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A Letter from ColdwaterJohn

My good friend ColdwaterJohn has recommended this article on HDR to me.  I went there and found a very well written basic multiple image HDR tutorial.  Not quite our thing, I know, but we must keep informed.  I was intrigued by this gentleman's (Harold Davis) previous article on using a single RAW image processed in different ways and then combining the versions with a layer mask and a gradient to reveal portions of underlying layers.   I know that that's really off the track of SIHDR but you should go there and read his article anyway.  I'll deal with his HDR ideas in time but first let's explore his ideas about multi-processed RAW images.  In short (but read it for yourself) he describes a basic method of processing a single RAW in different ways (in his example he's correcting for different brightness zones) and then combining these versions in Photoshop.  He takes one image in which the left is underexposed and, in Adobe Raw, he makes a version of that image in which the left is properly exposed.  He saves that first image.  He then makes a second version in Adobe Raw in which the overexposed portion on the right is corrected.  He now has two partially correct images and he layers the bright image over the dark image and then exposes part of the dark image by using Layer Mask Hide All and then erasing part of the mask (the bright image) with a gradient.

A gradient.

Hmmmmm.  It's the old neutral density filter trick but moved way up into post-processing.  There are circumstances in which this approach is effective (Mr. Davis demonstrates one) but for most circumstances it introduces more problems than it solves.  The basic problem is that the neutral density area is simply not the required shape.  Here's an example where something else has to be tried:

In Athens recently S and I went to the Kerameikos which is a kind of ancient cemetery (not far from Plato's Academy) and I made this ghastly shot.

This is a beautiful funeral stele but the shot is totally ruined because I was shooting into the sun so that the temporary roof in the foreground is way too bright and the rest is murky and dark.  A failed shot (Not to boast but I have a built-in advantage as a photo-blogger because I take so many bad shots that I always have something to fix).

Mr. Davis tells us to take the RAW and make a light and a dark version of it.  No sooner said than done.  Here's the light version:

The stele and the background look good but the roof is over-exposed.   Here's the dark version in which the roof will be properly exposed:

...and of course everything else is murk.

Now we put them together.  Of course it's very difficult to design a gradient that will combine these two.  There's a dark zone on the top, a bright zone in the middle, and another dark zone on the bottom.  Mr. Davis' shot had only a dark zone on the left and a light zone on the right.  His shot was made for a gradient.  This one isn't.  But just to show that I'm a good sport I tried it his way.  I layered the light layer over the dark layer and used a layer mask to hide the upper layer until the application of the gradient.    Here's the shot again with a gradient drawn from the upper left edge through the peak of the shadow and ending at the edge on the lower right.

In fairness this looks pretty good.  The shadow of the stele is more realistic than in any other of my versions but the stele sculpture is a little dark and the foreground shadow is way too dark (given that you were interested in it at all which I'm pretending that I am).

For my alternative version I approached this shot in the following way.  I layered the dark layer over the light layer and then I used the history brush in Photoshop to brush away everything except for the roof (and the shadow on the roof - very critical point).  The final result is this:

This is a highly-compressed jpg.  You have to take my word for it that the original TIF had more detail in the roof but I think you get the idea.  The only odd note here is that the shadow seems way too dark for the situation but I'd be interested in what my readers think about that.

In short, read Mr. Davis' article on RAW processing (it reads a little like my tutorial on dividing up an HDR into pieces and using each piece separately) but I'd take the gradient thing with a grain of salt.

