GDI+ Dithering Problem

Question

I have a C++ application that uses the Win32 API for Windows, and I'm having a problem with GDI+ dithering, when I don't know why it should be.

I have a custom control (custom window). When I receive the WM_PAINT message, I draw some Polygons using FillPolygon on a Graphics device. This Graphics device was created using the HDC from BeginPaint.

When the polygons appear on the screen, though, they are dithered instead of transparent, and only seem to show few colors (maybe 256?) When I do the same thing in C# using the .NET interface into GDI+, it works fine, which is leaving me wondering what's going on.

I'm not doing anything special, this is a simple example that should work fine, as far as I know. Am I doing something wrong?

Edit: Nevermind. It only happens over Remote Desktop, even though the C# example doesnt Dither over remote desktop. Remote Desktop is set at 32-bit color, so I don't know what's up with that.

Answer 1

It only happens over Remote Desktop

Many remoting applications will reduce colour depth in order to reduce bandwidth requirements. While I haven't used Remote Desktop, the same happens on certain VNC connections. I'd check your RD server and client settings.

Answer 2

Hmm... The filling capabilities are determined by the target device. When working over remote desktop, AFAIK Windows substitutes the display driver, so that can change the supported features of the display.

1. when drawing on wm_paint, you actually draw directly on the screen surface, while .net usually uses double buffering (draws to in memory bitmap and then blits the entire bitmap)
2. there are some settings in gdi+ that affect the drawing quality. maybe there are different defaults for on-screen, off-screen and remote painting?