Problem with Indexed Bitmaps

[HellRazor]

I'm writing a utility that converts map image files from Dragon format to Map Generator 2/UO Landscaper format.

I'm using a colormap to do the color conversion and it works great. But the problem I am having comes with saving the file. I'm having a problem saving the new file as an INDEXED bitmap. GDI+ doesn't seem to have any good methods built in for converting an image from true color to indexed bitmap.

I've played around with a few methods involving GetPixel but that is dog slow for these large bitmap files the program will be working with.

I've also tried this routine which I pulled from the web, it uses the GIF converter to convert from image to indexed bitmap. It is nice and fast, but the problem is that the resulting image comes out dithered, which makes it useless for my purposes.

        public static Image Image2Indexed8bpp(Image img)
        {
            System.IO.MemoryStream buffer = new System.IO.MemoryStream();
            img.Save(buffer, System.Drawing.Imaging.ImageFormat.Gif);
            buffer.Seek(0, System.IO.SeekOrigin.Begin);
            return Bitmap.FromStream(buffer);
        }

Anyone have any ideas on how to get around this shortcoming of GDI+?

[noobie]

well i have no experience in this, but this article might help you to get what you need. it shows how to manipulate the data and save it also as indexed bmp.

http://www.codeproject.com/csharp/steganodotnet11.asp

[Jeff]

Originally Posted by HellRazor

Anyone have any ideas on how to get around this shortcoming of GDI+?

I wouldnt call it a short coming, you need to learn to use pointers, the operation can be done very fast if you know how to use these, GetPixel and SetPixel use this but the reason it is slow is cause each time you use one of these operations it has to lock the bitmap into memory, do the method, then unlock it. In a sense, you yourself could do this with one lock if you learn how to manipulate bitmaps with pointers

[mordero]

http://www.codersource.net/csharp_image_Processing.aspx

i think this will help you get started

[HellRazor]

Originally Posted by mordero

http://www.codersource.net/csharp_image_Processing.aspx

i think this will help you get started

Thanks Modero, I had not come across this particular tutorial in my googling.

[HellRazor]

Originally Posted by Jeff

I wouldnt call it a short coming, you need to learn to use pointers, the operation can be done very fast if you know how to use these, GetPixel and SetPixel use this but the reason it is slow is cause each time you use one of these operations it has to lock the bitmap into memory, do the method, then unlock it. In a sense, you yourself could do this with one lock if you learn how to manipulate bitmaps with pointers

Yeah, pointers are very much unexplored territory for me. I have found a few tutorials on using lock and unlock with graphics but it has been a little daunting.

I was really hoping for an easier solution but I'll do some checking into pointers/locking/unlocking - maybe I can figure it out. Undoubtedly it is probably the BEST way, I'm just not sure if I can figure it all out on my own without some help.

Thanks Jeff!

[HellRazor]

Originally Posted by noobie

well i have no experience in this, but this article might help you to get what you need. it shows how to manipulate the data and save it also as indexed bmp.

http://www.codeproject.com/csharp/steganodotnet11.asp

Thanks noobie, this is helpful and I will check out the article.

[HellRazor]

Ok, I've found some sample code that will do what I want. However, it only works correctly if the source image is a GIF file. My source image is a BMP file.

I'm pretty sure it has to do with the formula being used to scan through the source image, but I don't know the correct formula to use for a BMP image.

Can someone point me in the right direction?

Bitmap TerrainMap = (Bitmap)Image.FromFile(@"C:/test2.bmp");
ColorPalette cp = TerrainMap.Palette;
if(cp!=null) 
    { 

    //Create a new 8 bit per pixel image 

    Bitmap bm=new Bitmap(TerrainMap.Width, TerrainMap.Height, PixelFormat.Format8bppIndexed); 

    //get it's palette 

    ColorPalette ncp=bm.Palette; 

    //copy all the entries from the old palette removing any transparency 
  
    int n=0; 
    foreach(Color c in cp.Entries) 
       ncp.Entries[n++]=Color.FromArgb(255,c); 

      //re-insert the palette 

      bm.Palette=ncp; 

      //now to copy the actual bitmap data 

     //lock the source and destination bits 

     BitmapData src=((Bitmap)TerrainMap).LockBits(new Rectangle(0,0,TerrainMap.Width,TerrainMap.Height),ImageLockMode.ReadOnly,TerrainMap.PixelFormat); 

     BitmapData dst=bm.LockBits(new Rectangle(0,0,bm.Width,bm.Height),ImageLockMode.WriteOnly,bm.PixelFormat); 

    //uses pointers so we need unsafe code. 
    //the project is also compiled with /unsafe 

    unsafe 
    { 
       //steps through each pixel 

      for(int y=0;y<TerrainMap.Height;y++) 
      for(int x=0;x<TerrainMap.Width;x++) 
      { 

          //transferring the bytes 

          ((byte *)dst.Scan0.ToPointer())[(dst.Stride*y)+x]=((byte *)src.Scan0.ToPointer())[(src.Stride*y)+x]; 

        } 
    } 

   //all done, unlock the bitmaps 

   ((Bitmap)TerrainMap).UnlockBits(src); 
   bm.UnlockBits(dst); 
   TerrainMap.Dispose(); 

   //set the new image in place 

   TerrainMap=bm; 
   cp=TerrainMap.Palette; 
        TerrainMap.Save("C:/testbit.bmp", ImageFormat.Bmp);
    }