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c#graphicsantialiasing

Change background colour of an anti-aliased image using C# with anti-aliasing

I have an image where I need to change the background colour (E.g. changing the background of the example image below to blue).

However, the image is anti-aliased so I cannot simply do a replace of the background colour with a different colour.

One way I have tried is creating a second image that is just the background and changing the colour of that and merging the two images into one, however this does not work as the border between the two images is fuzzy.

Is there any way to do this, or some other way to achieve this that I have no considered?

Example Image

Solution

  • Abstractly

    Each pixel in your image is a (R, G, B) vector, where each component is in the range [0, 1]. You want a transform, T, that will convert all of the pixels in your image to a new (R', G', B') under the following constraints:

    • black should stay black
      • T(0, 0, 0) = (0, 0, 0)
    • white should become your chosen color C*
      • T(1, 1, 1) = C*

    A straightforward way to do this is to choose the following transform T:

    T(c) = C* .* c  (where .* denotes element-wise multiplication)

    This is just standard image multiplication.

    Concretely

    If you're not worried about performance, you can use the (very slow) methods GetPixel and SetPixel on your Bitmap to apply this transform for each pixel in it. If it's not clear how to do this, just say so in a comment and I'll add a detailed explanation for that part.

    Comparison

    Compare this to the method presented by LarsTech. The method presented here is on the top; the method presented by LarsTech is on the bottom. Notice the undesirable edge effects on the bottom icon (white haze on the edges).

    Comparison

    And here is the image difference of the two:

    Difference

    Afterthought

    If your source image has a transparent (i.e. transparent-white) background and black foreground (as in your example), then you can simply make your transform T(a, r, g, b) = (a, 0, 0, 0) then draw your image on top of whatever background color you want, as LarsTech suggested.