Subjective, and true grays
Ven, you know me when I get the bug.
Rex, I gave a quick little description of how Doug's technique works.
Do photographers use the step-wedge thingie?
Grab a dowel, some paint, and step-wedge it.
Toss it into at least one photograph to get an idea of what corrections need to be done.
You could even do a dowel with RYGCBM.
Maybe I'll make a few and try to sell them on eBay...
White points, and unclipping tutorial
On a different subject, I have a question about your unclipping tutorial. After painting over the clippings, I have to restore the original contrast by applying the Brightness/Contrast tool a second time. Is there a way to get around this? Hate to spend a round trip on this tool.
Last edited by Duv; 03-03-2005 at 12:06 PM.
Lene revisited, Part 1
Sorry folks, this has taken some time, but Lene and I and my camera had to coincide in space and time After that, I simply needed time to pp again. Well, now it's Easter holiday and here we go!
In the meantime I've attended a colour calibration course with GretagMacbeth, the "Picture Perfect Color Seminar". I learnt one very important thing there that applies to this "project". RGB or CMYK values for flesh tones are absolutely worthless unless you state which colour space they apply to!
Back to Lene. What do I have to work with now?
- The original shot in ISO 1600, with the Red channel blown all over the place.
- A fresh, slightly underexposed facial shot from my KM Diamge A2, in sRGB using flash. This makes me fairly confident that the colour balance is correct.
- A pivotal understanding of the image having different colour problems in the shadows and highlights. (Thanks byRo)
- An understanding that CMYK or RGB values are worthless out of context, read colour space.
On page 82 of the second edition (2003) of her book, Katrin Eismann shows a technique for restoring a blown channel. Now who am I to stand up against Eismann? Let us just say that I am not convinced that this technique will always yield satisfactory results. It corrects the lost detail of the blown highlights without changing the colour (would that be hue?) of the image. But with one channel already blown, doesn't that throw the colour balance off? So why preserve it? I will try my personal approach.
I started thinking fresh. I tried to imagine what the image "looked" like. Here is my visualization:
First. let us imagine a b&w photo. When we look at it, we see intensities that vary from min to max. Min represents total black and Max represents total white. Everyone knows this, but bear with me while I'm stating the obvious.
Now think again. This b&w photo can be compared to a (rugged) landscape that we're flying over at high altitude. White represents mountains and high areas; black represents valleys and low areas. If we lay a vertical slice through this landscape, we get a height profile, just like you see in Tour de France or downhill skiing or whatever. Or like in CT (Computer Tomography X-ray).
If we lay such a slice across Lene's forehead in the Red channel, we get a nicely profiled curve in the temple area, but when we reach the forehead proper, the curve turns completely flat! It will look like those funny table top mountains you have in the US deserts. The Red channel has reached the technically maximum height (255), and is unable to show the real profile. What should have been there, is completely lost. Or is it?
The Green channel is not blown. In addition, the Green channel is usually almost synonymous with the Luminosity channel. In fact, my camera uses only Green for contrast focusing. Green is our luminosity and contrast friend.
I can make a mask from the Red channel, white where the channel is blown (255), black for the rest. I can then apply this mask to the Green channel. What does this represent? If again we apply a vertical slice, but this time to the masked Green channel, we see the profile of the "Green mountain", covering the same "aerial view" where the "Red mountain" was flat. We can think of it as if the water rose to the level represented by the mask, and only the unmasked area remained as an island.
I have now a fairly good guesstimate of what the Red channel should have looked like in the area where it is blown. There is just one problem; I cannot make the "Red mountain" taller :-( But what about "lowering the terrain"? Going back to the masked Green channel, I can use the histogram to see how much taller the "Green mountain" is than the area outside of the mask, or in the island metaphor, how tall is the tallest mountain on the island? If I "lower the terrain" to the same degree in all the channels and apply the "Green island" profile to the blown part of the Red channel, I should have re-created an approximation of what a correctly exposed photo would have looked like. This will of course not allow for colour variation in the skin, since the ratio of Green to Red will be static and unnatural. However, I hope that Blue (which I haven't touched) will take care of that, since it is notoriously blotchy and ... lively?
The difference between this method and Eismann's is that I don't touch the part of the image that is not overexposed.
Now I hopefully have a correctly exposed image, and can start to have a look at the colours again. Gaussing Lene's forehead from the flash shot should give me a usable swatch sample of her skin.
Ok, here we go.
1) Import and convert from Adobe RGB (the colour space used in the 10D) to sRGB (my working colour space)
2) Create a mask from the Red channel. Set Threshold to 255. Store the mask as an alpha channel. I will need it later. Name it "Blown".
