MATLAB and Simulink resources for Arduino, LEGO, and Raspberry Pi

# How to histogram equilize an image?

Asked by Manoj Murali on 24 Feb 2012

Suppose i have a set of face images I1,I2, ,IM ,size of each image being LxL,in the training set.Now how to histogram equalize all the images and represent them as row vectors of size L^2 (i mean L to the power 2)each...

## Products

No products are associated with this question.

Answer by Sean de Wolski on 24 Feb 2012
```doc histeq
doc reshape
```

Manoj Murali on 24 Feb 2012

Error:
??? Error using ==> iptcheckinput
Function HISTEQ expected its first input, I, to be two-dimensional.

Error in ==> histeq at 71
iptcheckinput(a,{'uint8','uint16','double','int16','single'}, ...

Error in ==> histeq1 at 3
J = histeq(I);

the code I wrote is:
clc;
J = histeq(I);
imshow(I)
figure, imshow(J)

Jurgen on 15 Nov 2012

Your error is because the matrix you want to equalize is not 2D. Im assuming its an RGB image. Either equalize all color channels separately (dangerous) or convert to a difference colorspace.

If you convert to e.g. L*a*b* or YCbCr you only need to equalize the luma channel, and then convert back again to RGB.

Note you can not represent an LxL RGB image as a vector of length L^2 without resorting to:

1. throwing away data (e.g. color)
2. indexing or some other form of encoding. This is because your RGB image contains LxLx3 elements.

Suppose you convert to grayscale then it is possible by using reshape or just J(:). To avoid columnwise dumping transpose first (permute for N-Dim.) e.g. (J = J').