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POIs StretchBlt HALFTONE == BILINEAR for all scaling?
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  1. POIs StretchBlt HALFTONE == BILINEAR for all scaling?
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    copied!<p>Can anyone clarify if the GDI <a href="http://msdn.microsoft.com/en-us/library/dd145120(VS.85).aspx" rel="nofollow noreferrer">StretchBlt</a> function for the workstation Win32 API performs bilinear interpolation for scaling to <strong>both</strong> larger and smaller images for 24/32-bit color images? And if not, is there a GDI (<em>not</em> GDI+) function that does this?</p> <p>The <a href="http://msdn.microsoft.com/en-us/library/dd145089(VS.85).aspx" rel="nofollow noreferrer">SetStretchBltMode</a> fn has a setting <strong>HALFTONE</strong> which is documented as follows:</p> <blockquote> <p><strong><em>HALFTONE</em></strong> Maps pixels from the source rectangle into blocks of pixels in the destination rectangle. The average color over the destination block of pixels approximates the color of the source pixels.</p> </blockquote> <p>I've seen <a href="https://stackoverflow.com/questions/1117537/how-to-improve-drawdibs-quality">references (see follow-up to first answer)</a> that this performs bilinear interpolation when scaling down an image, but no clear answer of what happens when scaling up.</p> <p>I have noticed that the <a href="http://msdn.microsoft.com/en-us/library/aa933043.aspx" rel="nofollow noreferrer">Windows Mobile CE SDK does support a BILINEAR flag</a> - which is documented exactly opposite of the HALFTONE comments (only works for scaling up).</p> <p>Note that for the scope of this question, I'm not interested in pursuing GDI+ (which has numerous interpolation options), OpenGL, DirectX, etc. as alternatives, so please don't bother with follow-ups regarding these other APIs or alternate image libraries.</p> <p>What I'm really hoping to find is some definitive MS/MSDN or other high-quality documentation that clearly documents this behavior of the Win32 (desktop) GDI behavior.</p> <p>Meanwhile, I'll try some experiments comparing GDI vs. Direct2D (which does have an explicit flag to control this) and post my findings.</p> <p>Thanks!</p>
 

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