PHP 对 png 图像进行缩放，支持透明背景

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function smart_resize_image( $file,$width = 0,$height = 0,$proportional = false,$output = 'file',$delete_original = true,$use_linux_commands = false )
    {
        if ( $height <= 0 && $width <= 0 ) {
            return false;
        }
        $info = getimagesize($file);
        $image = '';

        $final_width = 0;
        $final_height = 0;
        list($width_old,$height_old) = $info;

        if ($proportional) {
            if ($width == 0) $factor = $height/$height_old;
            elseif ($height == 0) $factor = $width/$width_old;
            else $factor = min ( $width / $width_old,$height / $height_old);  
            $final_width = round ($width_old * $factor);
            $final_height = round ($height_old * $factor);

        }
        else {       
            $final_width = ( $width <= 0 ) ? $width_old : $width;
            $final_height = ( $height <= 0 ) ? $height_old : $height;
        }

        switch ($info[2] ) {
            case IMAGETYPE_GIF:
                $image = imagecreatefromgif($file);
            break;
            case IMAGETYPE_JPEG:
                $image = imagecreatefromjpeg($file);
            break;
            case IMAGETYPE_PNG:
                $image = imagecreatefrompng($file);
            break;
            default:
                return false;
        }

        $image_resized = imagecreatetruecolor( $final_width,$final_height );

        if ( ($info[2] == IMAGETYPE_GIF) || ($info[2] == IMAGETYPE_PNG) ) {
            $trnprt_indx = imagecolortransparent($image);
            // If we have a specific transparent color
            if ($trnprt_indx >= 0) {
                // Get the original image's transparent color's RGB values
                $trnprt_color    = imagecolorsforindex($image,$trnprt_indx);
                // Allocate the same color in the new image resource
                $trnprt_indx    = imagecolorallocate($image_resized,$trnprt_color['red'],$trnprt_color['green'],$trnprt_color['blue']);
                // Completely fill the background of the new image with allocated color.
                imagefill($image_resized,$trnprt_indx);
                // Set the background color for new image to transparent
                imagecolortransparent($image_resized,$trnprt_indx);
            }
            // Always make a transparent background color for PNGs that don't have one allocated already
            elseif ($info[2] == IMAGETYPE_PNG) {
                // Turn off transparency blending (temporarily)
                imagealphablending($image_resized,false);
                // Create a new transparent color for image
                $color = imagecolorallocatealpha($image_resized,127);

                // Completely fill the background of the new image with allocated color.
                imagefill($image_resized,$color);

                // Restore transparency blending
                imagesavealpha($image_resized,true);
            }
        }

        imagecopyresampled($image_resized,$image,$final_width,$final_height,$width_old,$height_old);

        if ( $delete_original ) {
            if ( $use_linux_commands )
                exec('rm '.$file);
            else
                @unlink($file);
        }

        switch ( strtolower($output) ) {
            case 'browser':
                $mime = image_type_to_mime_type($info[2]);
                header("Content-type: $mime");
                $output = NULL;
            break;
            case 'file':
                $output = $file;
            break;
            case 'return':
                return $image_resized;
            break;
            default:
            break;
        }

        switch ($info[2] ) {
            case IMAGETYPE_GIF:
                imagegif($image_resized,$output);
            break;
            case IMAGETYPE_JPEG:
                imagejpeg($image_resized,$output);
            break;
            case IMAGETYPE_PNG:
                imagepng($image_resized,$output);
            break;
            default:
                return false;
        }

        return true;
    }

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