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  1. #1
    Join Date
    Feb 2005

    Read the transparent color in a GIF


    I have a little ActiveX control that reads gif images and displays them. Here is the code to load the images:

    IPicture *lpiPict;   // THE IMAGE IS LOADED HERE
    if( SUCCEEDED( ::OleLoadPicture( iStream, nFileSize, TRUE, IID_IPicture,(void**)&lpiPict) ) )
    	OLE_HANDLE ole_handle;
    	lpiPict->get_Handle( &ole_handle );
    	BITMAP bm;
    	GetObject( (HANDLE)ole_handle, sizeof(BITMAP), &bm );
    	return (HANDLE)ole_handle;  // RETURN THE IMAGE AS AN HBITMAP
    When this code returns I receive the image as an HBITMAP that I can then BitBlt for display.

    Now I need transparency support, I already have the method to draw the HBITMAP with a transparent color. Now the question: In a GIF image the color that must be transparent is contained in the image palette. How can I get find out which color is from the data loaded in the IPicture structure?? or from the HBITMAP structure??

    This way I can render the image with the transparent parts the creator intended and not a predefined transparent color chose by me...

    thanks for any tips...

  2. #2
    Join Date
    Dec 2003
    as far as I can tell either a gif is stored as rgba to indicate transparency. But I am not 100% sure about this.

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