Benefits
of the plugin

Repair a posterized histogram.

As the name says, this plug-in is for repairing damage. It should be used as soon as posterization appears, if much further editing is intended, in order to avoid further posterization. There is no point in doing histogram repair as a final operation since histogram repairing will not have any visual effect on the image. Histogram repairing is a way to prevent possible degradation due to much further editing.

The Histogram Repair plugin works with all image modes (Windows and Mac)...
8 & 16 bit / channel: All image modes: RGB, CMYK, Lab, Duotone, Grayscale, HSB, HSL

Histogram Repair
controls

The Histogram Repair plugin has no control panel.

It's effect is applied instantaneously and it is a very fast plugin.

Example - posterized image resulting from converting 8 bit to 16 bit. Fixed with the plugin

The Histogram Repair plugin is for repairing a posterized histogram.

The left image shows before applying the filter. The right image shows the result of filtering.

We developed means to avoid posterization during image-editing with the other Power Retouche plug-ins. Once that principle was in place it was natural to release an individual plug-in that did something similar as a repair of the damage, that might be created by editing images with other means than Power Retouche. It is of course best to avoid posterization within the image-editing algorithms, rather than to fix posterization after the editing has created it. So this plug-in is actually not intended to be used in conjunction with Power Retouche, because most Power Retouche plug-ins have anti-posterization built-in as an option. It is provided as a possibility to fix damage that other software has done. Also the Histogram Repair plug-in provides a means to turn an 8 bit image converted to 16 bit, into a true 16 bit image (more about this below).


Posterization is usually not visible since it in most cases only spans a single value. But further editing of a posterized image can widen the posterization gaps so much that the image appears as if divided into bands. When repairing an image where the posterization gaps are only a single value wide, you will not be able to see any difference, and this is how it should be.

So since the effect of repairing a mildly posterized histogram is not visible, there is no point in repairing it unless you intend to do further editing. In that sense this plug-in is far from essential, but as mentioned above, it was natural to release an individual plug-in that could repair posterization, once we had figured out how to avoid posterization in the first place.

Examples - 8 bit images

Original Photo

This picture is underexposed. As you can see from it's histogram, all the values are in the lower left side. The simplest way to improve it is to stretch the distribution of values to fill the entire range; this can be done with level adjustment in Photoshop.

Levels Adjusted

After stretching the histogram by adjusting levels in Photoshop we get this histogram. Notice the vertical lines indicating, that there are a lot of missing values. This is because the original only has about 140 levels to begin with and when they get redistributed over 256, then the gaps are inevitable. Photoshops level adjustments do not interpolate the missing values like the Power Retouche plug-ins allow you to do. Such a histogram is called posterized. Power Retouche Histogram Repair offers a solution for that.

Histogram Repair

After running Histogram Repair we get this histogram. As you can see, the empty bars are gone because the plugin has interpolated the msising value. The original image does of course not change in any noticable way.

Examples - 16 bit images

Do 16 bit images need histogram repairing?

Do 16 bit images need histogram repairing?

No, not if the image was originally photographed or scanned at 16 bit, but if it was made into 16 bits by converting an 8 bit image, then it most certainly does. This is a special problem with converting 8 to 16 bit, which few are aware of.

Problem reading histograms for 16 bit images

When Photoshop displays the 16 bit image's histogram, Photoshop converts the image to 8 bit, so it looks exactly like the 8 bit images nonposterized histogram. But this is not true! Since the original only had 256 levels and a 16 bit image has 32768, converting an 8 bit image to 16 bit will produce posterization gaps each of 128 levels. These huge gaps will not show in the Photoshop histogram, but will be clearly visible in the Power Retouche histogram..

Fixing Conversion from 8 to 16 bit

Original Photo
8 bit

Here's an 8 bit image and its histogram

Photoshop histogram

Converted
to 16 bit

We convert from 8 bits to 16 bits and check the histogram. In Photoshop it looks just like the smooth 8 bit images histogram shown above and this might trick you to beleive it is not posterized. But it is really peppered with gaps 128 levels wide and is in effect still an 8 bit image since it only contains 256 levels.

Histogram Repair

If you use Histogram Repair just after converting from 8 to 16 bit, you will get a true 16 bit image with all levels.

After Histogram repair