c# - Bitmap.LockBits 给出错误的值？

Question

我在将一些代码从 Bitmap.GetPixel 更改为使用 LockBits 返回的直接像素缓冲区时遇到问题。与 GetPixel 相比，LockBits 返回的数据似乎确实给了我不同的颜色值。

这很不幸，因为此更改确实会产生不同的颜色，这会破坏自动化单元测试。我有一个 29*30 像素的 png 文件，我确实将 Format32bppArgb 加载到位图中。这真的是 LockBits 和 GetPixel 返回的数据不同吗？我该如何解决这个问题？

这里是一些用加载的位图重现它的代码

public unsafe static Bitmap Convert(Bitmap originalBitmap)
{
    Bitmap converted = new Bitmap(originalBitmap);
    Rectangle rect = new Rectangle(0, 0, converted.Width, converted.Height);
    var locked = converted.LockBits(rect, ImageLockMode.ReadWrite, originalBitmap.PixelFormat);
    byte* pData = (byte*)locked.Scan0;

    // bytes per pixel
    var bpp = ((int)converted.PixelFormat >> 11) & 31;

    byte* r;
    byte* g;
    byte* b;
    byte* a;

    for (int y = 0; y < locked.Height; y++)
    {
        var row = pData + (y * locked.Stride);

        for (int x = 0; x < locked.Width; x++)
        {
            a = row + x * bpp + 3;
            r = row + x * bpp + 2;
            g = row + x * bpp + 1;
            b = row + x * bpp + 0;
            var col = Color.FromArgb(*a, *r, *g, *b);
            var origCol = originalBitmap.GetPixel(x, y);
            if (origCol != col)
            {
                Debug.Print("Orig: {0} Pixel {1}", origCol, col);
            }

        }
    }
    converted.UnlockBits(locked);

    return converted;
}

Orig: Color [A=128, R=128, G=128, B=255] Pixel Color [A=128, R=127, G=127, B=255]
Orig: Color [A=0, R=128, G=128, B=255]   Pixel Color [A=0, R=0, G=0, B=0]

Orig: Color [A=45, R=128, G=128, B=255]  Pixel Color [A=45, R=130, G=130, B=254]
                                                       ok     -2      -2     +1

大多数时候，似乎要进行一些舍入和转换。我可以强制 LockBits 像 GetPixel 返回的那样返回数据吗？

Answer 1

据我所知，PNG 可以包含颜色配置文件和 Gamma 校正信息以及任何其他可能影响像素最终颜色与其原始表示的内容。

即使我们忽略有关 PNG 的特定知识，一般来说 GetPixel 也可以返回与预期不同的值。

Bitmap.GetPixel 是这样实现的:

public Color GetPixel(int x, int y)
{

    int color = 0;

    if (x < 0 || x >= Width)
    {
        throw new ArgumentOutOfRangeException("x", SR.GetString(SR.ValidRangeX));
    }

    if (y < 0 || y >= Height)
    {
        throw new ArgumentOutOfRangeException("y", SR.GetString(SR.ValidRangeY));
    }

    int status = SafeNativeMethods.Gdip.GdipBitmapGetPixel(new HandleRef(this, nativeImage), x, y, out color);

    if (status != SafeNativeMethods.Gdip.Ok)
        throw SafeNativeMethods.Gdip.StatusException(status);

    return Color.FromArgb(color);
}

SafeNativeMethods.Gdip.GdipBitmapGetPixel 的定义是:

[DllImport(ExternDll.Gdiplus, SetLastError=true, ExactSpelling=true, CharSet=System.Runtime.InteropServices.CharSet.Unicode)] // 3 = Unicode 
[ResourceExposure(ResourceScope.None)]
internal static extern int GdipBitmapGetPixel(HandleRef bitmap, int x, int y, out int argb);

我们从中学到了什么 here是Gdiplus::Bitmap::GetPixel .该功能的文档说:

Remarks

Depending on the format of the bitmap, Bitmap::GetPixel might not return the same value as was set by Bitmap::SetPixel. For example, if you call Bitmap::SetPixel on a Bitmap object whose pixel format is 32bppPARGB, the pixel's RGB components are premultiplied. A subsequent call to Bitmap::GetPixel might return a different value because of rounding. Also, if you call Bitmap::SetPixel on a Bitmap object whose color depth is 16 bits per pixel, information could be lost during the conversion from 32 to 16 bits, and a subsequent call to Bitmap::GetPixel might return a different value.