Digital Image Processing

​The brightness of an image can be changed by adding (or subtracting) a fixed value from every single pixel value.

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​This shifts the histogram either left or right.

​Contrast describes how different the light parts of an image are to the dark parts.

Increasing contrast therefore makes the dark pixels darker and the light pixels lighter.​

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Increasing contrast spreads the histogram over a wider range of pixel values. The area remains constant as the number of pixels is fixed.

​Colour inversion switches each pixel colour for the opposite on the colour wheel.

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​The pixel value is subtracted from the maximum possible value for every pixel.

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​For RGB colour this happens for each of R,G and B separately.

​Edge detection colours the image black except where there is a sudden change in pixel value.

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​Each pixel value is multiplied by four and then has the N, S, E and W pixel values subtracted.

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​There is a one pixel boundary where the algorithm does not work.

​A median filter is a method for reducing random noise in an image.

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​Each pixel value is replaced with the median average of the surrounding pixels (N, S, E and W).

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​Using the median average, rather than the mean, reduces the amount of blurring that occurs.

​Noisy image

​Median filtered