How to change the color of the background of an image?

Question

I have an image, and I want to change the color of its background. For example, I want to change the color of the sky from blue to green (or just to pure-white, for simplicity). I have already get the edge of the house, i.e. imgEdges2 (my method of getting the imgEdges2 is very ad-hoc, and any advice for improvement is also welcome), how can I change the color of the sky?

Many thanks!

img = ExampleData[{"TestImage", "House"}]
imgEdges = EdgeDetect[img, 6.7, 0.11];
edgeData = ImageData@imgEdges;
edgeData[[246 ;; 256, All]] = 0;
edgeData[[80 ;; 120, 100 ;; 256]] = 0;
edgeData[[60 ;; 79, 90 ;; 154]] = 0;
edgeData[[50 ;; 59, 90 ;; 110]] = 0;
edgeData[[57 ;; 59, 111 ;; 115]] = 0;
imgEdges2 =Image@edgeData

Answer 1

img = ExampleData[{"TestImage", "House"}];

edge = ImageMultiply[EdgeDetect[ImageSubtract[#3, #] & @@ ColorSeparate[img], 4, 0.04],
    Image[SparseArray[{{i_, j_} /; Min[i, j] <= 100 -> 1}, ImageDimensions[img]]]]

binary = Binarize[ImageMultiply[ImageSubtract[#3, #] & @@ ColorSeparate[img],
    Image[SparseArray[{{i_, j_} /; Min[i, j] <= 100 -> 1}, ImageDimensions[img]]]], 0.12]

sky = Nest[ImageAdd[ImageFilter[Min, ImageFilter[Max, #, 1], 1], edge] &,
           ImageFilter[Min, ImageAdd[binary, edge], 1], 2]

ImageAdd[
   ImageMultiply[ImageApply[{#[[2]], #[[3]], #[[1]]} &, img], #],
   ImageMultiply[img, ColorNegate[#]]] & /@ {sky, sky - edge}

$\hspace{1.5cm}$

Answer 2

There are built in functions for this. Remove background

RemoveBackground[img]

Replace the background with a different color:

RemoveAlphaChannel[RemoveBackground[img], Green]

That works because removal of background is done with help of setting the alpha channel with mask:

AlphaChannel[RemoveBackground[img]]

Answer 3

Here is how I would do it:

Get the Image:

img = ExampleData[{"TestImage", "House"}];

Use RegionBinarize to get a mask, and then close up the holes:

backgroundMask = ColorNegate[FillingTransform[Closing[RegionBinarize[img, ColorNegate @ Binarize @ img, 0.25], 5]]];

Choose the color to be targetted based on what occurs in the mask:

backgroundColor = RGBColor[Median[DeleteCases[Flatten[ImageData[backgroundMask img], 1], {0., 0., 0.}]]];

Make the replacement through the whole image:

colorReplacedImg = ColorReplace[img, backgroundColor -> Red];

Arithmetic so the replacement is only kept inside the mask:

resultImg = colorReplacedImg backgroundMask + ColorNegate[backgroundMask] img