Until next time,

Bob

Dynamic-Photo HDR 4 and Halo-Matix

I promised another session with the HDR test sheets.  Here's a new sheet I designed.  It consists of some complicated black to white gradients.  Upon those I layered two articulated squares.  The first, in the upper left consists of a bottom layer of pure black followed by more layers each one brighter by 5% (the B in HSB) than the one preceeding.  The second, in the lower right, consists of a base of pure white and then additional layers; each one darker by 5% than the one underneath.  I then drew a number of pure white or black lines from one zone to the other.  Many of these lines were one line width apart.  At the place in the center lower left where the white lines and black lines come together I drew more lines in near-middle gray (136, 136, 136).  You can just barely see them.  Here's the chart:

It all looks very Piet Mondrian.  I then fed this to the HDR software which happens, in this case, to be Dynamic Photo HDR 4.  I used 'Auto Adaptive' and full HDR.  Then I used the Halo-Matix tone mapper and that produced this with default settings:

       

I tried Halo Matix because of the good separation between similar tones that it showed in the last test and here we are not disappointed.  All the dark squares are clearly separated; even the 100% and the 95% black square inside of it.  These could not be seen in the original.  The effect of the transformation has been to progressively exaggerate the lightness of the nested dark squares.  Contrariwise it has exaggerated the darkness of the nested white squares so that they, also, are separated.  The one pixel-width lines, black and white, are all much more clearly visible in this version than in the original.  If you go to the Picasa album here and enlarge the Halo-Matix version (click on 'Full Screen') you can also see some gray lines clustered just above the short black lines in the lower left center.  These gray lines are also more visible after HDR processing (particularly the vertically-oriented ones).

Now let's look at the same image when made bright and the halos increased to the maximum size:

Here the dark squares have maximum separation but the increased brightness has merged the brightest squares into each other with generally poor separation of tones - but better as they get darker.  Turning down the brightness of the image improves things as we see in the next image.  It is as dim as the settings allow but with max halo size:

The two lowest black squares blend into each other here to a much greater degree but the white squares are mostly clearly separated.  They are given a much greater range of darks.

   Now let's reduce the size of the halo setting to a minimum:

This image was produced with the brightest setting but with a minimum halo size.  Again the bright setting forces the brightest squares into each other and the small halo is clearly seen especially around the lines.  In the darkest squares each seems to run through exactly the same brightness ranges and the separation is achieved by enhancing the differences at the edges of the squares.

   Last of all comes the dim setting with minimum halo size:

One thing we notice in these low-halo settings is that they will certainly degrade tiny details that are close together.  Because each line has a halo around it these halos run together where the lines are close together and thus degrading the entire area.
   I created a chart to sum up what I think I've found and here it is:

 

If your eyes are like mine you'll have to click through to read it.  So it turns out that brightness has a relationship to tone separation for darks (bright) or lights (dim) and tonal separation can be achieved by radically altering the tones or by leaving the tones more or less the same and simply emphasizing the edges.  The chart should help you to pick the settings you need for whatever it is you're trying to achieve.  I think that for the next post I should find a desaturated image and experiment with it by trying these different settings to see what difference it makes 'in real life'.

Until next time...

HDR Test Sheets

Let's try to get this HDR thing on firmer ground.  We need a repeatable way to predict the performance of HDR for any set of parameters.  I'm going to use Dynamic Photo HDR 4 from MediaChance and I created the following simple test sheet with Photoshop CS4.  It consists of two black to white gradients at right angles.  In the dark portion of the test sheet I typed the word 'abracadabra' in black at three different locations.  In the light portion of the test sheet I typed the same word in white at three different locations.  We should be able to see, at least, some set of parameters that would let us read all six words.  Without further ado here is the test sheet (and if you can do better let me know):

I transformed this in Dynamic Photo HDR 4 using the Auto-Adaptive method of re-mapping and I selected 'Full HDR Process'.   When that was done I tone-mapped it using all seven algorithms at the default settings.  Here they are.  The first is 'Eye-Catching':

The next is 'Ultra Contrast':

Now comes the real surprise: 'Halo Matix':

I can't resist a gasp of surprise.  I would never have guessed that this algorithm (which seems to feature garish haloes and color distortions) is far and away the best at separating details from a similarly colored background.  Look how even the background tone is.  If anything it has slightly reversed the gradient.  And all six words visible as plain as day.  And notice that the white versions of 'abracadabra' are rendered in something like real white.  This distinguishes it from 'Human Eye' which picks out all three white 'Abracadabra's but in shades of gray.

