Ciderhouse is unavailable for a month since S and I are going to Barcelona and then into southern France (Arles, Nimes, Nice, Aix de Provence, etc.). We leave tomorrow and will be back on April 19. I'd love to see fans of Squinchpix or Ciderhouse on the way and we'll be twittering in the hope of making contact. Best to all of you until we come back and, in the meantime, you might like to look at some blog entries that I think might be helpful to you.
Real HDR vs. Pseudo HDR
FDRTools and the Compressor Algorithm
Dynamic Photo HDR and Photomatix: Color Compare
Why Photomatix is a poor choice
Dynamic Photo HDR and the Halo-Matix Algorithm
Doing HDR in pieces
A Simple HDR Workshop
Single Pixel shift in HDR alignment. What does it mean?
Noise in Single Image HDR. A Workshop.
Manipulating L-channel in Lab with HDR
Until we get back ...
Tuesday, March 16, 2010
Tuesday, March 2, 2010
Lars Roglin writes
One of my readers, Mr. Lars Roglin, of (I think) the Netherlands (see his gallery here) writes the following in response to one of my old posts:
Hi Bob,
interesting calculations, but I think you have one major flaw in your assumptions. You state: "Let’s say that the time to make all these exposures is 10 seconds."
With modern DSLRs this might be the case for long exposures and night shots but e.g. my EOS 40D is able to shoot at ~6 fps (the EOS 1D Mark IV even at ~12 fps) at full speed. So your 10 seconds change to .5 seconds for a series of -2EV,0EV,+2EV (0.25 or 1/4s respectively)!
In addition, I would assume that for most typical landscape shots clouds might be further away than a mile, so proper alignment should avoid stuttering.
If you want to push it to the limit, you could perhaps limit yourself to 2 exposures, which would push it to 1/8s which should even allow portrait shots (although perhaps with a higher reject rate - but shutter speeds of 1/8s and even 1/4s is not unusual in available light photography!)
Curious what you think about that.
Cheers,
Lars
My answer is as follows:
Hi Bob,
interesting calculations, but I think you have one major flaw in your assumptions. You state: "Let’s say that the time to make all these exposures is 10 seconds."
With modern DSLRs this might be the case for long exposures and night shots but e.g. my EOS 40D is able to shoot at ~6 fps (the EOS 1D Mark IV even at ~12 fps) at full speed. So your 10 seconds change to .5 seconds for a series of -2EV,0EV,+2EV (0.25 or 1/4s respectively)!
In addition, I would assume that for most typical landscape shots clouds might be further away than a mile, so proper alignment should avoid stuttering.
If you want to push it to the limit, you could perhaps limit yourself to 2 exposures, which would push it to 1/8s which should even allow portrait shots (although perhaps with a higher reject rate - but shutter speeds of 1/8s and even 1/4s is not unusual in available light photography!)
Curious what you think about that.
Cheers,
Lars
My answer is as follows:
Hi Lars,
Whatever you're doing your gallery shows that it's successful. I did use a large number for the time it would take to make multiple exposures. Great cameras like yours allow for much less time to accomplish the same number of exposures so the problems I outlined would be minimized for you.
Two points:
1. I shouldn't allow myself to be drawn into the 'average cloud thing' but I can't resist. So, in the spirit of fun, let's say that the average cloud height is about 2000 feet. And let's say that the angle to the nearest cloud in the landscape photo that you're taking is 45 degrees. Simple trig tells us that the hypotenuse of the resulting triangle (the hypotenuse is the direct distance from the camera to the nearest cloud as defined above) is 2828.4 feet. If that cloud is moving faster than about 1.92 miles per hour (in parallel with the sensor plane) then that's a displacement of 1 pixel in half a second. (I'll leave the math to you, Lars). It's not unusual for a cloud to travel about 20 mph and those conditions would translate into a 10 pixel offset even in half a second. These numbers are full of assumptions and I'm not trying to prove that you will necessarily have a problem with your equipment. But the fact is that we see a lot of cloud displacement (and foliage displacement) in multiple image HDR. It must be caused by something (perhaps it's not coming from Canons?) and the math (even highly speculative math like this) suggests what it is. I'm just not personally optimistic about such a technique but that means nothing if you can make it work.
