Brightness and contrast in color image

Question

Does, anyone know, how I can change brightness and contrast of color image. I know about vtkImageMapToWindowLevel, but after setting level or window of image in this class, the color image becomes grayscale.

Thanks for answers;

Answer 1

By definition, a color image is already color mapped, and you cannot change the brightness/contrast of the image without decomposition and recomposition.

First, define a pair of numbers called brightness and contrast in whatever way you want. Normally, I'd take brightness as the maximum value, and contrast as the ratio between minimum and maximum. Similarly, if you want to use Window/Level semantics, "level" is the minimum scalar value, and window is the difference between maximum and minimum.

Next, you find the scalar range - the minimum and maximum values in your desired output image, using the brightness and contrast. If you're applying brightness/contrast, the scalar range is:

1. Maximum = brightness
2. Minimum = Maximum / contrast

Assume a color lookup table (LUT), with a series of colors at different proportional values, say, in the range of 0 to 1. Now, since we know the brightness and contrast, we can setup the LUT with the lower value (range 0) mapping to "minimum" and the upper value (range 1) mapping to "maximum". When this is done, a suitable class, like vtkImageMapToColors can take the single-component input and map it to a 3 or 4 component image.

Now, all this can happen only for a single-component image, as the color LUT classes (vtkScalarsToColors and related classes) are defined only on single-component images.

If you have access to the original one-component image, and you're using vtkImageMapToColors or some similar class, I'd suggest handling it at that stage.

If you don't, there is one way I can think of:

1. Extract the three channels as three different images using vtkImageExtractComponents (you'll need three instances, each with the original image as input).
2. Independently scale the 3 channels using vtkImageShiftScale (shift by brightness, scale by contrast)
3. Combine the channels back using vtkImageAppendComponents

Another possibility is to use vtkImageMagnitude, which will convert the image back to grey-scale (by taking the magnitude of the three channels together), and re-applying the color table using vtkImageMapToColors and any of the vtkScalarsToColors classes as your lookup table.

The first method is better if your image is a real photograph or something similar, where the colors are from some 3-component source, and the second would work better if your input image is already using false colors (say an image from an IR camera, or some procedurally generated fractal that's been image mapped).