I need to do a histogram stretch

Question

I have an array of BitmapFrames and need to do a histogram stretch. I know this is different from a histogram equalization, and what the final outcome is... sorta. the problem i

Answer 1

If you have control over the lighting and or gain/offset of the camera, you can optimize those and stretch the histogram as you desire.

Answer 2

Don't forget to consider float. So a slight modification to dabonz413's answer:

maxVal = image.maximumValue() # 153
minVal = image.minumumValue() # 84
dynamic = maxVal-minVal
for pixel in image.Pixels():
    newPixel = ((float) (pixel-minVal)/dynamic)*255

Answer 3

Histogram stretching is the mapping of pixel values such that:

• the lowest value (e.g. 84 in the illustration) becomes 0
• the highest value (e.g. 153 in the illustration) becomes 255 (in 8bit images)
• and all the intermediate values are interpolated between that range (see illustration).

In other words histogram stretching means stretching the dynamic of the image data (84:153) to the greatest possible dynamic (0:255).

This should not affect the hight of the histogram peaks, but only their spread (the illustration is a bit misleading on this point).

histogram stretch http://cct.rncan.gc.ca/resource/tutor/fundam/images/linstre.gif

Image source

In practice this is the mapping you would apply to the pixels of the image (pseudocode):

maxVal = image.maximumValue() # 153
minVal = image.minumumValue() # 84
dynamic = maxVal-minVal
for pixel in image.Pixels():
    newPixel = ((pixel-minVal)/dynamic)*255