private Bitmap CopyToBpp(System.Drawing.Bitmap b, int bpp)
{
if (bpp != 1 && bpp != 8 && bpp != 4) throw new System.ArgumentException("1 or 4 or 8 ", "bpp");
// Plan: built into Windows GDI is the ability to convert
// Bitmaps from one format to another. Most of the time, this
// Job is actually done by the graphics hardware accelerator card
// And so is extremely fast. The rest of the time, the job is done by
// Very fast native code.
// We will call into this GDI functionality from C#. Our plan:
// (1) Convert our Bitmap into a GDI hbitmap (ie. copy unmanaged->managed)
// (2) Create a GDI monochrome hbitmap
// (3) Use GDI "BitBlt" function to copy from hbitmap into monochrome (as above)
// (4) Convert the monochrone hbitmap into a Bitmap (ie. copy unmanaged->managed)
int w = b.Width, h = b.Height;
IntPtr hbm = b.GetHbitmap(); // This is step (1)
//
// Step (2): create the monochrome bitmap.
// "BITMAPINFO" is an interop-struct which we define below.
// In GDI terms, it's a BITMAPHEADERINFO followed by an array of two RGBQUADs
BITMAPINFO bmi = new BITMAPINFO();
bmi.biSize = 40; // The size of the BITMAPHEADERINFO struct
bmi.biWidth = w;
bmi.biHeight = h;
bmi.biPlanes = 1; // "planes" are confusing. We always use just 1. Read MSDN for more info.
bmi.biBitCount = (short)bpp; // Ie. 1bpp or 8bpp
bmi.biCompression = BI_RGB; // Ie. the pixels in our RGBQUAD table are stored as RGBs, not palette indexes
bmi.biSizeImage = (uint)(((w + 7) & 0xFFFFFFF8) * h / 8);
bmi.biXPelsPerMeter = 1000000; // Not really important
bmi.biYPelsPerMeter = 1000000; // Not really important
// Now for the colour table.
uint ncols = (uint)1 << bpp; // 2 colours for 1bpp; 256 colours for 8bpp
bmi.biClrUsed = ncols;
bmi.biClrImportant = ncols;
bmi.cols = new uint[256]; // The structure always has fixed size 256, even if we end up using fewer colours
if (bpp == 1)
{
bmi.cols[0] = MAKERGB(0, 0, 0); bmi.cols[1] = MAKERGB(255, 255, 255);
}
else if (bpp == 4)
{
// For 8bpp we've created an palette with just greyscale colours.
// You can set up any palette you want here. Here are some possibilities:
// Rainbow:
bmi.biClrUsed = 16;
bmi.biClrImportant = 16;
int[] colv = new int[16] { 8, 24, 38, 56, 72, 88, 104, 120, 136, 152, 168, 184, 210, 216, 232, 248 };
//
for (int i = 0; i < 16; i++) bmi.cols[i] = MAKERGB(colv[i], colv[i], colv[i]);
}
else if (bpp == 8)
{
// For 8bpp we've created an palette with just greyscale colours.
// You can set up any palette you want here. Here are some possibilities:
// Rainbow:
bmi.biClrUsed = 216; bmi.biClrImportant = 216; int[] colv = new int[6] { 0, 51, 102, 153, 204, 255 };
for (int i = 0; i < 216; i++) bmi.cols[i] = MAKERGB(colv[i / 36], colv[(i / 6) % 6], colv[i % 6]);
// Optimal: a difficult topic: http:// En.wikipedia.org/wiki/Color_quantization
}
//
// Now create the indexed bitmap "hbm0"
IntPtr bits0; // Not used for our purposes. It returns a pointer to the raw bits that make up the bitmap.
IntPtr hbm0 = CreateDIBSection(IntPtr.Zero, ref bmi, DIB_RGB_COLORS, out bits0, IntPtr.Zero, 0);
//
// Step (3): use GDI's BitBlt function to copy from original hbitmap into monocrhome bitmap
// GDI programming is kind of confusing... nb. The GDI equivalent of "Graphics" is called a "DC".
IntPtr sdc = GetDC(IntPtr.Zero); // First we obtain the DC for the screen
// Next, create a DC for the original hbitmap
IntPtr hdc = CreateCompatibleDC(sdc); SelectObject(hdc, hbm);
// And create a DC for the monochrome hbitmap
IntPtr hdc0 = CreateCompatibleDC(sdc); SelectObject(hdc0, hbm0);
// Now we can do the BitBlt:
BitBlt(hdc0, 0, 0, w, h, hdc, 0, 0, SRCCOPY);
// Step (4): convert this monochrome hbitmap back into a Bitmap:
Bitmap b0 = System.Drawing.Image.FromHbitmap(hbm0);
//
// Finally some cleanup.
DeleteDC(hdc);
DeleteDC(hdc0);
ReleaseDC(IntPtr.Zero, sdc);
DeleteObject(hbm);
DeleteObject(hbm0);
//
return b0;
}
}
