Digital Light & Color

Perhaps I am not understanding Levels and Color.

I made a minor change to an image (48 bit comprised of only shades of grey).

To better see the change, I Composited the new image with the old, using Absolute Difference. The result was an all black (appearing) image with a tiny spot which was very light grey tone(s).

Again, to better see the change, I then used Levels and Color. The Dialogue showed both sliders at 0.0%. I changed the Dynamic Range to Full (the black remained at 0.0% and the white moved to 100.0%), expecting the grey would be made brighter/whiter, however the image turned all black and the greyish area disappeared. I even tried changing the DR slider manually (which was a bit of a pain in itself as I could not "grab" the white slider without first moving the black slider away from the white, to the left), but I could not get the grey area to brighten and the smallest movement of the white slider turned the image black.

The only way I could think of to enhance/whiten that grey spot was to Composite the Abs Diff image with itself, using the Subtractive Filter, which required an extra, needless(?) step.

Is Levels and Color working correctly when it does this?

Marv

There are several ways to amplify the differences computed by Absolute Difference.

The simplest is to just increase the Strength value in the Composite (or Blend or Compare) transformation.

Levels and Color uses algorithms to locate the white and black points in the input image and sets the dynamic range based in part on these values. While this works well for most photographic images, the result of Absolute Difference with only very minor differences between the images may cause fail to locate a usable white point.

Other ways to exaggerate the differences are:

The Brightness transformation with Preserve set to Black, then increase the Brightness setting.

The Brightness Curve transformation with a curve that starts at the lower left corner and rises steeply