这种棕褐色调转换算法有什么问题？

Question

我似乎有一种几乎正常工作的棕褐色调.由于某种原因,图像的一部分原来是柠檬绿!有谁知道我可能做错了什么？方法发布在下面.

private void SepiaBitmap(Bitmap bmp)
{
    Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
    System.Drawing.Imaging.BitmapData bmpData = bmp.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadWrite,
        System.Drawing.Imaging.PixelFormat.Format32bppRgb);

    IntPtr ptr = bmpData.Scan0;

    int numPixels = bmpData.Width * bmp.Height;
    int numBytes = numPixels * 4;
    byte[] rgbValues = new byte[numBytes];

    System.Runtime.InteropServices.Marshal.Copy(ptr, rgbValues, 0, numBytes);
    for (int i = 0; i < rgbValues.Length; i += 4)
    {
        rgbValues[i + 2] = (byte)((.393 * rgbValues[i + 2]) + (.769 * rgbValues[i + 1]) + (.189 * (rgbValues[i + 0]))); //red
        rgbValues[i + 1] = (byte)((.349 * rgbValues[i + 2]) + (.686 * rgbValues[i + 1]) + (.168 * (rgbValues[i + 0]))); //green
        rgbValues[i + 0] = (byte)((.272 * rgbValues[i + 2]) + (.534 * rgbValues[i + 1]) + (.131 * (rgbValues[i + 0]))); //blue

        if ((rgbValues[i + 2]) > 255)
        {
            rgbValues[i + 2] = 255; 
        }

        if ((rgbValues[i + 1]) > 255)
        {
            rgbValues[i + 1] = 255;
        }
        if ((rgbValues[i + 0]) > 255)
        {
            rgbValues[i + 0] = 255;
        }
    }

    System.Runtime.InteropServices.Marshal.Copy(rgbValues, 0, ptr, numBytes);
    this.Invalidate();
    bmp.UnlockBits(bmpData);

}

Answer 1

您的算法中有2个问题(至少,如果您从此处遵循算法描述).

首先,正如其他人所指出的,你有字节类型溢出.其次,所有输出颜色值必须基于输入颜色值,而不是按顺序计算.

这是固定的主循环代码:

        for (int i = 0; i < rgbValues.Length; i += 4)
        {
            int inputRed = rgbValues[i + 2];
            int inputGreen = rgbValues[i + 1];
            int inputBlue = rgbValues[i + 0];

            rgbValues[i + 2] = (byte) Math.Min(255, (int)((.393 * inputRed) + (.769 * inputGreen) + (.189 * inputBlue))); //red
            rgbValues[i + 1] = (byte) Math.Min(255, (int)((.349 * inputRed) + (.686 * inputGreen) + (.168 * inputBlue))); //green
            rgbValues[i + 0] = (byte) Math.Min(255, (int)((.272 * inputRed) + (.534 * inputGreen) + (.131 * inputBlue))); //blue
        }

请注意,在Min函数内部我将颜色值转换为doubleto,int否则Min(double, double)调用重载,255首先转换为double,然后可能返回到byte,包括额外的舍入.

如果有人需要一个示例控制台应用程序棕褐色转换器,这里是我的最终代码:

namespace ConsoleApplication8_Sepia
{
    using System;
    using System.Drawing;
    using System.Drawing.Imaging;

    class Program
    {
        static void Main(string[] args)
        {
            Bitmap b = (Bitmap)Bitmap.FromFile("c:\\temp\\source.jpg");
            SepiaBitmap(b);
            b.Save("c:\\temp\\destination.jpg", ImageFormat.Jpeg);
        }

        private static void SepiaBitmap(Bitmap bmp)
        {
            Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
            BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.ReadWrite, PixelFormat.Format32bppRgb);
            IntPtr ptr = bmpData.Scan0;

            int numPixels = bmpData.Width * bmp.Height;
            int numBytes = numPixels * 4;
            byte[] rgbValues = new byte[numBytes];

            System.Runtime.InteropServices.Marshal.Copy(ptr, rgbValues, 0, numBytes);
            for (int i = 0; i < rgbValues.Length; i += 4)
            {
                int inputRed = rgbValues[i + 2];
                int inputGreen = rgbValues[i + 1];
                int inputBlue = rgbValues[i + 0];

                rgbValues[i + 2] = (byte)Math.Min(255, (int)((.393 * inputRed) + (.769 * inputGreen) + (.189 * inputBlue))); //red
                rgbValues[i + 1] = (byte)Math.Min(255, (int)((.349 * inputRed) + (.686 * inputGreen) + (.168 * inputBlue))); //green
                rgbValues[i + 0] = (byte)Math.Min(255, (int)((.272 * inputRed) + (.534 * inputGreen) + (.131 * inputBlue))); //blue
            }

            System.Runtime.InteropServices.Marshal.Copy(rgbValues, 0, ptr, numBytes);
            bmp.UnlockBits(bmpData);
        }
    }
}

Answer 2

要解决此问题,请更改此循环:

for (int i = 0; i < rgbValues.Length; i += 4)
{
    int red = rgbValues[i + 2];
    int green = rgbValues[i + 1];
    int blue = rgbValues[i + 0];

    rgbValues[i + 2] = (byte)Math.Min((.393 * red) + (.769 * green) + (.189 * blue), 255.0); // red
    rgbValues[i + 1] = (byte)Math.Min((.349 * red) + (.686 * green) + (.168 * blue), 255.0); // green
    rgbValues[i + 0] = (byte)Math.Min((.272 * red) + (.534 * green) + (.131 * blue), 255.0); // blue
}

计算中出现算术溢出,这就是错误颜色的原因.在将类型表达式与255进行比较之前,double将显式地转换为表达式,因此它永远不会大于255.byte