Histogram Levels

The Histogram Levels module provides a way to control the histogram of an image in a more customized way by compressing, stretching and bending the histogram to achieve a desired effect. The modification can be done on multiple channels or a single channel.

Interface

Instructions

1. Edit - you can edit all RGB channels together, the Intensity only, Red channel, Green channel, or Blue channels. Selecting RGB will change all the RGB values of the histogram simultaneously. Selecting Intensity will only change the luminosity of the image. Selecting Red, Green or Blue will only change that particular color channel. Note that each channel modification is performed in this order. Thus if the Intensity channel is chosen and a modification is made the subsequent channels (Red, Green, Blue) will make their modifications after the intensity is adjusted. This allows for each channel to specify its modifications prior to other channels being processed.

2. Range max/min - changing the values in the output range will compress the histogram values within the max-min range. Reducing the max will prevent pixel values from exceeding that value. I.e. it reduces the maximum intensity of the image. Similarly increasing the min range will increase the lowest possible intensity value which increases the overall intensity of the image.

3. Midtones compress - to bend the image towards lighter and darker intensity values select a new midtone by entering a value into the text box or by dragging the slider up or down. Increasing the midtones moves the image closer to a high contrast image, reducing the midtones reduces the contrast towards a more gray image.

4. Stretching - by moving the triangle markers below the histogram you can threshold the image to the specified minimum or maximum values. All pixels that are below (darker) than the minimum (dark triangle) will be set to zero. All pixels whose values are greater than the maximum (white triangle) will be set to 255. Instead of dragging the markers you can enter in values into the text boxes below the histogram. You can change the values directly in the provided text boxes or by sliding the white and black markers up and down the intensity range.

5. Gamma - you can change the image gamma value by dragging the middle triangle marker (gray). The Gamma value can also be set by entering a number (typically around 1.0) into the provided text box.

Example

Source	Histogram Levels (gamma 2.43, midtone -5)

See Also

Equalize
Normalize

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