PixelFormat.Format32bppArgb 似乎有错误的字节顺序 [英] PixelFormat.Format32bppArgb seems to have wrong byte order

问题描述

我尝试从位图(System.Drawing.Bitmap)中获取所有字节值.因此我锁定字节并复制它们:

I try to get all byte values from a Bitmap(System.Drawing.Bitmap). Therefore I lock the bytes and copy them:

public static byte[] GetPixels(Bitmap bitmap){
    if(bitmap-PixelFormat.Equals(PixelFormat.Format32.bppArgb)){
        var argbData = new byte[bitmap.Width*bitmap.Height*4];
        var bd = bitmap.LockBits(new Rectangle(0, 0, image.Width, image.Height), ImageLockMode.ReadOnly, bitmap.PixelFormat);
        System.Runtime.InteropServices.Marshal.Copy(bd.Scan0, argbData, 0, bitmap.Width * bitmap.Height * 4);
        bitmap.UnlockBits(bd);
    }
}

我使用在 Photoshop 中创建的带有像素(红色、绿色、蓝色、白色)的非常简单的 2x2 PNG 图像测试了此图像.由于格式的原因，我希望 argbData 中有以下值:

I tested this Image with a very simple 2x2 PNG image with pixels (red, green, blue, white) that I created in Photoshop. Because of the format, I expected the following values within the argbData:

255 255   0   0    255 0   255   0 
255 0     0 255    255 255 255 255 

但我得到了:

0     0 255 255     0 255   0 255
255   0   0 255   255 255 255 255

但这是一种 BGRA 格式.有人知道为什么字节似乎交换了吗?顺便说一句，当我将图像直接用于 Image.Source 时，如下所示，图像显示正确.那我的错是什么?

But this is a BGRA format. Does anybody know why the bytes seems swapped? By the way, when I use the image directly for a Image.Source as shown below, the Image is shown correctly. So what's my fault?

<Image Source="D:/tmp/test2.png"/>

推荐答案

像素数据为 ARGB，1 字节为 alpha，1 为红色，1 为绿色，1 为蓝色.Alpha 是最高有效字节，蓝色是最低有效字节.在像您和许多其他机器一样的小端机器上，小端首先存储，因此字节顺序是 bb gg rr aa.所以 0 0 255 255 等于蓝色 = 0，绿色 = 0，红色 = 255，alpha = 255.那是红色.

Pixel data is ARGB, 1 byte for alpha, 1 for red, 1 for green, 1 for blue. Alpha is the most significant byte, blue is the least significant. On a little-endian machine, like yours and many others, the little end is stored first so the byte order is bb gg rr aa. So 0 0 255 255 equals blue = 0, green = 0, red = 255, alpha = 255. That's red.

当您将 bd.Scan0 转换为 int*(指向整数的指针)时，此字节序详细信息将消失，因为整数也以小字节序存储.

This endian-ness order detail disappears when you cast bd.Scan0 to an int* (pointer-to-integer) since integers are stored little-endian as well.

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