3) Create an Art layer from the Green channel, for instance using the Channel Mixer.
4) Apply the mask from above. We now have a strange, grey, ghostly shape of most of Lene's face. The Red channel was worse than I had thought.
5) Create a Black filled layer underneath this one.
6) Merge the grey ghost face down into the black layer. Now we have a ghost surrounded by black.
7) Show the histogram for this layer. It actually has values from about 150 to 225. That means the "Green mountain" is as much as 75 levels high.
8) 75 is 1/2 of 150 and 1/3 of 225. The "tallest Green island mountain" within the blown Red area is 50% higher than "sea level" at 150. It means that this image is overxposed by exactly one half f-stop. It needs to be pulled back 1/3.
9) Use the Channel mixer and set all three channels to 65%, about a half f-stop lower exposure. Now we have the levels where they should be, and there is room to re-expand the Red channel.
That was the easy part. I will not need any of the layers anymore, except the "Blown" alpha mask, so I can throw them away.
Now how do I partially replace one channel? And it should be seamless. Hmmm... I have this uncanny suspicion why Eismann prefers that ... other method
After some hours of trial and terror, I think I have something.
10) Working on specific channels is easier in Greyscale. Create a greyscale image, same size. Drag the Red and Green channels from the working document to the Greyscale document. Drag the Blown mask too.
11) The channels become alpha channels. Disappointment. I thought they might instantly turn into layers. Copy-paste into Layers, "Red" and "Green".
12) Create Levels clipping Adjustment layers for both the Red and Green layers. Adjusting the right slider for Green to 230 looks ok to me. Only a few reflexes reach 255. The Green layer receives the Blown mask.
13) For the Red layer I adjust the output level (the one at the bottom) to approx 175, then use the arrow keys to do fine adjustment. I end up at 176. This number depends on the number from the Green levels.
14) I Gauss the Green (Blown) mask with a radius of 3.0. Better.
15) Still some mismatch. I use a brush at 10% on the Green mask a few places. Left eyelid (her right), left cheek (her right), lip where it turns into shadow. I think I have a new Red channel!
Below the two versions. They uploaded in the wrong order, so the strong yellow one to the right is the original, and the darker one to the left has a reconstructed Red channel withut blown highlights.
The rest of the story in the next post, hang on...
Lene revisited, Part 2
To create a good flesh tone swatch of Lene, I took my flash shot of her, Gaussed it at 50 pixels radius, and increased the exposure to a "sensible" level (flash reflection on forehead approaching 255). I then simply clicked around on her forehead, jotted down the readings in Excel and sorted them. NB! These are sRGB values.
R G B G/R B/R
164 113 93 69% 57%
165 116 95 70% 58%
171 123 100 72% 58%
179 123 103 69% 58%
180 124 104 69% 58%
191 133 110 70% 58%
192 135 114 70% 59%
206 145 123 70% 60%
209 152 130 73% 62%
217 161 139 74% 64%
225 169 150 75% 67%
235 181 162 77% 69%
238 180 162 76% 68%
Ahhh, this doesn't look good. The tabs are lost.
I then Gaussed my reconstructed Lene somewhat, and was actually able to place a control point that was both on her forehead and on the forehead of the flash shot. Then it was just a matter of correcting the Colour balance until the two images matched. I used the Channel mixer.
Ok, from the left, the hall where the first shot was taken. Both Lene and I were sitting on the bench along the wall. Second, the flash shot of Lene, Gaussed. Third, the reconstructed Lene, Gaussed to make adjustment easier. And finally Lene with the same colour values as the flash version.
This is where I lie down and cry
I am no expert lol but i do like to play photoshop challenge...
I used PSP9 to correct the colours, then i used curves to enhance the lips, fur, hair, then i used the patch brush...
By the way, I see red cast in a previous posted image of mine.
So...just second try...here it is. What do you think?
Last edited by venivedi; 03-28-2005 at 12:36 PM.
Well I tried my hack method again. Her left eye has a clear expanse on the cornea, so I used that as a gray point. Dropper showed red 252 (the whole red channel is pretty blown out), green 189, and blue 117.
So I adjusted the brightness in the color channels, red down 63, blue up 72. The high number of these adjustments is a giveaway that this method is not optimal for this kind of fix, but I tried my novice skills with levels and curves and came up with something even worse.
To me the lighting conditions are the real killer. I'm beginning to suspect that the necessary color information is just not there, in which case manual colorization in spots might be the way out.
Just to clarify
I should return to Beethoven instead. That would at least make my piano teacher happy...
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