# Documentation

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# imhist

Histogram of image data

## Syntax

``imhist(I)``
``imhist(I,n)``
``imhist(X,map)``
``````[counts,binLocations] = imhist(I)``````
``````[counts,binLocations] = imhist(gpuarrayI,___)``````

## Description

example

````imhist(I)` calculates the histogram for the intensity image `I` and displays a plot of the histogram. The number of bins in the histogram is determined by the image type.```
````imhist(I,n)` calculates the histogram, where `I` specifies the number of bins used in the histogram. `n` also specifies the length of the colorbar displayed at the bottom of the histogram plot.```
````imhist(X,map)` displays a histogram for the indexed image `X`. This histogram shows the distribution of pixel values above a colorbar of the colormap `map`. The colormap must be at least as long as the largest index in `X`. The histogram has one bin for each entry in the colormap.```
``````[counts,binLocations] = imhist(I)``` returns the histogram counts in `counts` and the bin locations in `binLocations` so that `stem(binLocations,counts)` shows the histogram. For indexed images, `imhist` returns the histogram counts for each colormap entry. The length of `counts` is the same as the length of the colormap.```

example

``````[counts,binLocations] = imhist(gpuarrayI,___)``` performs the histogram calculation on a GPU. The input image and the return values are `gpuArray`s. This syntax requires the Parallel Computing Toolbox™. When the input image is a `gpuArray`, `imhist` does not automatically display the histogram. To display the histogram, use `stem(binLocations,counts)`.```

## Examples

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Read a grayscale image into the workspace.

`I = imread('pout.tif');`

Display a histogram of the image. Since `I` is grayscale, by default the histogram will have 256 bins.

`imhist(I)`

`load mristack`

Display the histogram of the data. Since the image is grayscale, `imhist` uses 256 bins by default.

`imhist(mristack)`

Create array of class `uint16`.

`I = gpuArray(imread('pout.tif'));`

Calculate histogram. Because `imhist` does not automatically display the plot of the histogram when run on a GPU, this example uses `stem` to plot the histogram.

```[counts,x] = imhist(I); stem(x,counts); ```

## Input Arguments

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Input intensity image, specified as a numeric array. `I` can be 2-D, 3-D, or N-D.

Example: `I = imread('cameraman.tif');`

Data Types: `single` | `double` | `int8` | `int16` | `int32` | `uint8` | `uint16` | `uint32` | `logical`

Number of bins, specified as a numeric scalar. If `I` is a grayscale image, `imhist` uses a default value of 256 bins. If `I` is a binary image, `imhist` uses two bins.

Example: `[counts,x] = imhist(I,50);`

Data Types: `single` | `double` | `int8` | `int16` | `int32` | `int64` | `uint8` | `uint16` | `uint32` | `uint64`

Input indexed image, specified as a numeric array. `X` can be 2-D, 3-D, or N-D.

Example: `[X,map] = imread('trees.tif');`

Data Types: `single` | `double` | `uint8` | `uint16` | `logical`

Colormap associated with indexed image, specified as a p-by-3 array.

Example: `[X,map] = imread('trees.tif');`

Data Types: `single` | `double` | `int8` | `int16` | `int32` | `uint8` | `uint16` | `uint32` | `logical`

Input image, specified as a `gpuArray`.

Example: `gpuarrayI = gpuArray(imread('cameraman.tif'));`

## Output Arguments

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Histogram counts, returned as a numeric array.

Bin locations, returned as a numeric array.

## Tips

• For intensity images, the `n` bins of the histogram are each half-open intervals of width A/(n−1). In particular, the pth bin is the half-open interval

`$\frac{A\left(p-1.5\right)}{\left(n-1\right)}-B\le x<\frac{A\left(p-0.5\right)}{\left(n-1\right)}-B,$`
where x is the intensity value. The scale factor A and offset B depend on the type of the image class as follows:

doublesingleint8int16int32uint8uint16uint32logical
A112556553542949672952556553542949672951
B001283276821474836480000