What ACR/Photoshop controls really do to the image levels

[Guillermo Luijk]

This afternoon I reverse engineered some ACR tools to find out what the Exposure, Brightness, Contrast and Shadows controls really do to the image. I had a feeling they were just curves applied to the image data, without any RGB differentiation. Representing them in terms of curves is very clarifying:


ACR (v3.7)

- Exposure (exposición): is an expected scaling by an integer multiplier, with a particular behaviour when the adjustment is done to reduce exposure since it keeps the blown areas white. When increasing exposure it just blows information away (Note this was the only curve calculated in gamma 1.0 to properly see what was going on in the preserved highlights).
- Brightness (brillo): is some gamma-like curve, being linear in the low end (i.e. the same as an exposure adjustment), but ending in (255,255) so it will always preserve the highlights.
- Contrast (contraste): a very typical 'S' curve crossing the middle point (128,128).
- Shades (sombras): just a black point clipping curve.



I did all the calculations exporting to Adobe RGB images, but still it is noticeably that ACR did something different to expected in the deepest shadows. I wonder if this was intended to improve the result or it's just because the gamma curve in ACR is always sRGB like instead of a pure 2.2 gamma curve (I read that LR plots histograms in the ProPhoto profile, but uses a sRGB gamma curve; perhaps ACR does the same regarding the gamma curve).


COMPARING ACR (v3.7) TO PHOTOSHOP (CS2)

Photoshop's Exposure control performs the same as ACR's except that it does not preserve the blown areas when reducing exposure. I found that the parameter number you set is not exactly the amount of EV applied but a different scale.

Exposure +2



Photoshop's Brightness and Contrast are totally different to ACR's, they are curves that can easily clip information so should be avoided unless we know very well what we are doing:

Bright +40  and  Contrast +40
 .  



The Levels tool is not found in ACR and corresponds to a black and white clipping points, plus a medium control to increase/reduce levels in the inner range:

Levels adjustment tool




BR

[Alistair]

This afternoon I reverse engineered some ACR tools to find out what the Exposure, Brightness, Contrast and Shadows controls really do to the image.
BR

What version of ACR and PS were you analysing. I had a the impression PS brightness and contrast controls were improved in CS3.

[Guillermo Luijk]

What version of ACR and PS were you analysing. I had a the impression PS brightness and contrast controls were improved in CS3.

CS2 and ACR v3.7

[Peter_DL]

I did all the calculations exporting to Adobe RGB images, but still it is noticeably that ACR did something different to expected in the deepest shadows. I wonder if this was intended to improve the result or it's just because the gamma curve in ACR is always sRGB like instead of a pure 2.2 gamma curve (I read that LR plots histograms in the ProPhoto profile, but uses a sRGB gamma curve; perhaps ACR does the same regarding the gamma curve).

Maybe it’s the slope limiting feature of the color engine (see last page).

Peter
--

[Hening Bettermann]

Hi Guillermo,

thank you for sharing this very useful information!

[Lisa Nikodym]

I've often wondered myself what the various PS controls did in terms of curves, and bemoaned the fact that reference books never seem to include it.  Thanks very much for telling us about your findings!

Lisa

[madmanchan]

Ultimately, Camera Raw has a single tone curve. It is fundamentally different from PS in that PS's curve operates on RGB channels independently. CR's tone curve uses a hue-preserving method (to minimize visual color shifts) in HSV space. Brightness, Contrast, Point Curve, and Parametric Curve ultimately all affect this single tone curve. Exposure & Blacks (referred to above as shades) are NOT part of this tone curve; they are simply white and black clipping points and are applied much earlier in the image processing chain. Keep this in mind as technical info only, not necessarily practical advice.

The practical advice is to work your way through the controls in ACR roughly from top to bottom starting with the Basic tab, going from the coarse adjustments to the fine adjustments. e.g., Brightness and Contrast are much coarser than Point and Parametric curves. Generally you can get 90% of the work done with the former and do the last 10% with the latter set.

[evogel99]

Ultimately, Camera Raw has a single tone curve. It is fundamentally different from PS in that PS's curve operates on RGB channels independently. CR's tone curve uses a hue-preserving method (to minimize visual color shifts) in HSV space. Brightness, Contrast, Point Curve, and Parametric Curve ultimately all affect this single tone curve. Exposure & Blacks (referred to above as shades) are NOT part of this tone curve; they are simply white and black clipping points and are applied much earlier in the image processing chain. Keep this in mind as technical info only, not necessarily practical advice.

Eric - this is truly wonderful to know, but where do you find this kind of info on internals? I'd love to see documents on Photoshop itself. I have seen a number of people spend a lot of time trying to reverse engineer this stuff so they can understand what is really going on -- and explain some weird effects that turn up. Some architecture notes would great - has Adobe made any of this available?

Thanks

Eric Vogel

[francois]

Eric - this is truly wonderful to know, but where do you find this kind of info on internals?
…

Eric works for Adobe...  

[Hening Bettermann]

[quote name='evogel99' date='Jan 21 2009, 11:44 AM' post='253595']
 Some architecture notes would great -

Or better still - wouldn't it save all of us a lot of time and frustration, if The Profile Editor/ACR/PS *displayed* this one curve at all times? It makes my head hurt to keep track of how these curves interfere with each other across apps.

[evogel99]

Eric works for Adobe...  

Ah, well then. I see.

If so, let me put it this way, I would appreciate it if Adobe would see fit to publish internals. Or at least publish functional definitions of the controls that define what they really do.

Having been a developer, I know that won't happen. After all, that is "secret sauce" and keeps everyone on their toes. So, never mind.

Although, if they want to sell it for forensic/scientific purposes, I would think they should include internals documentation??? Or maybe "extended" has same? That might make it worth it.

Thanks Francois.

[jdemott]

Or better still - wouldn't it save all of us a lot of time and frustration, if The Profile Editor/ACR/PS *displayed* this one curve at all times? It makes my head hurt to keep track of how these curves interfere with each other across apps.

It seems to me that Adobe has done us a favor by providing an interface that displays the histogram in terms of the familiar RGB values seen in Photoshop.  As a user, my primary concern is what the tone curves will look like when the raw conversion is done and the image is opened in PS.  I assume that it required some effort by Adobe to provide that sort of interface while operating mathematically on linear data that is not in separate color channels. However, I also appreciate the info from Eric.