Clever Geek Handbook

Accessing direct pixels of a GDI drawing context

I want to read and write pixels directly from / to the drawing context, i.e. during a drawing operation on the window.

I understand there are GetPixel

/ functions in Windows GDI SetPixel

, but for large operations it would be much better to be able to read and write pixel data directly into memory.

How can I do this using standard GDI?

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2 answers


You can create a compatible DC, Bitmap:

HDC     hMemDC = CreateCompatibleDC(hdc);
HBITMAP hBmp   = CreateCompatibleBitmap(hdc, WIDTH, HEIGHT);
SelectObject(hMemDC, hBmp);

Next, there is the GetDIBits function that you can use to get the bits:



int GetDIBits(
  _In_     HDC hdc,
  _In_     HBITMAP hbmp,
  _In_     UINT uStartScan,
  _In_     UINT cScanLines,
  _Out_    LPVOID lpvBits,
  _Inout_  LPBITMAPINFO lpbi,
  _In_     UINT uUsage
);

NOTE. You may need to set lpvBits to NULL to get the dimensions and aspect ratio via BITMAPINFO (lpbi parameter).

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Use GetObject

to get a structure BITMAP

that contains a pointer to the bitmap data.



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