Wednesday, July 09, 2014

AutoTonal Fixes & Jpeg

M
ost packages these days offer an autofix solution for “correcting” the tonal range in an image, which normally means stretching the range by increasing the contrast and manoeuvring the exposure/brightness. They mostly work, underexposed images will be lightened and overexposure images will be darkened, and some extra detail will appear. However these processes/filters can come spectacularly unstuck when the exposure is heavily clipped (clipping is an adobe-centric term to described when the exposure intensity falls below the minimum recordable, ie black or above the maximum recordable ie white). This clipping will be visible as vertical lines on the right or left hand edges of the intensity histogram. The image I’m using for this series is clipped a little on the black side, but not that significantly.
IMGP3368 original IMGP3368_auto_levels IMGP3368 pefectly clear
aftershot jpeg as shot histo aftershot jpeg auto_levels histo aftershot jpeg perfectly clear histo

Original Jpeg (as Shot)

After Auto Levels

Perfectly Clear

If you look at the exposure intensity histograms under each image you see what has been done by each step. The original jpeg image does not capture the full range of exposures, they are grouped to the left og the histogram. The auto level fix has stretched the available pixels to fill the histogram. Because there is less detail in the information to begin with, when this is stretched across the full range there will be some new intensities that get missed, the new locations for originally adjacent exposure in a nice gradient of light will now be two or more illumination values apart, leaving a little gap, which can produce stripping in the nice gradient, and sometimes this becomes very obvious and distracting. The perfect clear filter has done a similar stretch but has less mini peaks.
All on these images still have a very dark shadow in the foreground and probably a bit more post processing could be justified.
AfterShot Pro’s Perfectly Clear filter, is a bit of a secret sauce, but I find I prefer using it to any other AutoFix solution because it gives me a better idea of a what an image is capable of. The ones I want to rank as worthy of further work in the future. On occasion I need a jpeg version to post on the web straight away, and perfectly clear probably does the best job in most situations, with just a single click. (Perfectly Clear also works with RAW images).
Auto Enhance options in Google+ SettingsGoogle+ photos has a featured called Auto Enhance which is turned on and set to normal by default. It works well most of the time, but if you think your images is just to contrasty, or looks posterized you can turn it off or change it individually on the photo page under edit, or change the default setting under the google+ settings. If you are only uploading jpeg photos, from your phone the normal setting will probably do a good job 90% plus of the time.
So be careful with Autofix as a solution to extend your dynamic range. There are better solutions to come in this series but it is well worth having a play with these settings/filters in your package of choice.


Cambridge in colour has a good introduction to Understanding Image Histograms
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