Equalization implies mapping one distribution (the given histogram) to another distribution (a wider and more uniform distribution of intensity values) so the. Histogram equalization is a technique for adjusting image intensities to enhance contrast. normalized histogram of f with a bin for each possible intensity. So. Histogram is a graphical representation of the intensity distribution of an image. In simple terms, it represents the number of pixels for each.

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The resulting binary image is equalisatikn to the previous one. The background is black and is representing a big part of the image. For example, if applied to 8-bit image displayed with 8-bit gray-scale palette it will further reduce color depth number of unique shades of gray of the image.

To call the function, you need to use the following prototype and parameters: So, to be able to model the two distributions, we historam to know these parameters.

On the previous figure, we can see that the input picture is really dark. Indeed, a linear cdf is associated to the uniform histogram that we want the resulting image to have. Indeed, we want to be able to specify the number of bins and also its range. There may be some cases were histogram equalization can be worse. But there is an important thing to notice, in the previous case, we didn’t use a “scaling” factor according to the area of the image associated to each color.

Histogram Equalization

In simple terms, it represents the number of pixels for each intensity value considered. I decided to run the histogram equalization technique on other images with different types of histogram to see how this is affecting them.

However, applying the same method on the Red, Green, and Blue components of an RGB image may yield dramatic changes in the image’s color balance since the relative distributions of the color channels change as a result of applying the algorithm.


Histogram is a graphical representation of the intensity distribution of an image. So, here we can see many things, firstly that there is an issue in the display of the cdf which is not ending at the right value.

Histogram equalization – Wikipedia

It returns a vector containing the relative frequencies associated to the histogram. In this part, we implemented a pipeline to segment the checkerboard according to a threshold computed from the two Gaussian pdf. So the idea here is to get more information about these two Gaussian noise distributions.

But here it’s a bit more tricky than before to get accurate patches, this is why on eqaulisation following plot, the Gaussian distributions are not describing correctly the histogram. This page was last edited on 18 Mayat The last thing we need to know that we know the threshold is to create the output image by assigning to each pixel it’s new value.

The histoggram is not computationally intensive. Corresponding histogram red and cumulative histogram black.

Histogram equalization

The only thing we can say is that the resulting cdf is no longer linear, which could be explained by the two “dirac” a 0 and which are probably influencing the whole thing.

After applying we got the following image and its following histogram. An histogram being a distribution of the number of pixel according to their intensities, we have in this part to analyze the image to determine this distribution. The objective of this first project is to develop and implement some image processing functions based on the histogram of an Image. In terms equzlisation statistics, the value of each output image pixel characterizes the probability that the corresponding input pixel group belongs to the object whose histogram is used.

Again, pixel values that do not contribute to an increase in the cdf are excluded for brevity. In order to map the values back into their original range, the following simple transformation needs to be applied on the result:. A signal transform equivalent to histogram equalization also seems to happen in biological neural networks so histogrxm to maximize the output firing rate of the neuron as a function of the input statistics.


To do so we are going to use the same pipeline of equalization as before except that the resulting image is going to be a linear blending of the original image and the equalized one according to the following formula:. The operation can be expressed as P M I where I is the original image, M is histogram equalization mapping operation and P is a palette. This allows for areas of lower local contrast to gain a higher equwlisation.

On the previous result, we can see that the input image has an histogram occupying the whole range of gray levels and with a cumulative distribution which equalidation going up really smoothly without big steps.

The Hjstogram function computing the mean when we give it a 2 dimensional array our image is returning an average per column. Each image can be decomposed into histogrxm different layers according to the three color channels encoded: The cumulative distribution function cdf is shown below. So the idea here is clearly to blend them linearly according to what we want to have. This method usually increases the global contrast of many images, especially when the usable data of the image is represented by close contrast values.

The probability of an occurrence of a pixel of level i in the image is.