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Date Posted: 07:12:48 04/05/00 Wed
Author: Michel
Subject: Re: Scaling/Displaying very large bitmaps
In reply to: Andrew Pitts 's message, "Scaling/Displaying very large bitmaps" on 15:32:25 04/04/00 Tue

Hi Andrew,

Working with such large bitmaps can indeed be problematic. Envision is design using the VCL framework, TGraphic and TPicture/TImage which assume entire images are loaded and displayed. Using a striped or tiled approach is much more complex, and may be supported by some high end image libraries.

As far as I know, the upper limit is really system dependent. I think, the operating system (9.X, NT), video driver, and video mode have an effect.

The only possible solution which might work is to use the TResizeTransform but with Interpolated set to True. This will make sure the StretchDiBits call is not used, although this will be time consuming.

Best regards,


> Using your graphics library, I am unable to load a
> very large bitmap (3000x3000) and shrink it to full
> screen. I want to end up with a TMemoryStream of the
> bitmap that has been shrunk to a reasonable screen
> size (1024x768).
> The basic code is:
> bmapgraphic.LoadFromFile (xxx)
> bfinal.NewImage (MaxW, MaxH,ifTrueColor,nil,0,0);
> SetStretchBltMode( bfinal.DC, Windows.COLORONCOLOR );
> StretchDiBits( bfinal.DC, 0, 0, 1024, 768,
> bmapgraphic...)
> ...
> The StetchDiBits call fails (I have looked at a lot of
> postings that state bitmaps larger than the screen
> have to be done as stripes, or chunks).
> Do you know of a way to take these large bitmaps and
> shrink them to a manageable size for viewing only ?
> I have also tried it using your TResizeTransform and
> that fails also.

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