Why do I see pixels when zooming WPF application in Windows?

Question

I have developed a GUI for a random application using WPF. I have a bunch of out of box WPF controls laid on the application window. I haven't customized anything, didn't use bitmaps, etc.

When running my application and zooming using Magnifier application in Windows 7 (Win key + Plus key, the magnified GUI is showing pixels.I am probably wrong, because I can't explain it otherwise, but isn't WPF supposed to provide vector like control rendering?

Thanks for participating in the discussion.

Bonus Reading

• Tim Sneath: Magnifier: An Interesting Discovery (archive)
• WPF Vector based interface *(screenshot of WPF being vector scaled by Magnifier)
• MSDN Blogs: Greg Schechter explains why it longer happens (archive)

Answer 1

Back when Vista first shipped, and when WPF was on version 3.0, zooming with the built-in magnifier would actually do vector-based scaling.

This stopped working when WPF 3.5 service pack 1 shipped. (It worked in 3.5 before sp1.) The reason it worked before then is that the DWM (Desktop Window Manager) - the part of Windows responsible for presenting everything you see on screen - uses MILCORE.DLL to do its rendering. Version 3.0 and 3.5 of WPF also used this same component to render - this meant that all WPF content was native content, so to speak. (In fact, on Windows XP, which doesn't have the DWM, MILCORE.DLL is something that WPF puts on your system for its own benefit. But it's built into Vista and Windows 7.) When WPF was using MILCORE.DLL to render on Vista, any effects applied by the DWM such as scaling would also apply in the way you want to WPF - it really did scale without pixelating.

Unfortunately, this is no longer the case. And the reason is that WPF started adding new rendering features. In 3.5 sp1, the new feature in question was support for custom pixel shaders. To enable that, Microsoft had to release an update to the MIL. (The Media Integration Layer - the bit that does the actual rendering.) However, they weren't really in a position to update MILCORE.DLL, because that's part of Windows - it's how everything you see on screen gets to be on screen. Releasing a new version of MILCORE.DLL effectively means pushing out an update to Windows. The release schedule for Windows is independent of that for .NET, and so the only way the WPF team could reasonably add new features was to ship a new MIL. (In theory they could have done it via Windows Update, but since WPF is now owned by a different division of Microsoft than Windows, that sort of thing doesn't seem to happen in practice.)

As of .NET 3.5 sp1, the MIL is in a different DLL called wpf_gfx_vXXXX.dll where vXXXX is the version number. In .NET 4.0, it's wpf_gfx_v0400.dll.

The upside is that WPF gets to add new rendering features with each new version, without needing Windows itself to be updated. The downside is that WPF's rendering is no longer as tightly integrated with Windows as it was briefly back when Vista shipped. And the upshot is, as you've seen, that magnifying is not as much fun as it used to be.

Answer 2

The magnifier application implements its own zoomed image rendering, so that's why you are seeing pixels. WPF does use vector graphics, but in this situation it's not the WPF application itself that is rendering the zoomed image.

If you use something like Snoop you can see zoomed and scaled WPF vector graphics rendering in action.