改变位图颜色深度 (转)

最新推荐文章于 2024-08-26 21:47:57 发布

转载最新推荐文章于 2024-08-26 21:47:57 发布 · 283 阅读

本文介绍了一种位图颜色深度转换的方法，特别是针对大型位图（例如8000x8000像素），将其从高色彩深度减少到较低色彩深度（如4位每像素）。该方法通过将大位图分割成小块进行处理，并直接读取和保存位图文件来实现。同时，文章还提供了一个用于获取实际使用的颜色数量和创建颜色调色板的方法。

改变位图颜色深度 (转)[@more@]

Changing Bitmaps Color Depth

This article was contributed by Mailto:admin@gerdesser.de">Gerd Esser, Germany.

Environment: MSvc 6.0

The basic problem of this code was to decrease a color depth of great bitmaps (like 8000 x 8000 pixels) to - for example - 4 bits per pixel and solving nearly (! see below) the original color palette. The first attempt was to use the method as shown in DdbtoDIB. But such great bitmaps produce an error: They do nothing sencefull in functions like GetDIBits. So I handle the bitmap in bands of smaller bitmap data and make direct file reading/saving operations of the bitmaps.

Because of the internal mapping of the color palette to the system palette I got some strange results. Also, by realizing the whole number of really used colors, I ve got another problem with the previous banding of the bitmap, which show me only a small part of the overall used colors. This was the second problem. So I create a color palette by counting and adding the colors of each band to a global color palette of the bitmap.

A small problem still remains: By mapping the source bitmap into memory, the color palette cannot be solved exactly. If the color palette cannot be mapped into the system palette, the result color palette may differ by a view percent. But I didn't spend more time to this fact because the present solution was sufficient for me.

The result is a class BitmapLib with two (static) methods, ChangeColorDepth() and GetUsedColors().

BOOL CBitmapLib::ChangeColorDepth(CString const& stSrcBitmapFile, CString const& stDstBitmapFile, word const wDstBitCount, CProgressCtrl* pProgressCtrl=NULL);

ChangeColorDepth() changes a color depth of a file based bitmap and saves the result into a destination file. The operation is done by increasing or decreasing the bits per pixel through the whole range from 1 to 32 (thus converting also from device dependent into device independent or the way back). The progress control is only used if nescessary and showing the work progress (great bitmaps last a few seconds ...)

int CBitmapLib::GetUsedColors(CString const & stSrcBitmapFile, RGBQUAD * pColorTable, WORD const wMaxColorTableSize, CProgressCtrl *pProgressCtrl = NULL); int CBitmapLib::GetUsedColors(BITMapiNFO const * pBmi, BYTE const * pBits, UINT const nStartScanLine, UINT const nScanLines, RGBQUAD * pColorTable, WORD const wMaxColorTableSize, WORD & wUsedColors );

A byproduct of the operation ChangeColorDepth() is the method GetUsedColors(), which count the the really used colors and produces a color table of the used colors.

Both methods abort its operation, if the destination color depth is not sufficient enough for the number of used colors in the source bitmap.