Using Win32 functions in Visual FoxPro Image Gallery
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How to convert a bitmap file to monochrome format (1 bpp)

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Before you begin:
It all started with a question a developer posted in the Universal Thread Visual FoxPro forum:
Does anyone know how to create a monochrome bitmap or monochrome tiff file from a VFP report?
...
I need to automate the process. We are using this for a faxing application and it must work in a multi-user environment
Well, there is ReportListener object in VFP9 that can solve at least a part of this task. Its OutputPage method can render the output of a report to a bitmap file. The code is very simple (requires VFP9):

LOCAL oListener As ReportListener, nPageIndex
oListener = CREATEOBJECT("ReportListener")
oListener.ListenerType=3 && renders all pages at once

* make sure the report can load and run
REPORT FORM MyReport PREVIEW OBJECT oListener

FOR nPageIndex=1 TO oListener.PageTotal
        cOutputFile = "tmp"+TRANS(nPageIndex)+".bmp"
        oListener.OutputPage(nPageIndex,;
                cOutputFile, 105, 0,0,768,1024) && 105=bitmap
NEXT

I run the code and voila! It created one bitmap file for each page of my report. But the output files were nothing but small: 32 bits per pixel, 3.28 MB each. Still I was half way through.

Then I spent some time in Visual FoxPro trying in vain to convert bitmaps to monochromes using GDI+ API along with several GDI calls like CreateCompatibleBitmap, BitBlt and so on. The GdipGetImageGraphicsContext absolutely refused to accept monochrome HBITMAP handles.

After that I tried to find a solution using C# .NET, any solution at all. The Bitmap object did not help either. It failed to Clone to the PixelFormat.Format1bppIndexed, each time producing the System.OutOfMemoryException. That appeared consistent with GpStatus=3 returned by the GdipGetImageGraphicsContext function in my VFP tests.

Finally I gave up on GDI+ and switched to good old GDI API calls. And they worked perfectly, credits to the LoadImage for doing the actual conversion.
 
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5268 bytes  
Created: 2005-09-08 15:31:56  
Modified: 2010-01-14 12:01:18  
Visits in 7 days: 81  
Listed functions:
CloseHandle
CreateCompatibleDC
CreateFile
DeleteDC
DeleteObject
GetDIBits
GetObject
GlobalAlloc
GlobalFree
LoadImage
SelectObject
WriteFile
ZeroMemory
Printer friendly API declarations
My comment:
The code above implements:

PROCEDURE SaveBitmapAsMonochrome(srcfile As String, dstfile As String)

To create HBITMAP from a bitmap file I use the LoadImage with LR_LOADFROMFILE and LR_MONOCHROME flags. This way, it takes care of all color-to-monochrome mumbo-jumbos, which fact I am very happy with.

The source HBITMAP is selected into a memory device context. Then both of them are passed to the SaveBitmapToFile procedure.

The procedure is a bit hard-coded, which means it produces monochrome bitmap files only. It creates BITMAPINFOHEADER and BITMAPFILEHEADER structures, extracts bitmap binary data using the GetDIBits function and then saves everything to a file.

* * *
The writing and reading from temporary file makes the C# code noticeably slow on large files, which appears to be an unavoidable pay-off for the utter simplicity of this approach.
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User Contributed Notes:
A.M. | 2007-10-31 11:05:13
Bit2Mono.exe Changes Color Bitmaps to Monochrome
http://support.microsoft.com/kb/q77282/

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