2. I want to approach this from a different angle. Let's say that you wrote me a letter and said "Bob, I invented this great new way to shoot photographs! Instead of using f8 at 250th of a second for normal lighting, I've pioneered what I call the 'slow shutter'(tm) technique. The benefits of a small f-stop are so obvious to me that I always shoot at f78 at one/half second even, I repeat, in normal lighting conditions." And then you would carry on, perhaps, about the incredible depth-of-field and that everyone should always shoot at speeds of 0.5 seconds or even longer. Well, if I received (or if you received) such a letter we would suspect that our correspondent was cracked. But that's what you seem to actually be advocating.
However, I know that the f64 school of American photographers (Weston and Adams and the like) actually did shoot this way so maybe I need to go back and review what they actually did and what problems they encountered. Perhaps you would be interested enough to look into it and write back about what you find? Thanks for your letter.
Best,
Bob
Monday, March 1, 2010
Real HDR vs. Pseudo HDR
There's a lot of discussion on the web about the difference between 'Real' HDR and something called 'Pseudo' HDR. You know how it goes; 'Real' HDR is made with more than one image and 'Pseudo' HDR is made with only one image. To all of which I say 'Balls!' If you use HDR tone-mapping algorithms to get everything you can out of a single image then it's as real as HDR can be. In fact, not a lot rides on the outcome of this discussion (it's a variation of the Essentialist fallacy) but I had an experience recently that made me think again about what 'pseudo' HDR might really mean.
I was on Crete last year and, at the palace of Knossos, I took about 600 shots and all of them were more or less defective. It is the Mediterranean after all and so all the shots were either over-exposed or under-exposed but usually both. And yet, while looking at them, I felt that all the detail was really there although pushed to the edges. If only I could find a way to retrieve the detail on both ends of the brightness range. I process my pictures in stages; the first stage employs Lightroom 2.6. So the problem was to find a method of working in Lightroom that would take the rough edges off. Here, for example, is a shot of a wall and a buttress in Knossos that will illustrate a typical starting point.
In this shot the darks are not too bad but what to do about the highlights?
The first thing I did was set the 'Recover' slider to 100. Here's the result:
'Recover' moves brightnesses on the extreme bright end back towards the middle. In other words the effect is to manipulate one end of the histogram without modifying the other .. or at least not very much. By doing this we can see that there is some detail in the brightest stones on the right. They're not completely burnt out. The next step is to turn the brightnesses all the way to the bottom. Here we go:
By doing this we've regained all the detail in the highlights that we're likely to get. In the next step we turn up the fill slider as much as possible consistent with the current state of the brights. Here it is:
Now we have a pretty dim image but all the brightnesses are in agreement with each other. The shadows are suitably dark with respect to the brights. There's full detail in the darks AND full detail in the lights. We're light-years away from where we started. Now we're going to turn up the brightness gradually until the brightest areas of the photo are just under where they should be. Here we go:
Now the steps that I've outlined typically degrade the contrast. The fill slider and the contrast slider are really opposites. The contrast has to be fixed particularly in the lights. I do that in parts. I begin by moving the 'Black' slider up to about 15 (way too high for an ordinary photo). Here's the result:
Before we fix the rest of the Contrast problem I'm going to correct the color cast. If I don't fix the blue cast in the shadows now then changing the contrast will only worsen it. In Lightroom I just decided to use the 'Saturation' slider and I pulled the cursor downward in the shadows on the tops of the rocks on the left. Here's the result:
I present the whole Lightroom screen so you can see what the effect was of the Saturation change. The shadows are now monochromatic (pretty much) and upping the Contrast in the next step shouldn't worsen that. Now it's time to fix the contrast. By moving the Contrast or the Clarity sliders (or both) to the right we push the brights up and the shadows down so the trick in doing this is to increase the contrast without finally burning out the lights and pushing the shadows down so that you're back where you started from.
