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Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved. This file is offered as-is, without any warranty.


This source code provides an implementation of the "midway" image equalization, as described in IPOL.

The 'bin/midway' program reads two PNG images, then apply contrast changes to these images such that the output images have the same distributions (this is done channel by channel). This common distribution is built as the midway between the two original ones. If H1 and H2 are the cumulative histograms of the input images (for the considered channel), the midway histogram is defined as (0.5*(H1^{-1} + H2^{-1}))^{-1}. The contrast changes to be applied are thus respectively x -> 0.5*(x + H2^{-1} o H1 (x)) for the first image and x -> 0.5*(x + H1^{-1} o H2 (x)) for the second image. Both resulting images are written as PNG images.

Only 8bit GRAY/RGB PNG images are handled.


The code is written in ANSI C, and should compile on any system with an ANSI C compiler.

The libpng header and libraries are required on the system for compilation and execution. On Linux, just use your package manager to install it:

sudo apt-get install libpng

For more information, see


Simply use the provided makefile, with the command 'make'. The makefile will produce a program called : 'bin/midway'.

The 'midway' program is used to compute the midway equalization of two images.




The 'bin/midway' program is used to apply the midway equalization over two images. It takes 4 parameters with other optional:

bin/midway in1.png in2.png out1.png out2.png [options]
  • in1.png :: first input image.
  • in2.png :: second input image.
  • out1.png :: first output image.
  • out2.png :: second output image.

Options :

  • -dithering sigma :: apply a Gaussian noise of standard deviation sigma before applying the midway equalization.
  • -verbose :: activate the verbose mode

Usage examples

If you want to compute the midway equalization between the files 'input/bat1.png' and 'input/bat2.png' without dithering and save the resulting images into 'out1.png' and 'out2.png', you can use the command:

bin/midway input/bat1.png input/bat2.png out1.png out2.png

The command:

bin/midway input/bat1.png input/bat2.png out1.png out2.png -dithering 2

will compute the midway equalization of the files 'input/bat1.png' and 'input/bat2.png' with a dithering of sigma=2 and save the resulting images into 'out1.png' and 'out2.png'.

Bugs reports

You can report any bug with the github interface:


Midway implementation sources for the IPOL demo





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