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POBilinear interpolation - DirectX vs. GDI+
 

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  1. POBilinear interpolation - DirectX vs. GDI+
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    <p>I have a C# app for which I've written GDI+ code that uses Bitmap/TextureBrush rendering to present 2D images, which can have various image processing functions applied. This code is a new path in an application that mimics existing DX9 code, and they share a common library to perform all vector and matrix (e.g. ViewToWorld/WorldToView) operations. My test bed consists of DX9 output images that I compare against the output of the new GDI+ code.</p> <p>A simple test case that renders to a viewport that matches the Bitmap dimensions (i.e. no zoom or pan) <em>does</em> match pixel-perfect (no binary diff) - but as soon as the image is zoomed up (magnified), I get very minor differences in 5-10% of the pixels. The magnitude of the difference is 1 (occasionally 2)/256. I suspect this is due to interpolation differences.</p> <p><b>Question</b>: For a DX9 ortho projection (and identity world space), with a camera perpendicular and centered on a textured quad, is it reasonable to expect <a href="http://msdn.microsoft.com/query/dev10.query?appId=Dev10IDEF1&amp;l=EN-US&amp;k=k(MICROSOFT.DIRECTX.DIRECT3D.TEXTUREFILTER);k(TargetFrameworkMoniker-%22.NETFRAMEWORK%2cVERSION%3dV3.5%22);k(DevLang-CSHARP)&amp;rd=true" rel="nofollow">DirectX.Direct3D.TextureFilter.Linear</a> to generate identical output to a GDI+ TextureBrush filled rectangle/polygon when using the <a href="http://msdn.microsoft.com/query/dev10.query?appId=Dev10IDEF1&amp;l=EN-US&amp;k=k(SYSTEM.DRAWING.DRAWING2D.INTERPOLATIONMODE);k(TargetFrameworkMoniker-%22.NETFRAMEWORK%2cVERSION%3dV3.5%22);k(DevLang-CSHARP)&amp;rd=true" rel="nofollow">System.Drawing.Drawing2D.InterpolationMode.Bilinear</a> setting?</p> <p>For this (magnification) case, the DX9 code is using this (MinFilter,MipFilter set similarly):<br> <code>Device.SetSamplerState(0, SamplerStageStates.MagFilter, (int)TextureFilter.Linear);</code></p> <p>and the GDI+ path is using: <code>g.InterpolationMode = InterpolationMode.Bilinear;</code></p> <p>I thought that "Bilinear Interpolation" was a fairly specific filter definition, but then I noticed that there is another option in GDI+ for "HighQualityBilinear" (which I've tried, with no difference - which makes sense given the description of "added prefiltering for shrinking")</p> <p><b>Followup Question:</b> Is it reasonable to expect pixel-perfect output matching between DirectX and GDI+ (assuming all external coordinates passed in are equal)? If not, why not?</p> <p><b>Clarification:</b> The images I'm using are opaque grayscale (R=G=B, A=1) using Format32bppPArgb.</p> <p>Finally, there are a number of other APIs I could be using (Direct2D, WPF, GDI, etc.) - and this question generally applies to comparing the output of "equivalent" bilinear interpolated output images across any two of these. Thanks!</p>
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    1. COI don't get it. How can you expect pixel-perfect values when you use interpolation? The point of interpolation is to *change* pixel values. If your point is that DX and GDI+ don't interpolate the same way: no they don't. Different code.
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    2. CODifferent code certainly, but as the definition of bilinear interpolation is a simple algorithm (http://en.wikipedia.org/wiki/Bilinear_interpolation) to find the weighted contributions of adjacent pixels, why should they generate different results? The results should be equivalent (within floating point rounding error, at least) if they are using the same formula. I'm looking for any specific knowledge on how the formulas may differ between DX9 and GDI+.
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    3. COBecause this kind of code is optimized to be quick before being accurate. You'd have to call Microsoft Support if you want to find somebody with the specific knowledge. I only know that GDI+'s interpolation is a bit noisy, taking advantage of the human eye not being able to observe small differences in order to gain speed. That really mattered 10 years ago.
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