Here's my final version.
Now this photograph will clearly never interest very many people. Even now it's not really successful but it could be of some small interest to an archaeologist whose specialty happens to be this particular wall. It could use a lot more work. The local operator brush could probably be employed to good effect in the highlights. But what really interested me after doing this more than 550 times was how much like HDR the results of this kind of processing actually look. The final example looks like HDR (at least the shadowed areas do) and next I present another example of before and after shots in which the kind of processing outlined above gave the result an 'HDR' look.
Here's a 'before' shot of Knossos' famous colonnade as it came from the camera (complete with sensor dirt).
Here's the 'after' shot in which I used an aggressive 'fill' as described above.
And also this example. Here's the before picture.
The same picture after processing as I described. If I told you that this 'after' picture was HDR you'd probably have believed me. It just looks like HDR.
The following photo actually was processed with HDR software (FDRTools). The non-HDR version actually looks more like HDR (or our common expectation of it) than the one that actually is HDR.
HDR is always described as having a 'unique look'. But what is that look? Lots of people suppose that it has something to do with aggressive saturation of the image. Sometimes we're told that it makes photos look like paintings although the people saying this have no idea what makes a painting look like a painting. So let's answer that question first. What does make a painting look like a painting? Simple. A reduced tonal range. For technical reasons most painters never, and have never, employed pure blacks or whites. So that giving up those extreme tones right away reduces the tonal range. Also painters do not paint to obscure. They will never (or rarely) paint shadows as dark as they are in real life. In fact one of the chief goals of the painter is not only to paint clearly in the shadows but to paint full color in the shadows. Therefore any picture that appears to have a reduced tonal range will look painterly. (As in everything there are many exceptions. There are certain chiaroscuro painters who DO employ pure blacks and whites and virtually nothing in between. Such paintings gain in drama but relatively few painters actually paint this way.)
In photography HDR algorithms produce a reduced tonal range; in many cases a shorter range than can actually be handled by the output media. The resulting images feature weakened shadows; shadows no darker than if one had simply poured water on the ground. Doing this darkens the ground but does not obscure it and many shadows in HDR look this way. Also many HDR shots seem to show us a perpetual dawn.
So the moral of the story is that if you want 'pseudo' HDR pump up the fill slider.
Tuesday, February 2, 2010
FDRTools and the Compressor Algorithm: An approach
In one of the recent posts I promised to explore FDRTools' Compressor Algorithm. The 'Compressor' is, according to FDRTools, a local toning operator. That means that it covers the surface point by point and it tries to adjust each pixel according to the brightnesses of neighboring pixels within a certain radius. The FDRT manual says this "The Compressor algorithm works as follows: the intensity of each pixel is regulated individually and depends on the intensities of neighbouring pixels. If neighbouring pixels differ strongly in their respective intensity, e.g. in the surroundings of a strong light source, then these differences are strongly attenuated. Where the differences are small, e.g. in shadowy areas, there is little or no attenuation, evtl. the difference is even increased. As a result the dynamic range of the HDR image is strongly reduced while local tonal value differences are preserved. Local contrast and thus the perceptibility of details are considerably improved compared to global tone mapping methods like Simplex or Receptor."
The authors of this manual are, I think, Germans and I'd like to rephrase this as follows: "Compressor examines each individual pixel and changes it depending on what it finds in neighboring pixels (within a certain radius). If the immediate neighborhood has a big dynamic range then it is reduced. If the neighborhood has a relatively narrow dynamic range then it is increased. The result is that the dynamic range of the entire image is significantly reduced but in such a way that local relative differences are preserved and local contrast is even improved relative to Simplex or Receptor."
