How to use PixelFormats.IndexedX with RenderTargetBitmap?

Question

I am looking to render a DrawingVisual (visual in the example) to a bitmap using RenderTargetBitmap with the view to set this bitmap as the background to a Canvas as below:

var bmp = new RenderTargetBitmap(2000, 50, 120, 96, PixelFormats.Indexed2);
bmp.Render(visual);
var brush = new ImageBrush(bmp) { Stretch = Stretch.Fill };
Canvas.Background = brush;

When using PixelFormats.Default as the last argument to RenderTargetBitmap, the image renders as expected. However, when I choose PixelFormats.Indexed2 (or any of the PixelFormats.IndexedX), my code seems to exit the method without an exception, the bmp.Render line is never called and hence the image is not displayed on the Canvas.

How to use the IndexedX pixel formats with RenderTargetBitmap? Or are there other ways to reduce the memory footprint of the image? It only uses three colors, so using a palette rather than 32bit RGB seemed the way to go.

Lifesaving

Answer 1

You can't. RenderTargetBitmap only supports the Pbgra32 pixel format. That's because WPF's rendering system works entirely in 32 bits per pixel. That's the format in which it generates images, and it's also the format in which it prefers images to be in if you want to render them. (If you provide it with a bitmap in any other format, it'll need to convert it into a 32 bit per pixel representation first.)

What are you planning to do with this bitmap? If you want to render it in a WPF application, it'll need to be converted to a 32bpp format first in any case, so you risk using more memory if you attempt to hold it internally in any other format. (You'll have your supposedly memory-efficient representation and the version WPF's actually able to work with.) Not to mention the extra CPU time spent converting between your chosen format and a format WPF can work with.

Dominique

Thanks for the info Ian, I didn't realise that image had to be held in memory as Pbgra32 at some point in the render process. Looks like I am going to have to live with the extra memory usage. – Lifesaving

Out of interest, the bitmap is going in a scrollviewer to provide a scrollable chart. I haven't been able to get the performance I want out of vector drawings, so have resorted to a bitmap. – Lifesaving

@Simon_Weaver it does say so. When I try this, it throws an ArgumentException with the message 'Indexed2' PixelFormat is not supported for this operation. Parameter name: pixelFormat Seems clear enough. Notice that the OP says that the bmp.Render line is never called. So I think he is mistaken when he concludes that it "exit[s] the method without an exception". I would guess that what actually happened is something higher up in his call stack swallowed the exception, meaning the debugger didn't report it. – Dominique

See RenderTargetBitmap(Int32,Int32,Double,Double,PixelFormat) in .NET 4.6.2 reference source. – Sherwin