Convert BitmapImage to grayscale, and keep alpha channel

Question

I'm having an issue with converting a BitmapImage (WPF) to grayscale, whilst keeping the alpha channel. The source image is a PNG.

The MSDN article here works fine, but it removes the alpha channel.

Is there any quick and effective way of converting a BitmapImage to a grayscale?

Answer 1

if relying on API calls fails. You can always try the 'do it yourself' approach: Just get access to the RGBA bytes of the picture, and for every RGBA replace it with MMMA, where M = (R+G+B)/3;

If you want it more perfect, you should add weights to the contribution of the RGB components. I believe your eye is more receptive for green, and as such that value should weigh more.

Answer 2

You should have a look at image transformation using matrices.

In particular, this article describes how to convert a bitmap to grayscale using a ColorMatrix. (It is written in VB.NET, but it should be easy enough to translate to C#).

I haven't tested if it works with the alpha channel, but I'd say it's worth a try, and it definitely is a quick and effective way of modifying bitmaps.

Answer 3

While not exactly quick and easy, a ShaderEffect would do the job and perform quite well. I've done it myself, and it works great. This article references how to do it and has source associated. I've not used his source, so I can't vouch for it. If you run into problems, ask, and I may be able to post some of my code.

Not every day you get to use HLSL in your LOB app. :)