Good enough. But is it true and how do we use it? Let's concentrate on a small number of details just at first. We start with the gross image of the column drum that I used last time. Here it is:
Yow.
What we see with defaults is this:
Now this is another low-contrast image. All the brightnesses are squeezed into the center of the histogram as we see. We know that we need to draw in the left and right sides (black point and white point) to more nearly enclose the actual values. The result is this:
You may have to experiment but, at least in this case, the right side (RHS) of the curve had to be steepened so that there would be more output grays in a very small horizontal brightness distance. That's a very clumsy way of putting it but it's the best way that I can think to explain it. Let's pretend that the brights are o.k. and examine the darks. In this particular case I really couldn't find any better setting for the darks (the gravel) and so I left it alone. There are several more important controls and let's look at the Compressor slider. This slider, according to the tool tips, regulates the strength of tonal length compression. I confess that I don't know what that means in practical terms so we're going to have to approach that empirically. When I moved it all the way to the left (0.0) the histogram shrunk symmetrically around its center. When I moved it all the way to 10.0 the histogram expanded around the center. The effect was, at 0.0, to lower contrast, at 10.0 to heighten contrast. The tool tips say that you should use values from 0.0 to 5.0 for a single raw (which this is) and 5.0 to 10.0 for multiple images. I found that this all looked best when I moved it up to 8.0 or even 10.0. Here it is:
HDR gets details out of burned-out or from under-exposed areas. Once you get that it's always in low contrast. The whole problem of HDR, as I'm sure you're aware, is to increase the contrast (that is, to increase the range of grays) on the result of HDR processing without going all the way back to the original picture. That was our challenge with the simplex method. But FDRTools Compressor seems to do the trick. What do you think? Let me know in the comments section if I've grossly mishandled this or if you have better ideas. I'm going to finish up by processing this result and then post-processing in Photoshop. I gave it a lick with Curves to increase the contrast, sharpened the L layer, and here's the final result:
The authors of this manual are, I think, Germans and I'd like to rephrase this as follows: "Compressor examines each individual pixel and changes it depending on what it finds in neighboring pixels (within a certain radius). If the immediate neighborhood has a big dynamic range then it is reduced. If the neighborhood has a relatively narrow dynamic range then it is increased. The result is that the dynamic range of the entire image is significantly reduced but in such a way that local relative differences are preserved and local contrast is even improved relative to Simplex or Receptor."
Good enough. But is it true and how do we use it? Let's concentrate on a small number of details just at first. We start with the gross image of the column drum that I used last time. Here it is:
Yow.
What we see with defaults is this:
Now this is another low-contrast image. All the brightnesses are squeezed into the center of the histogram as we see. We know that we need to draw in the left and right sides (black point and white point) to more nearly enclose the actual values. The result is this:
Now it's no longer grossly low-contrast. In fact the gravel looks pretty good. But the brights have lost all the definition even of the default settings and somehow that needs to be restored. Another way of saying this is that the brights are mapped into too narrow a range and to spread out that range you steepen the curve on the bright/right side, like this:
You may have to experiment but, at least in this case, the right side (RHS) of the curve had to be steepened so that there would be more output grays in a very small horizontal brightness distance. That's a very clumsy way of putting it but it's the best way that I can think to explain it. Let's pretend that the brights are o.k. and examine the darks. In this particular case I really couldn't find any better setting for the darks (the gravel) and so I left it alone. There are several more important controls and let's look at the Compressor slider. This slider, according to the tool tips, regulates the strength of tonal length compression. I confess that I don't know what that means in practical terms so we're going to have to approach that empirically. When I moved it all the way to the left (0.0) the histogram shrunk symmetrically around its center. When I moved it all the way to 10.0 the histogram expanded around the center. The effect was, at 0.0, to lower contrast, at 10.0 to heighten contrast. The tool tips say that you should use values from 0.0 to 5.0 for a single raw (which this is) and 5.0 to 10.0 for multiple images. I found that this all looked best when I moved it up to 8.0 or even 10.0. Here it is:
It'll never be a great picture but it's a perfectly good illustration of a column drum. If I say so myself there's nothing at all wrong now. It's a clear and adequate picture with a full and reasonably distributed range of grays. Perfect for squinchpix.com. The sunlit wedge in the right foreground is a potential source of trouble (just after the top of the drum itself) but here it gave no problem.
