Question

Say I have a jpeg file and I want to set some pixels to a certain color. When I save the jpeg, I am losing color and I see aliasing around my new pixels, even if I set quality to 100. I know it s a lossy format, but I don t want to re-compress the picture, just set a few pixels.

// Create the GD resource
$img = imagecreatefromjpeg($filename);

// Set the first pixel to red
$color = imagecolorallocate($img, 255, 0, 0);
imagesetpixel($img, 0, 0, $color);

// Save the jpeg - is this where I m wrong? I see the red pixel but it s the wrong color and is blurred.
imagejpeg($img,  foo.jpg , 100);

// Lossless format works fine, red pixel is bright and accurate.
imagepng($img,  foo.png );

因此,要走什么路? 我确实需要改变一些餐厅的颜色,在节省时,需要准确。如果不依靠全球投资框架、国家投资计划或联合项目2000,是否有办法做到这一点?

Answer 1

As you said yourself, JPEG is a lossy format. It doesn t actually store "pixels" directly. If you make a change to the image, the image has to be re-compressed. There is no way around this.

您的红皮书是“大色”和“蓝色”的,其原因是,日本航天学会如何压缩。同样,它也不储存粉。它强调亮度的变化,实际的肤色信息如此重要。

I m not positive, but you may be able to only re-compress the few blocks that are affected by your change. You would not be able to do this with any standard functions, and would have to dig into the format and compression schemes yourself.

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