c# - 显示内存中的 RGBA 图像

Question

我有一个包含 RGBA 编码图像的 C# 字节数组。在 WPF 中显示此图像的最佳方式是什么？

一个选项是从字节数组创建一个 BitmapSource 并将其附加到一个图像控件。但是，创建 BitmapSource 需要 RGBA32 的 PixelFormat，这在 Windows 中似乎不可用。

byte[] buffer = new byte[] { 25, 166, 0, 255, 90, 0, 120, 255 };
BitmapSource.Create(2, 1, 96d, 96d, PixelFormats.Bgra32, null, buffer, 4 * 2);

我绝对不想交换字节数组中的像素。

Answer 1

在深入研究 WIC 的细节之前，您可以考虑将字节交换封装在一个简单的自定义 BitmapSource 中，如下所示。它采用 RGBA 字节数组并在覆盖的 CopyPixels 方法中交换像素字节以生成 PBGRA 缓冲区。

您可以通过提供 RGBA 缓冲区和位图宽度来简单地创建此自定义 BitmapSource 的实例

var buffer = new byte[] { 25, 166, 0, 255, 90, 0, 120, 255 };
var bitmapSource = new RgbaBitmapSource(buffer, 2);

实现如下:

public class RgbaBitmapSource : BitmapSource
{
    private byte[] rgbaBuffer;
    private int pixelWidth;
    private int pixelHeight;

    public RgbaBitmapSource(byte[] rgbaBuffer, int pixelWidth)
    {
        this.rgbaBuffer = rgbaBuffer;
        this.pixelWidth = pixelWidth;
        this.pixelHeight = rgbaBuffer.Length / (4 * pixelWidth);
    }

    public override void CopyPixels(
        Int32Rect sourceRect, Array pixels, int stride, int offset)
    {
        for (int y = sourceRect.Y; y < sourceRect.Y + sourceRect.Height; y++)
        {
            for (int x = sourceRect.X; x < sourceRect.X + sourceRect.Width; x++)
            {
                int i = stride * y + 4 * x;
                byte a = rgbaBuffer[i + 3];
                byte r = (byte)(rgbaBuffer[i] * a / 256); // pre-multiplied R
                byte g = (byte)(rgbaBuffer[i + 1] * a / 256); // pre-multiplied G
                byte b = (byte)(rgbaBuffer[i + 2] * a / 256); // pre-multiplied B

                pixels.SetValue(b, i + offset);
                pixels.SetValue(g, i + offset + 1);
                pixels.SetValue(r, i + offset + 2);
                pixels.SetValue(a, i + offset + 3);
            }
        }
    }

    protected override Freezable CreateInstanceCore()
    {
        return new RgbaBitmapSource(rgbaBuffer, pixelWidth);
    }

    public override event EventHandler<DownloadProgressEventArgs> DownloadProgress;
    public override event EventHandler DownloadCompleted;
    public override event EventHandler<ExceptionEventArgs> DownloadFailed;
    public override event EventHandler<ExceptionEventArgs> DecodeFailed;

    public override double DpiX
    {
        get { return 96; }
    }

    public override double DpiY
    {
        get { return 96; }
    }

    public override PixelFormat Format
    {
        get { return PixelFormats.Pbgra32; }
    }

    public override int PixelWidth
    {
        get { return pixelWidth; }
    }

    public override int PixelHeight
    {
        get { return pixelHeight; }
    }

    public override double Width
    {
        get { return pixelWidth; }
    }

    public override double Height
    {
        get { return pixelHeight; }
    }
}

正如评论中所指出的，您可以通过实现不安全的复制操作来提高性能，该操作可能如下所示:

unsafe public override void CopyPixels(
    Int32Rect sourceRect, Array pixels, int stride, int offset)
{
    fixed (byte* source = rgbaBuffer, destination = (byte[])pixels)
    {
        byte* dstPtr = destination + offset;

        for (int y = sourceRect.Y; y < sourceRect.Y + sourceRect.Height; y++)
        {
            for (int x = sourceRect.X; x < sourceRect.X + sourceRect.Width; x++)
            {
                byte* srcPtr = source + stride * y + 4 * x;
                byte a = *(srcPtr + 3);
                *(dstPtr++) = (byte)(*(srcPtr + 2) * a / 256); // pre-multiplied B
                *(dstPtr++) = (byte)(*(srcPtr + 1) * a / 256); // pre-multiplied G
                *(dstPtr++) = (byte)(*srcPtr * a / 256); // pre-multiplied R
                *(dstPtr++) = a;
            }
        }
    }
}