PIL jpeg，如何保留像素颜色 [英] PIL jpeg, how to preserve the pixel color

问题描述

我有一些JPEG实验，文档说100完全禁用JPEG量化阶段。

I have some experiments with JPEG, the doc said "100 completely disables the JPEG quantization stage."

然而，我在保存期间仍然进行了一些像素修改。这是我的代码：

However, I still got some pixel modification during saving. Here is my code:

import Image
red = [20,30,40,50,60,70];
img = Image.new("RGB", [1, len(red)], (255,255,255))
pix = img.load()

for x in range(0,len(red)):
    pix[0,x] = (red[x],255,255)

img.save('test.jpg',quality=100)

img = Image.open('test.jpg')
pix = img.load()

for x in range(0,len(red)):
    print pix[0,x][0],

我得到了意想不到的输出： 22 25 42 45 62 65
我应该怎么做才能保留像素值？请注意，我也使用 imagejpeg 尝试使用PHP，它给了我正确的价值当质量= 100时。

I got unexpected output: 22 25 42 45 62 65 What should I do to preserve the pixel value ? Please note that I also tried with PHP using imagejpeg and It gives me the correct value when quality=100.

我可以使用 png 来保存，但我想知道这背后的原因和如果有任何选择可以避免

I can use png to preserve, but I want to know the reason behind this and if there is any option to avoid

推荐答案

JPEG由许多不同的步骤组成，其中许多步骤会引入一些损失。通过使用仅包含红色的样本图像，您可能遇到了最严重的罪犯 - 下采样或色度子采样。一半的颜色信息被丢弃，因为眼睛对亮度变化比颜色变化更敏感。

JPEG consists of many different steps, many of which introduce some loss. By using a sample image containing only red, you've probably run across the worst offender - downsampling or chroma subsampling. Half of the color information is thrown away because the eye is more sensitive to brightness changes than color changes.

某些JPEG编码器可以配置为关闭子采样，但我不喜欢我认为PIL的情况就是如此。在任何情况下，它都不会给你一个完全无损的文件，即使你可以，因为还有其他步骤引入了损失。

Some JPEG encoders can be configured to turn off subsampling, but I don't think that's the case for PIL. In any case it won't give you a completely lossless file even if you could, since there are still other steps that introduce a loss.

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