"标准" RGB到灰度转换 [英] "Standard" RGB to Grayscale Conversion

问题描述

我正在尝试编写一个转换器算法，该算法采用JPEG图像并返回其PGM（便携式灰度图）版本。
问题是我无法理解官方JPG-> PGM转换器如何根据从经典RGB格式开始分配给最终像素（我猜，0-> 255）的值来工作。

I'm trying to write a converters algorithm that takes a JPEG image and returns its PGM (Portable Gray Map) version. The problem is that I can't understand how the "official" JPG->PGM convertitors work in terms of what value to assign to the final pixel (i guess, 0->255) starting from the classic RGB format.

一开始，我使用了这个公式（它与OpenCV的CV_RGB2GRAY转换使用的公式相同）：

At the beginning, I used this formula (it's the same used by OpenCV's CV_RGB2GRAY conversion):

0.30 * R + 0.59 * G + 0.11 * B = val

0.30*R + 0.59*G + 0.11*B = val

我写了一个简单的代码来测试我的结果：它需要一个彩色图像及其PGM版本（已经转换）使用GIMP）。然后它使用前面的公式转换彩色图像。目标是使像素到像素的灰度图像等于PGM输入。

I wrote a simple code to test my results: it takes a color image and its PGM version (already converted using GIMP). Then it converts the color image using the previous formula. The goal is to have a grayscale image that is pixel-to-pixel equal to the PGM input.

此时，它不会返回相同的值。你可以帮我吗？

At this point, it does not return the same values. Can you help me?

推荐答案

问题是我无法理解官方JPG-> PGM转换器根据从经典RGB格式开始分配给最终像素（我猜，0-> 255）的值来工作。

The problem is that I can't understand how the "official" JPG->PGM convertitors work in terms of what value to assign to the final pixel (i guess, 0->255) starting from the classic RGB format.

在官方工具正在使用的转换中可能存在伽马调整。
即，它不仅仅是线性变换。

There is likely a gamma adjustment in the conversion those "official" tools are using.
That is, it is not just a linear transform.

有关详细信息，请参阅此维基百科部分： 将颜色转换为灰度

See this Wikipedia section for the details: Converting color to grayscale

我相信你想使用 C srgb 。
尝试一下，看看它是否与您期望的结果相符。

I believe you want to use the formula for Csrgb.
Try it out and see if it matches the results you're expecting.

基本上，你会这样做：

1. 取 R，G，B 颜色（每个 [0,1] 范围）
   
   
   
   • 如果他们是我在范围 0..255 中，只需除以 255.0
   
   

1. Take R, G, B color (each in [0,1] range)
   • If they're in the range 0..255 instead, simply divide by 255.0

• 这可能是你之前申请的线性变换

• 这是您缺少的非线性伽玛校正件


• 查看 此WolframAlpha情节


• C srgb = 12.92 C linear 当 C linear < = 0.0031308


• C srgb = 1.055 C linear 1 / 2.4 - 0.055 当 C < sub> linear > 0.0031308

• This is the nonlinear gamma correction piece you were missing
• Check out this WolframAlpha plot
• Csrgb = 12.92 Clinear when Clinear <= 0.0031308
• Csrgb = 1.055 Clinear1/2.4 - 0.055 when Clinear > 0.0031308

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