Normally, one would use the histogram function for finding out about the distribution of tonal values of an image. But analyzing the effects on tonal values of a specific image processing tool would involve the effortful task of comparing before and after histograms and this would not be very precise, either.
It is often more illuminating to look at what happens to the image's gradation. The gradation describes a "before-after" in itself, e.g. the amount of blackening evoked by a specific amount of light hitting the photo paper. The "gradation curves" tool that is part of any image processing program (under different names) allows for precise brightening or darkening of pixels having particular tonal values (brightnesses). These adjustments are displayed at the same time for all tonal values.
It is quite intuitive to present changes in brightness that have been affected by other image processing tools in the same way. Quite a number of these tools merely change the brightness of pixels - sometimes separately for each color channel like the popular tool "color balance". If this is the case, the exact same results can be achieved by using the "curves" tool.
Measuring gradation shifts is only possible with a special test image or at least a band that is placed into the to-be-processed image. You can find more information on that in the PDF file Self-made gradation curve as a "measuring device" (German) and below.
Kind and intensity of global tonal value changes can be analyzed in a simple way using a test image consisting merely of a linear gradient from black to white. A tricky combination of layers produces of the gradient's left-to-right-increasing brightness a left-to-right-increasing curve. The unchanged gradient produces a straight line increasing with an angle of 45 degrees.
Test image and the resulting gradation curve
Every change in brightness will be displayed clearly as a shift, bend or even fragmentation of the curve. If you are familiar with Photoshop's "Curves" tool or similar tools in other programs you will not have difficulties in interpreting the curve because there are hardly any visual differences. However, while "curves" is a tool for modifying tonal values, the gradation curve presented here is a measuring device: It displays the changes the tonal values have undergone. For doing experiments on your own you can download the test image including the complete layers construction necessary for evaluating the changes as a ZIP archive using the link above. It contains versions for all popular image processing programs and is free of charge.
The ZIP archive first contains the test image as a PSD file. This version uses adjustment layers which can only be used by Photoshop and Paint Shop Pro. For all other programs (PhotoImpact, Photoline 32, Photo-Paint and GIMP) I included test images not containing adjustment layers.
Modified test image and the resulting gradation curve
Color changes can be displayed, too.
This is a correction made with "color balance".
An improved and much more comfortable version of this tool uses a Photoshop plugin and Photoshop actions for evaluation. Thus all it takes to produce the curve is a single click with the mouse, another click will thicken the curve points and add a grid layer to the diagram for the finishing touch.
To this test kit belongs a small test band that can be build right into any image and after processing tells you exactly what tonal value changes happened to it. The test kit can be used in all programs that are compatible with Photoshop plugins. Support of actions and layers are not absolutely necessary, but lacking them you will lose some comfortableness (e.g. finishing the diagrams, if wanted, must be done by hand in this case).
You will find ordering information concerning the test kit in one of the boxes on top of the page.
The test band shows after evaluation that the image has gone through an increase in contrast and a slight color correction (brightening of mid-tones in the green channel).
gradationstest.zip (39 KB)
Test image consisting of multiple layers for displaying gradation changes caused by image processing tools. Works with Photoshop, Paint Shop Pro, Photo-Paint, Picture Publisher, PhotoImpact, PhotoLine 32 and GIMP (Windows, Mac-OS, Linux...).
The test constructions are Freeware and can be copied and distributed freely as long as no charges are being taken.
Now for free!
Contains a test image and a test band for evaluation of gradation changes happening to an image during processing. It further contains a plugin for creation of the resulting gradation curve and supplementary Photoshop actions.
(for Windows or Mac OS)