Also the purple grid marks are what FDRTools puts over pictures when you don't have a license.
So:
1. bring in the curve ends so that they enclose the histogram.
2. Evaluate the gray range in the brights. If it's too narrow (which it was) then increase the range there by steepening the right end of the curve. Steeper means more grays. Shallower means fewer grays.
3. Evaluate the gray range in the darks and increase the gray range if you need to by making the left end of the curve steeper.
4. Experiment with an optimal setting of the 'Compressor' slider. By this stage you should have an reasonably distributed tonal range of grays. That's your real goal for HDR processing. If you get to that point you're successful.
5. Finish up in Photoshop to get complete control over the result in the way you're used to. You can adjust contrast, correct color casts, etc.
I'm mindful that color is a distraction and what I'd like to do next is to use FDRTools on just the L-layer of a Lab version of this picture. I've done things like this in the past and it's time to do that again. In the next day or two.
And there's something that I now realize that I assumed throughout but never actually stated:
With HDR Always think in terms of brightness - Never think in terms of color. HDR is not an idea about color. It's an idea about brightness. Always, always, always!!!
Till next time...
Also the purple grid marks are what FDRTools puts over pictures when you don't have a license.
So:
1. bring in the curve ends so that they enclose the histogram.
2. Evaluate the gray range in the brights. If it's too narrow (which it was) then increase the range there by steepening the right end of the curve. Steeper means more grays. Shallower means fewer grays.
3. Evaluate the gray range in the darks and increase the gray range if you need to by making the left end of the curve steeper.
4. Experiment with an optimal setting of the 'Compressor' slider. By this stage you should have an reasonably distributed tonal range of grays. That's your real goal for HDR processing. If you get to that point you're successful.
5. Finish up in Photoshop to get complete control over the result in the way you're used to. You can adjust contrast, correct color casts, etc.
I'm mindful that color is a distraction and what I'd like to do next is to use FDRTools on just the L-layer of a Lab version of this picture. I've done things like this in the past and it's time to do that again. In the next day or two.
And there's something that I now realize that I assumed throughout but never actually stated:
With HDR Always think in terms of brightness - Never think in terms of color. HDR is not an idea about color. It's an idea about brightness. Always, always, always!!!
Till next time...
Bob
Saturday, January 30, 2010
HDRCreme shot of the day: A Review
When listing the areas in which multiple-image HDR has a competency I did leave one out: the still life. Nowhere is that borne out better than in the image which graces HDRCreme's front page. See it here. It is a photo of an old spinning wheel placed against a background of colored wool swatches and hand-embroidered fabrics. The photographer hasn't given us any details about it and I'm assuming that it originated with multiple images. It is very low-key and even-contrast. When I downloaded the largest version and ran it through Photoshop it produced this histogram.
The spike on the right is the over-exposed swatch of wool. Take a good look at this histogram. This smooth and even shape, just barely bell-shaped, is the mark of many HDR photos that I've seen. In fact when we see a histogram as smooth as this one we can be pretty sure that the photo which produced it was created with HDR. I wish I could pin this specific smooth histogram to a specific HDR tone-mapper but I can't yet. Perhaps my readers know more than I do about this? I may devote entries in the future to relating histograms to specific tone mappers.
The creator of this photo identifies him (her?) self as 'Akun2500' and there's a small gallery of his work on HDRCreme. You can ignore the motorcycles which are beginning efforts but the others are splendid. Sighisoara is a popular tourist destination in Romania which is where 'Akun2500' lives. Clearly people go there (among other things it is the home of Vlad the Impaler - the original Dracula) to look at the antiques. The delicacy of this rendering, the absence of any visible HDR artifacts, the over-all violet tonality lend it an inexpressible charm and it is a strong advertisement for multiple-image HDR (if that's in fact what it is) - at least under controlled conditions. Vermeer would have been proud to do this one.
