Perceptual Gamma Space Evaluation

This page provides charts for evaluating the perceptionally uniform gamma space, an issue that is largely misunderstood. Before evaluation please verify that the black-point of your monitor is correctly set.

The graph on the right contains all the 256 gray levels (0 ... 255) in an arrangement that smoothly gradates from black to white. The graph does not have embedded monitor gamma compensation so it directly shows the gradation of the system it is viewed on. When this graph is viewed in a steep gamma space like 1.8 to 2.5 then in the black-end there is a large group of levels that all appear the same for the eye, pure black and the mid-tones as well as the highlights are very coarse.

Perceptually uniform gamma space would be such that:

1. shows all the patches and
2. renders the tonality as smoothly as possible.

Photoshop users can evaluate the perceptually uniform gamma-space easily while viewing the chart in the browser. Open AdobeGamma and adjust the desired gamma value until the chart shows perceptually uniform distribution of the gray codes. There is no need to save the AdobeGamma profile since AdobeGamma changes the gamma-space immediately when the gamma value is changed. After evaluation just press Cancel to Adobe Gamma.

When the chart is shown on black background (as above) gamma-spaces that are higher than 1.4 will compress the black-end of the range heavily so that a large amount of codes in the deep shadows can not be differentiated from each other.

Next we will experiment in the other contrast situation, the same graph is shown below but on a white background.

When the chart is shown on white background then in a steep gamma space (1.8 ... 2.5) there is even more codes in the black-end that appear as pure black, one can almost discern all the patches in the white-end. Again experiment the perceptually uniform gamma-space using AdobeGamma, now even lesser gamma-space than the 1.4 is require to render the chart as perceptually uniform.

A closer look at Contrast Sensitivity

The below contrast sensitivity chart contains 36 squares that each have a split circle. The gray levels are: background levels as shown in the table. lighter half-circe = background level + 4 levels darker half-circe = background level - 4 levels
In otherwords for each element the background level is always halfway in between the lightness of its half-circles. This arrangement allows the eye to adapt freely so that maximum contrast sensitivity is utilized.

In a steep gamma space (1.8 ...2.5) the above chart does not appear perceptionally uniform. The difference between the half-circles are discernible in the white-end but in in the dark end there are many squares where the contrast is either invisible or very weak. Again use the AdobeGamma to experiment what is that gamma-space that show the contrast between the half-circles in all the the 36 patches perceptually unifrom (as having equally strong contrast).

Next we will look what happens to the contrast sensitivity when the background of the above chart is replaced with the background of the most light square, level 251 (same half-circles different background):

The black-end goes much deeper, there are a lot of circles that where the contrast is either invisible or very weak. At the white-end the contrast becomes very strong. Again evaluate using AdobeGamma.

Below we will see what happens to the contrast sensitivity when the background of the chart is replaced with the background of the most darkest patch, level 4 (again same half-circles different background):

Contrast in the white-end is not affected much by this change, in the black-end we now can discern the contrast a little better, but there still are nabt circles where the contrast between is either invisible or weak. Once a gain, evaluate using AdobeGamma.

Photographic images are never that black, this chart puts the vision into very very narrow adaptation but even with this chart the perceptually uniform gamma-space would be maybe 1.6 at maximum.

Contrast sensitivity and lighness perception in one chart

Again using AdobeGamma it is easy to notice that there is no gamma value that makes match well with the perception and anything over about gamma 1.6 will make the highlight coarse and waste codes in the dark end.

The Dreaded Banding Issue

The banding issue is related to the perceptually uniform gamma-space. A steep gamma space (like 1.8 ... 2.5 is said to prevent the banding in the shadows.

It is very easy to show the dreaded banding, the below experiment shows the smoothest gradation in the shadows that can be achieved using linear calibration. The step wedge contains levels 0 to level 8 linear swatch that was then compensated for gamma space 2.2 viewing. Set AdobeGamma to 2.2 for this demonstration.