Save Image object as Group4 B&W Tiff with 300dpi

Hi everyone,

I've been struggeling with this problem for some time now.
I'm starting from 4 image objects of 24bpp with a resolution of 300dpi. Then I have to save these object into 1 Tiff-file with the following specifications:

Multi-page Tiff
Black&white
Group4 compression
1bpp
Must maintain the dpi-settings of the original Image object (300dpi)

I found a sub, that converts an Image object to a black&white 1bpp image object, but there are two problems:

It's too slow (1.5sec/image on a fast computer)
It changes the dpi-settings to 96dpi

At the moment, I just reset the resolution to 300dpi and save the file to disk. This works fine, but the image-quality isn't good enough to do ICR (Intelligent Character Recognition).
So I really have to get a Black&White image of 300dpi.

Here is the sub I am using at the moment, but I have no idea how to modify the code so I can maintain the original 300dpi in stead of the 96dpi.
I hope someone can help me with this, or show me a better way to do this!

The code:

#region "Converting 32bpp to 1bpp image"
[System.Runtime.InteropServices.DllImport("gdi32.dll")]
public static extern bool DeleteObject(IntPtr hObject);

[System.Runtime.InteropServices.DllImport("user32.dll")]
public static extern IntPtr GetDC(IntPtr hwnd);

[System.Runtime.InteropServices.DllImport("gdi32.dll")]
public static extern IntPtr CreateCompatibleDC(IntPtr hdc);

[System.Runtime.InteropServices.DllImport("user32.dll")]
public static extern int ReleaseDC(IntPtr hwnd, IntPtr hdc);

[System.Runtime.InteropServices.DllImport("gdi32.dll")]
public static extern int DeleteDC(IntPtr hdc);

[System.Runtime.InteropServices.DllImport("gdi32.dll")]
public static extern IntPtr SelectObject(IntPtr hdc, IntPtr hgdiobj);

[System.Runtime.InteropServices.DllImport("gdi32.dll")]
public static extern int BitBlt(IntPtr hdcDst, int xDst, int yDst, int w, int h, IntPtr hdcSrc, int xSrc, int ySrc, int rop);
static int SRCCOPY = 0x00CC0020;

[System.Runtime.InteropServices.DllImport("gdi32.dll")]
static extern IntPtr CreateDIBSection(IntPtr hdc, ref BITMAPINFO bmi, uint Usage, out IntPtr bits, IntPtr hSection, uint dwOffset);
static uint BI_RGB = 0;
static uint DIB_RGB_COLORS = 0;
[System.Runtime.InteropServices.StructLayout(System.Runtime.InteropServices.LayoutKind.Sequential)]

public struct BITMAPINFO
{
public uint biSize;
public int biWidth, biHeight;
public short biPlanes, biBitCount;
public uint biCompression, biSizeImage;
public int biXPelsPerMeter, biYPelsPerMeter;
public uint biClrUsed, biClrImportant;
public byte blue0, green0, red0, reserved0;
public byte blue1, green1, red1, reserved1;
}

public static System.Drawing.Bitmap CopyTo1bb(System.Drawing.Bitmap b)
{
// 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 = 1; // ie. 1bpp
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); // number of bytes. each scanline is padded to a multiple of 4
//bmi.biXPelsPerMeter = 11811; // not really important
//bmi.biYPelsPerMeter = 11811; // not really important
bmi.biClrUsed = 2; // our mono bitmap uses both colours in the RGBQUAD table
bmi.biClrImportant = 2; // and both are important
// Now for the RGBQUAD table. Its first entry is "black". Its second entry is "white".
bmi.red0 = 0; bmi.green0 = 0; bmi.blue0 = 0; bmi.reserved0 = 0;
bmi.red1 = 255; bmi.green1 = 255; bmi.blue1 = 255; bmi.reserved1 = 255;
// Now create the monochrome 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 monochrome bitmap
// GDI programming is kind of confusing...
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:
System.Drawing.Bitmap b0 = System.Drawing.Bitmap.FromHbitmap(hbm0);
//
// Finally some GDI cleanup tasks
DeleteDC(hdc);
DeleteDC(hdc0);
ReleaseDC(IntPtr.Zero, sdc);
DeleteObject(hbm);
DeleteObject(hbm0);

//I Added the following rule to do ICR on the images (the program only accepts 300dpi images)
b0.SetResolution(300, 300);

return b0;
}
#endregion

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