Over and over I've tried to make the point that HDR does away with the subject and creates a fully articulated surface over which the eye may wander with pleasure. This picture is what I mean. The spinning wheel is deemphasized by the calm and even light, a light which (I'll bet) is purely imaginary - the product of HDR alone; it (the wheel) fades into the background. The eye takes in the sublime needlework,the old wood, the textures on the walls and the wool. It is a decorated surface and nothing more. This picture restores pleasure to the act of looking at the surface of a photograph. We learn nothing - we enjoy everything.
I won't give it a '12 out of 10' but I can hardly see how it could be improved.
Bob
The spike on the right is the over-exposed swatch of wool. Take a good look at this histogram. This smooth and even shape, just barely bell-shaped, is the mark of many HDR photos that I've seen. In fact when we see a histogram as smooth as this one we can be pretty sure that the photo which produced it was created with HDR. I wish I could pin this specific smooth histogram to a specific HDR tone-mapper but I can't yet. Perhaps my readers know more than I do about this? I may devote entries in the future to relating histograms to specific tone mappers.
The creator of this photo identifies him (her?) self as 'Akun2500' and there's a small gallery of his work on HDRCreme. You can ignore the motorcycles which are beginning efforts but the others are splendid. Sighisoara is a popular tourist destination in Romania which is where 'Akun2500' lives. Clearly people go there (among other things it is the home of Vlad the Impaler - the original Dracula) to look at the antiques. The delicacy of this rendering, the absence of any visible HDR artifacts, the over-all violet tonality lend it an inexpressible charm and it is a strong advertisement for multiple-image HDR (if that's in fact what it is) - at least under controlled conditions. Vermeer would have been proud to do this one.
Over and over I've tried to make the point that HDR does away with the subject and creates a fully articulated surface over which the eye may wander with pleasure. This picture is what I mean. The spinning wheel is deemphasized by the calm and even light, a light which (I'll bet) is purely imaginary - the product of HDR alone; it (the wheel) fades into the background. The eye takes in the sublime needlework,the old wood, the textures on the walls and the wool. It is a decorated surface and nothing more. This picture restores pleasure to the act of looking at the surface of a photograph. We learn nothing - we enjoy everything.
I won't give it a '12 out of 10' but I can hardly see how it could be improved.
Bob
FDR Tools. The Simplex Algorithm
I've been intrigued by the performance of FDRTools and I put up a small post two days ago to show how it handled the color sheet. Now I want to consider the Simplex algorithm which, according to FDR, is a simple tone-mapping operator. Let's start with a black and white test sheet and see how it performs. Straight out of the box with default settings it produces this:
This has good separation in the blacks and not so good separation in the whites. No problems with the lines or their intersections and - until it reaches the brights - realistic rendition of the gradient with some banding.
I looked for a simple manipulation of the contrast curve that would improve things and so I steepened the slope on the left side hoping to increase separation among blacks. That worked pretty well so I did the same thing on the light side. That resulted, of course, in the classic low-contrast reverse S-curve and, well, see what it did:
This simple change produced low contrast (see previous) but a huge improvement overall in terms of rendition and separation. Now that I felt that I was on the right track I tried to optimize it:
I brought in the outer bounds so that they more tightly bounded the observable values on the histogram and I steepened the sides of the curves in the darkest and brightest areas. I also brought the middle of the curve way down so that it produced the darker band in the center of the gradient - in so doing it helped to intensify the separation in the lights (you probably couldn't do this with a real photo). I think the result looks really good but, at least, by knowing that we're searching for some reverse S-curve to improve the scene we have a clue as to how to proceed.
