WPF rendering a Viewport3D to RenterTargetBitmap changes RGB values

Question

I'm trying to render a Viewport3D with textured triangles to a RenderTargerBitmap.

But the colors in the bitmap are different than in the viewport:

• Right side: an actual in-app Viewport3D.
• Left side: the RenderTargetBitmap shown in a Image Stretch="None". All colors have lower RGB values from the viewport (and the original textures) The antialiased grays are a lot darker than the original, making it seem like aliasing issues (which it isn't). But also the filling colors are shifted.

I use this code to make the bitmap:

var renderTarget = new RenderTargetBitmap(500, 500, 144, 144, PixelFormats.Default);  
renderTarget.Render(SceneViewport);  
yield return renderTarget;  

I have a 150% zoom set in Windows, so I use 144 dpi (gotten via PresentationSource), but it's the same issue with 96: the pixel colors shift.

I converted my entire app from WPF .net core to .net framework hoping it was a bug in core, but i get the exact same color shifting.

2 questions:

• Any ideas how to fix this?
• Alternatives to get the actual pixel values of the Viewport3D?

Answer 1

I have done more testing in a small new WPF app. It seems all rendering of WPF visuals to a RenderTargetBitmap are off

These are 2 real buttons, and below it, a bitmap render of those 2 buttons:

I'm starting to think the RenderTargetBitmap.Render uses an entirely different rendering pipeline (software maybe?), showing a lot of differences.

So, unusable for my animation app.. :(