Now comes 'Smooth Compressor':

What's going on with 'Smooth Compressor' also seems clear.  I believe that this algorithm simply takes the entire tonal range and compresses it into the number of output bits available (7 or 8).  We'd expect this to be a low-contrast solution which would make the words hard to read.  That's what we actually seem to see.  See this post for my ideas about reduction of contrast under some compressive HDR algorithms.

After that comes 'Auto Adaptive':

'Photographic' is next:

And, finally, 'Human Eye':

I'm going to put this on-line right now.  Next post I'll present the seven results from performing what MediaChance calls 'Eye-Catching' remapping.

Breaking up the HDR-processed image

We were in southern Greece in the middle of October and, one day, Sue and I found ourselves in an old Mycenaean tomb.  These are sometimes called 'tholos' or 'beehive' tombs; they're made of rock, domed, and covered with earth except for the entrance.  This particular tomb had fallen in centuries ago and nothing was left except the circular wall up to a height of about four feet and the entrance which had a gigantic lintel over it.  I pointed the camera upward and took a shot of the lintel but because the sky was overcast and bright the underside of the lintel was underexposed.  What to do?  I could see enough texture under the lintel so I supposed that HDR might retrieve something.  Here's the original:

Pretty grim.

I then proceeded to expose this mess to my HDR software.  I used Dynamic Photo HDR 4 and the settings were as follows: I chose 'Eye-catching' over 'Adaptive' and then selected 'Full HDR Process'. Once that was complete I chose the 'Auto Adaptive' tone-mapping process with the following parameters: Vivid Colors: 0.27, Light Radius: 4.1, Light Strength: 0.52. Other choices are possible.

 

I felt that I came out too bright but this is still usable. But I want to use the underside of the lintel separately from the rest of the picture.  The underside of the lintel is closer to what I really want but the rest has to be toned way down.  First I'm going to move the under side of the lintel and modify it by setting the opacity.  Then I'm going to use a different opacity for the rest of the picture.

First I used a mask to select just the underside of the lintel.  Here's a Photoshop screen shot.

Once it was a selection I then copied it and pasted it over the original.  It was way too bright there so I moved the opacity of the underside of the lintel down to 64%.  Here's the result.

Now the underside of the lintel is where it should be but the rest of the photograph could benefit by having the rest of the HDR-processed version layered over it.  I had saved the selection for the underside of the lintel.  I got that back and inversed it to select the rest of the picture without the lintel.  I then copied that and pasted it over the ongoing composite.  Here's a screen-shot of the result; it shows the composited lintel after opacity adjustment along with the rest of the  picture before opacity adjustment.  If you can make out the layer menu in this next shot you can see that the original is the bottom layer, the lintel (only) is the middle layer, and everything else in the HDR shot is the top layer.  Here it is at this stage:

Believe it or not this is  progress.  Yes the composite is garishly too bright but we're going to fix that by adjusting the opacity just for the part of the image that isn't the lintel underside.  I took the lower two layers (original and lintel underside) and merged them.  That leaves two layers.  The composite is on the bottom and the HDR shot minus the lintel is on the top layer.   Now simply adjust the opacity of this top layer down to 48%.  That leaves enough brightness so that details can be made out and it is appropriate to the lintel underside.  Here's what it looked like after I adjusted opacity.  Remember my goal in these sessions is clarity, not beauty.  Because I've been able to rescue the clarity this picture is now suitable for Squinchpix where you can see what I actually uploaded.

...and here's the original again so that you can compare:

The moral of the story is that you can cut up the product of any HDR transform any way you like and re-composite it back on the original with any adjustments you choose.

And because you've been such good students here is your reward:  This is a shot of the west coast of the Mani peninsula in southern Greece on a stormy day.  It was also processed with a modified HDR procedure.  You're looking south here towards Areopolis.

Until next time,

Bob