Now keep this reverse S-curve in mind because the next thing I wanted to do was to see if I could apply just this one thing to a real-life scene. I chose the following awful picture that I took in Argos, in Greece.
Here's a column-drum lying on the ground. The lights are on the edge of being burnt-out but everything else, almost, is in inky blackness. I'm going to use FDR tools on this and just take the default settings first. What does Simplex do to this without my doing anything?
Lots of detail here and overall very low contrast. Here's something to work with. Let's apply the reverse S-curve and see where we are but, it's clear, we have to move in the upper and lower curve bounds on the histogram itself. Here are the settings that I found and the resulting image.
I found that it's not just a matter of applying any old reverse S curve. You have to experiment with the shape on each end until it looks as good as possible. I moved in the upper and lower bounds (do that first) on the curve until they just touched the values on the highest and lowest end and that makes a crucial difference. So the thought for today is, when using FDR tools, the Simplex processor will do a reasonable job and very quickly. Once you've applied it inspect the histogram and move in the upper and lower bounds appropriately. After that you need to find the right reverse S-curve to bring out the shadow detail with adequate contrast and then the same on the lights in a way that doesn't burn them out. In most circumstances you should be able to find a reasonable compromise. The resulting image won't be perfect. It will probably be low-contrast relative to the ideal and you'll need to correct that in Photoshop.
But that should get you started with FDR tools. Next time I'll look at the other two tone-mapping algorithms, Receptor and Compressor.
Till next time,
Bob
This has good separation in the blacks and not so good separation in the whites. No problems with the lines or their intersections and - until it reaches the brights - realistic rendition of the gradient with some banding.
I looked for a simple manipulation of the contrast curve that would improve things and so I steepened the slope on the left side hoping to increase separation among blacks. That worked pretty well so I did the same thing on the light side. That resulted, of course, in the classic low-contrast reverse S-curve and, well, see what it did:
This simple change produced low contrast (see previous) but a huge improvement overall in terms of rendition and separation. Now that I felt that I was on the right track I tried to optimize it:
I brought in the outer bounds so that they more tightly bounded the observable values on the histogram and I steepened the sides of the curves in the darkest and brightest areas. I also brought the middle of the curve way down so that it produced the darker band in the center of the gradient - in so doing it helped to intensify the separation in the lights (you probably couldn't do this with a real photo). I think the result looks really good but, at least, by knowing that we're searching for some reverse S-curve to improve the scene we have a clue as to how to proceed.
Now keep this reverse S-curve in mind because the next thing I wanted to do was to see if I could apply just this one thing to a real-life scene. I chose the following awful picture that I took in Argos, in Greece.
Here's a column-drum lying on the ground. The lights are on the edge of being burnt-out but everything else, almost, is in inky blackness. I'm going to use FDR tools on this and just take the default settings first. What does Simplex do to this without my doing anything?
Lots of detail here and overall very low contrast. Here's something to work with. Let's apply the reverse S-curve and see where we are but, it's clear, we have to move in the upper and lower curve bounds on the histogram itself. Here are the settings that I found and the resulting image.
I found that it's not just a matter of applying any old reverse S curve. You have to experiment with the shape on each end until it looks as good as possible. I moved in the upper and lower bounds (do that first) on the curve until they just touched the values on the highest and lowest end and that makes a crucial difference. So the thought for today is, when using FDR tools, the Simplex processor will do a reasonable job and very quickly. Once you've applied it inspect the histogram and move in the upper and lower bounds appropriately. After that you need to find the right reverse S-curve to bring out the shadow detail with adequate contrast and then the same on the lights in a way that doesn't burn them out. In most circumstances you should be able to find a reasonable compromise. The resulting image won't be perfect. It will probably be low-contrast relative to the ideal and you'll need to correct that in Photoshop.
But that should get you started with FDR tools. Next time I'll look at the other two tone-mapping algorithms, Receptor and Compressor.
Till next time,
Bob
Thursday, January 28, 2010
Look what FDR tools can do:
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