Question

Addendum: it seems to run correctly when I uncheck “optimize code” which leads me to believe it is some quirky configuration problem

Firstly I am trying to run unmanaged code. I have “allow unsafe code” checked. It is pointing to this line of code here where I am trying to read a bitmap without using the relatively slow getpixel:

byte[] buff = { scanline[xo], scanline[xo + 1], scanline[xo + 2], 0xff };

Entire snippet is below. How can I correct this problem?

private const int PIXELSIZE = 4;              // Number of bytes in a pixel

BitmapData mainImageData = mainImage.LockBits(new Rectangle(0, 0, mainImage.Width, mainImage.Height), ImageLockMode.ReadOnly, mainImage.PixelFormat);
List<Point> results = new List<Point>();

foundRects = new List<Rectangle>();

for (int y = 0; y < mainImageData.Height
{
        byte* scanline = (byte*)mainImageData.Scan0 + (y * mainImageData.Stride);

        for (int x = 0; x < mainImageData.Width; x++)
        {
            int xo = x * PIXELSIZE;
            byte[] buff = { scanline[xo], scanline[xo + 1], 
                    scanline[xo + 2], 0xff };
            int val = BitConverter.ToInt32(buff, 0);

            // Pixle value from subimage in desktop image
            if (pixels.ContainsKey(val) && NotFound(x, y))
            {
                Point loc = (Point)pixels[val];

                int sx = x - loc.X;
                int sy = y - loc.Y;
                // Subimage occurs in desktop image 
                if (ImageThere(mainImageData, subImage, sx, sy))
                {
                    Point p = new Point(x - loc.X, y - loc.Y);
                    results.Add(p);
                    foundRects.Add(new Rectangle(x, y, subImage.Width,
                                                               subImage.Height));
                }
          }
        }

Answer 1

It’s hard to say with the limited information we have, but I see a couple of obvious problems, one of which addresses your issue directly:

1. You are not checking the pixel format, but are assuming that it is 32bppRGB. It is likely 24bppRGB, and that would explain the error.

2. You are reading the RGB values incorrectly; Windows internally stores bitmaps in BGR order.

3. You are not calling UnlockBits at the end of the method.