除了使用imagejpeg或imagepng之外，还有更高质量的将PHP图像资源转换为jpeg或png的方法吗？

Question

目前我正在使用imagepng将基于原始图像的图像资源转换为png图像类型。事实证明，这比imagejpeg函数为我的jpg创建的质量更高。

然而，你可以清楚地看到这两个函数中的压缩算法的质量不如Photoshop那样高。

有谁知道如何获得更高质量的jpeg？是否有关于图像资源格式以及如何将其转换为jpeg或png的文档？如果我有这些信息，我至少可以尝试实现更好的算法。

//Create image
        $im = apppic($filename, $colors);
        imagepng($im, "images/app/".rtrim($path_array[$count],"jpeg")."png", 0);

function promopic ($filename, $colors){


    if(@imagecreatefromjpeg($filename))
            $img = imagecreatefromjpeg($filename);
    elseif(@imagecreatefrompng($filename))
            $img = imagecreatefrompng($filename);
    elseif(@imagecreatefromgif($filename))
            $img = imagecreatefromgif($filename);
    $width = imagesx($img);
    $height = imagesy($img);
    $imHeight = 625;

    if( $width/$height > 4/5){$imWidth = 800; $imHeight = $height/$width*$imWidth ; }
    else {  $imWidth = $width/$height*$imHeight;  }

    $colorWidth = 1200 - $imWidth;
    $numColors = count($colors);
    $colorHeight = ceil($imHeight/$numColors) - 2;
    $imHeight = ceil($imHeight/$numColors)* $numColors - 2;


    $im = @imagecreate(1200, $imHeight+50)
        or die("Cannot Initialize new GD image stream");
    $background_color = imagecolorallocate($im, 250, 250, 250);
    $text_color = imagecolorallocate($im, 255, 255, 251);
    $blue_color = imagecolorallocate($im, 40, 5, 251);


    //Add color boxes
    for ($i = 1; $i <= $numColors; $i++) {
        $imcolorbox = @imagecreate($colorWidth, $colorHeight);
        $rgb = hex2rgb($colors[($i-1)]);

        $colorbox_color = imagecolorallocate($imcolorbox, $rgb[0], $rgb[1], $rgb[2]);
        $dest_x = 1200-$colorWidth;
        $dest_y = ($colorHeight + 2) * ($i-1);

        $copy_image = imagecopy($im, $imcolorbox,  $dest_x, $dest_y, 0, 0, $colorWidth, $colorHeight);
        imagedestroy($imcolorbox);
        //imagestring($im, 5, 1075, 2+$dest_y,  "Reinvogue.com", $text_color);
        //imagestring($im, 5, $imWidth+5, 2+$dest_y,  "Reinvogue.com", $text_color);
        //imagestring($im, 5, 1050, 17+$dest_y,  "Hex: ".$colors[($i-1)], $text_color);
        //imagestring($im, 5, 1050, 2+$dest_y,  "RGB: ".$rgb[0]." ".$rgb[1]." ".$rgb[2], $text_color);
    }

     imagestring($im, 5, 10, $imHeight+15,  "Powered by the Reinvogue.com Colorway Engine", $blue_color);


    $copy_image = imagecopyresampled($im, $img,  0, 0, 0, 0, $imWidth-2, $imHeight, $width, $height);
    return $im;
}

Answer 1

这应创建一个白色背景的高质量图像：

$im = @imagecreatetruecolor(1200, $imHeight+50)
$white = imagecolorallocate($im, 255, 255, 255);
imagefill($im, 0, 0, $white);

如果您正在使用png图片，您甚至可以查看imagecolorallocatealpha（http://www.php.net/manual/en/function.imagecolorallocatealpha.php）

希望这有帮助。

Answer 2

对于高质量的PNG8使用pngquant2。 PHP的内置libgd fork具有糟糕的调色板转换。

引用：http://pngquant.org/php.html

<?php

/**
 * Optimizes PNG file with pngquant 1.8 or later (reduces file size of 24-bit/32-bit PNG images).
 *
 * You need to install pngquant 1.8 on the server (ancient version 1.0 won't work).
 * There's package for Debian/Ubuntu and RPM for other distributions on http://pngquant.org
 *
 * @param $path_to_png_file string - path to any PNG file, e.g. $_FILE['file']['tmp_name']
 * @param $max_quality int - conversion quality, useful values from 60 to 100 (smaller number = smaller file)
 * @return string - content of PNG file after conversion
 */
function compress_png($path_to_png_file, $max_quality = 90)
{
    if (!file_exists($path_to_png_file)) {
        throw new Exception("File does not exist: $path_to_png_file");
    }

    // guarantee that quality won't be worse than that.
    $min_quality = 60;

    // '-' makes it use stdout, required to save to $compressed_png_content variable
    // '<' makes it read from the given file path
    // escapeshellarg() makes this safe to use with any path
    $compressed_png_content = shell_exec("pngquant --quality=$min_quality-$max_quality - < ".escapeshellarg(    $path_to_png_file));

    if (!$compressed_png_content) {
        throw new Exception("Conversion to compressed PNG failed. Is pngquant 1.8+ installed on the server?");
    }

    return $compressed_png_content;
}