pointers and bitmaps

Question

hi i'm trying to program an image filter. since pointers are the fastest thing to do that i'm wondering how i can access each pixel's RGB value through pointer arithmetic... Thanks!

bilnaad · Answer

Okay I found a real solution to the problem. What you really need is are two functions from the bitmap object. LockBits() and UnLockBits(). When you lock the bits of a bitmap al bitmapdata is stored in memory for super fast access via pointers. So here is a little sample that inverts the colors in an image of 24bpp and 32bpp. using System; using System.Drawing; using System.Drawing.Imaging; namespace ImageTransform { public class ImageFilter { public unsafe static Image ReverseColor(Image image) { short pixelSize = 0; switch(image.PixelFormat) { case PixelFormat.Format32bppArgb: { pixelSize = 4; }break; case PixelFormat.Format24bppRgb: { pixelSize = 3; }break; default: { pixelSize = 4; }break; } Bitmap bitmap = new Bitmap(image); BitmapData bits = bitmap.LockBits ( new Rectangle(00image.Width  image.Height)ImageLockMode.ReadWrite image.PixelFormat ); for(int y = 0; y < bits.Height; y++) { for(int x = 0; x < bits.Width; x++) { byte* pixel = (byte*)bits.Scan0+(y*bits.Stride)+(x*pixelSize); pixel[0] = (byte)(255 - pixel[0]); pixel[1] = (byte)(255 - pixel[1]); pixel[2] = (byte)(255 - pixel[2]); } } bitmap.UnlockBits(bits); return Image.FromHbitmap( bitmap.GetHbitmap() ); } } }

bilnaad · Answer

As a solution you could use code like this. using System; using System.Drawing; using System.Drawing.Imaging; namespace ImageTransform { unsafe struct UColor { byte r g b; public UColor(Color color) { r = color.R; g = color.G; b = color.B; } public byte R { get{return r;} } public byte G { get{return g;} } public byte B { get{return b;} } } public class ImageFilter { public unsafe static Image ReverseColor(Image image) { Bitmap bitmap = new Bitmap(image); for(int y = 0; y R); byte g = (byte)(255 - c->G); byte b = (byte)(255 - c->B); bitmap.SetPixel(x y Color.FromArgb(rgb)); } } return Image.FromHbitmap( bitmap.GetHbitmap() ); } } } The reason you can't use a pointer to a Color is because Color has the property named Name. Which is a string and strings are managed types. And you can't point to structs that contains a managed type. So.... as a solution you could use something like the code sample I provided. I used a custom valueType named UColor(unsafe Color). Which just stores the rgb values of a color. To this type a pointer can be created and used for the color manipulation. This is the best solution I can think of. But if it's fater.... I don't know for sure ^_^

bilnaad · Answer

oepsie I'm sorry you can't take the address of a Color struct. You can only take the address of structs that has unmanaged fields only! This means only fields of type intuintbytesbytedoublechar etc... If some field in a struct is a string or some other managed type you can't take the address of it :S Sorry

bilnaad · Answer

public unsafe static void Main() { Color color = Color.Azure; fixed(Color* c = &color) { c->A = 0; c->B = 255; c->G = 32; c->R = 234; } }