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Author Topic: Photoshop CS2 High Dynamic Range (HDR)  (Read 3330 times)
Concorde-SST
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« on: December 03, 2005, 05:44:19 AM »
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Hello,

I just got my update for Photoshop CS2 for Mac. Great Program!

I was experimenting with the "Merge to HDR" feature. I know
MR described it at his website - but what I donŽt understand
fully is the curves dialog when converting to 16bit mode (HDR
Conversion).

I must admit I always was weak in maths - can someone explain
me how to read the "Toning Curve and Histogram" in the submenu
"Local Adaption"?

I know I can make changes - I can try and see but IŽd like to
understand it more thoroughly. For example, if I select and ad-
just the lower part of the curve - what does that do?

Are there some guidelines (the so-called S-Curve?) how to obtain
a realistic tonal change?

IŽd appreciate any help highly!

happy weekend,

yours
Andreas
Concorde-SST
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jdemott
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« Reply #1 on: December 03, 2005, 12:13:42 PM »
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Hello Andreas,

I have tried a number of experiments with HDR.  You are not alone in finding the process to be difficult.  Even after doing some reading and working on a number of images, I have yet to find an image that gives me better results (or even equivalent results) in HDR than I could obtain with typical blended exposure techniques.  I will be interested to see if you get responses that explain how to get better results.

Here is a summary of what I have learned about the Local Adaptation conversion to 16 bit mode:  

First, the default settings for Radius and Threshold are almost certainly too low.  Try setting the Radius quite high (100-300) and then experiment with the Threshold.  Radius is essentially how big an area is considered "local" when the program evaluates brightness.  

The Toning Curve and Histogram dialog essentially is a typical Photoshop Curves dialog superimposed over a histogram that shows the frequency distribution of pixels at each level of brightness, with the darkest at the left and the brightest at the right.  The red marks at the bottom of the scale each represent one EV (approx. one stop), so you can see there is a wide range possible. In a typical case, all the pixels may occupy only a small portion of the possible range.   A reasonable starting point for the Curve adjustment would be to move the bottom end of the curve to the right so that it corresponds with the darkest pixel shown in the histogram, and move the upper end of the curve to the left so that it corresponds with the brightest pixels in the histogram.  That will give a steeply sloped, straight line extending from top to bottom.   Those end points will become the darkest and brightest levels respectively (0 and 255) when the conversion is made.  You can fine tune the curve  by adjusting the straight line to an S or other curved shape.  The general rule of thumb is to make the steepest part of the curve correspond with the region of the most important detail in the photo.  If you click in the preview image while the dialog is open, the Curve display will show you where that particular pixel falls along the curve so you can judge where the steepest part of the curve should fall.
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John DeMott
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« Reply #2 on: December 05, 2005, 09:25:49 AM »
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Hi John,

thank you very much for your lines! I tried again and
found some more insight into it. It really helped. None-
theless I found out that there is no standard routine
for this sort of editing.

Every picture is just unique! So I guess I should keep
practising!

happy week!

Andreas.
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Grys
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« Reply #3 on: December 26, 2005, 04:41:18 AM »
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If you are interested in HDRI, I would recomend you also to try out the application HDR-Shop ( http://gl.ict.usc.edu/HDRShop/ ) which is free for personal use and is still the most advanced tool for HDRI-editing.
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