callable( array( $this->image, 'getImageDepth' ) ) && is_callable( array( $this->image, 'setImageDepth' ) ) ) {
				if ( 8 < $this->image->getImageDepth() ) {
					$this->image->setImageDepth( 8 );
				}
			}

			if ( is_callable( array( $this->image, 'setInterlaceScheme' ) ) && defined( 'Imagick::INTERLACE_NO' ) ) {
				$this->image->setInterlaceScheme( Imagick::INTERLACE_NO );
			}
		} catch ( Exception $e ) {
			return new WP_Error( 'image_resize_error', $e->getMessage() );
		}
	}