About bitmap compare alogrithm

I have implemented a Windows WinMine game auto run algorithm. And I need to read Winmine Game cell content in my program. My method is compare every bitmap of the game resource with the bitmap capture from screen, if there is a resource bitmap same with screen resource, then I know the content of this cell.
My OS is windows 2k, and screen properties is 1028 * 768 * 16bit, large font. On this setup, my program run well. But when I modify system font to small. My program can not run. The bitmap campare error, it cannot found the same one. I don't know what's wrong with it. Anyone can help me?

hello. if u have the solution can u give it to me..
i have an assignment to comparing two bmp images using visual c++
help me.

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