imagetruecolortopalette忽略颜色参数

Question

如何在不丢失所有颜色的情况下创建图像？它正在创建一个带有斑点的图像，并且更改颜色编号并没有什么区别。我开始使用的PNG图像（通过PHP上传）非常清晰。

//Create an image from the source
$srcImage = imagecreatefromstring( file_get_contents( $sourcePath ) );

//Get the source width and height
list( $width, $height ) = getimagesize( $sourcePath );

//Create image to paint on
$canvas = imagecreatetruecolor( $width, $height );

//Add alpha for transparency
$bga = imagecolorallocatealpha( $canvas, 0, 0, 0, 127 );
imagecolortransparent( $canvas, $bga );
imagefill( $canvas, 0, 0, $bga );

//Convert to palette (ignores the number of colors)
imagetruecolortopalette( $canvas, true, 1000 );

//Retain transparency
imagesavealpha( $canvas, true );

//Save as PNG to file
imagepng( $canvas, $destPath );

//Remove temporary data
imagedestroy( $canvas );

如何创建具有更多颜色的调色板图像？