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Linear Processing of RAW Image Files .

Luis A Guevara , Oct 24, 2008; 12:46 p.m.

Some images carry their message in their lower tones and some do it with their midlle tones and highlights, like this one:

This image was shot RAW and processed as a 16 Bit LINEAL RAW in Adobe 
Prophoto Color Space with a Custom Gamma 1.0 profile to properly render the extended amount of upper Tones.

Normal RAW processing spreads the image tones evenly across its Dynamic Range , thus , abnormally ,compressing the highlights and expanding the Shadows , something that will not work well with this type of imagery .

Linear processing workflows , instead ,compress the shadows and devote the rest of the Color Space to Midtones and highlights making it the perfect Complement to this type of imagery ,that results from frontal lighting.

I am inviting all members to discuss this advanced technique and share their experiences with the rest of us.

Responses


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Ellis Vener , Oct 24, 2008; 01:26 p.m.

Choosing a linear response curve is an option in all versions of Adobe Photoshop Lightroom. For some iamges it is useful for making the photo look the way yu want it to look.

Roger Smith , Oct 24, 2008; 04:22 p.m.

That's a very delicately lit image, nice work with it. Can you say more about the workflow you use for "linear processing?"

I use Lightroom and generally use the "linear" setting for curves but I'm not sure if that's doing what you describe.

Luis A Guevara , Oct 24, 2008; 07:31 p.m.

Sure Roger , that is the whole idea of this posting. This image was processed using dcRAW-X which is an alternate RAW converter for the Macintosh , but versions exist for other platforms . It was output as a 16 Bit Linear PSD file . The file was opened in photoshop CS3 and asigned a Custom Adobe Prophoto profile that was only modified by changing its Gamma setting to 1.0.

If you dont use a Custom Profile , since Linear RAW files are not Tagged to any color space , Photoshop , unless instructed to do otherwise, will convert them to the working Color Space , and if none has been selected in the preferences it would default to sRGB , the most restrictive color space. In essence turning the LINEAL raw back to a plain vanilla NON LINEAR RAW file with the sRGB Gamma value of 1.8 , not different than what the camera would have produced .

I use Lightroom and its Linear option, but it does not work out the same . When I opened the image in Photoshop seem to have followed the same process that I just described , since Photoshop obviously knows about Lightroom exotic options . The only difference , if any, is that you dont have to asign a Linear Custom Profile.

When I open dcRAW-X files the situation is very different , the images are dark and the Histogram is concentrated at the lower end , as any Linear image should . When you asign or convert to the Custom Linear Profile of Gamma 1.0 you are not really changing the distribuition of Tones , but giving meaning to its Colors . You will have to do a Manual Tone Mapping (Or Re Mapping if you wish ) to expand the tones that have been concentrated in the Shadow portion of the Histogram .

I will be creating a Tutorial that I will post in my website when ready.

Frans Waterlander , Oct 25, 2008; 12:41 p.m.

Luis,

Most image editing programs have a curve function with linear being the default and tweaking it is easy to do. So what exactly is the point you are trying to make? It seems to me that you are reinventing the wheel.

Luis A Guevara , Oct 25, 2008; 01:54 p.m.

Hi Frans . Those curves that you are refering to are really just describing the behavior of the Midtones of the image, in fact the end points are fixed and your options are limited to defining the Slope of the curve at the Mid Point , which as you described , by default is lineal , meaning no transformation takes place. However the Data that you are trying to work on is already non lineal , since as soon as you open it in a RAW Converter , before it is even separated into our familiar 3 RGB chanels , a Gamma of 0.45 is imposed on it, to counteract the Display Gamma of 2.2 , a very necessary step , since we expect the image to be ,overall ,a lineal representation of the Scene, in spite that the Display is highly unlineal. If you choose a Midtone Curve of Lineal value , then all the intrinsic non linearity of the converted RAW file is passed along untouched


What this means is that our RAW Data that was Linearly captured ,thanks to the painstaking efforts of the Sensors Designers ,is artificially DISTORTED when it hurts the most , right before the Color Channels and the Color Balance are created from it. The RAW file is just a Matrix of Data with no Color Channels , Color Spaces or even Color , for that matter.


So what is LINEAR PROCESSING AGAIN?


Simple. I am sure that you will agree that is simple. Linear processing is removing the introduction of this Inverse Gamma at the point where it hurts the most ,and reintroduce it as one of the last steps , where it hurts the least.


You can do this in the Adobe products Lightroom and ACR , but only if you go deep into the advanced preferences for conversion ,and choose LINEAL , instead of the default embed Original RAW file.

This is very different than the tweakings that you are talking about , since by removing the compression of tones introduced so early in the process you will have more tones to TWEAK .


So the point that I am trying to make is that when Utmost Image Quality matters , linear processing is the way to go. Lineal Image Converters , like dcRAW-X and others , not only remove Gamma but also , Compression , Sharpening ,Denoising , Antialiasing and Interpolation. What you want is that your chanels are made from data that has been touched the least . Ideally ,not touched at all.


All this , Compression , Sharpening ,Denoising , Antialiasing and Interpolation, can be best done taylored to the image type, under the control of the Artist and not by invisible factory presets that only work for Generic ,Average Scenes.

Luis

Frans Waterlander , Oct 25, 2008; 03:23 p.m.

Luis,

Maybe you could start by posting an example of an image processed both ways to demonstrate possible advantages.

Luis A Guevara , Oct 25, 2008; 04:10 p.m.

Good Idea . Here you have it one NON LINEAR RAW and the best one , at the end ,the LINEAR RAW:

IN THIS ONE THE TONES HAVE BEEN SPREAD EVENLY ACROSS THE HISTOGRAM BY THE GAMMA FUNCTION

IN THIS OTHER ONE THE LINEAL RENDERING PUTS THE TONES AS THEY WHERE ON THE ORIGINAL SCENE:


You tell me which one you like best, bost have the same "tweaking" except for the different Gammas

Luis

Tim Lookingbill , Oct 25, 2008; 10:25 p.m.

Luis,

Thanks for explaining such a somewhat complicated subject in a not so complicated manner. Very informative.

What versions of ACR can you get this linear output through its preferences?

And what profile did you assign to the bottom image?

Was it the custom 1.0 gamma ProPhoto RGB which can be made using Photoshop's CustomRGB within Color Settings?

How was color temp/white balance applied?

Did you apply further tweaks to contrast after assigning the 1.0 gamma profile to the bottom image before posting it here?

Very interesting. A much better and more accurate looking rendering.

Roger Smith , Oct 26, 2008; 08:54 p.m.

Would you be willing to post or email the original raw file so I can try it myself and compare?


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