Source

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

public function set_quality( $quality = null ) {
		if ( null === $quality ) {
			/**
			 * Filters the default image compression quality setting.
			 *
			 * Applies only during initial editor instantiation, or when set_quality() is run
			 * manually without the `$quality` argument.
			 *
			 * The WP_Image_Editor::set_quality() method has priority over the filter.
			 *
			 * @since 3.5.0
			 *
			 * @param int    $quality   Quality level between 1 (low) and 100 (high).
			 * @param string $mime_type Image mime type.
			 */
			$quality = apply_filters( 'wp_editor_set_quality', $this->default_quality, $this->mime_type );

			if ( 'image/jpeg' === $this->mime_type ) {
				/**
				 * Filters the JPEG compression quality for backward-compatibility.
				 *
				 * Applies only during initial editor instantiation, or when set_quality() is run
				 * manually without the `$quality` argument.
				 *
				 * The WP_Image_Editor::set_quality() method has priority over the filter.
				 *
				 * The filter is evaluated under two contexts: 'image_resize', and 'edit_image',
				 * (when a JPEG image is saved to file).
				 *
				 * @since 2.5.0
				 *
				 * @param int    $quality Quality level between 0 (low) and 100 (high) of the JPEG.
				 * @param string $context Context of the filter.
				 */
				$quality = apply_filters( 'jpeg_quality', $quality, 'image_resize' );
			}

			if ( $quality < 0 || $quality > 100 ) {
				$quality = $this->default_quality;
			}
		}

		// Allow 0, but squash to 1 due to identical images in GD, and for backward compatibility.
		if ( 0 === $quality ) {
			$quality = 1;
		}

		if ( ( $quality >= 1 ) && ( $quality <= 100 ) ) {
			$this->quality = $quality;
			return true;
		} else {
			return new WP_Error( 'invalid_image_quality', __( 'Attempted to set image quality outside of the range [1,100].' ) );
		}
	}