Level Correction
Since things are relatively quiet in the Mac community right now, I thought I'd answer another question I'm asked quite often: how to "fix" images. When people ask this question, what they're really needing to know is how to manipulate an image's Levels — the red, green and blue mixtures that make up the image. I took a shot of my white Apple keyboard to use as an extreme example and fired-up Photoshop CS2 to answer this. If you don't Photoshop, any image manipulation application that supports level modification will work (iPhoto, Aperture, etc.).
The original image is below. Notice how dull and dark it is.
To fix this, I open my Levels window and instead of manipulating the RGB channel, I choose "Red" from the Channel popup menu. What you'll see in the window is a spike in the middle and no histogram data in the right (toward white) or left (toward black). In a nutshell, what this tells us is that our Red channel doesn't know what it should consider pure white or pure black, so we need to help it out. We do this by dragging the white and black point sliders to where the first "real" white and black appear in the channel (I ignore histogram flat-lines and move the sliders to the point where the graph begins to move upward). In the three images below, you'll see the original Red channel, the corrected Red channel, and what the image looks like after making the change.
With that done, select "Green" from the Channel popup menu and do the exact same thing. The three images below show the original green channel, the corrected one, and the result.
With that done, select "Blue" from the Channel popup menu and do the exact same thing, one last time. The three images below show the original blue channel, the corrected one, and the result.
That's really about it. Simple, wasn't it? Most applications worth their salt have some sort of auto-level tool and it basically tries to do the exact same thing I just showed you how to do. However, sometimes the "mathematical" results aren't so good, so you need to know how to do this yourself.
Lastly, let's see the original image and the "fixed" image back to back:
A note: if your image manipulation program doesn't let you modify each color channel, you'll have to follow the same above technique on the combined RGB channel histogram.
The original image is below. Notice how dull and dark it is.
To fix this, I open my Levels window and instead of manipulating the RGB channel, I choose "Red" from the Channel popup menu. What you'll see in the window is a spike in the middle and no histogram data in the right (toward white) or left (toward black). In a nutshell, what this tells us is that our Red channel doesn't know what it should consider pure white or pure black, so we need to help it out. We do this by dragging the white and black point sliders to where the first "real" white and black appear in the channel (I ignore histogram flat-lines and move the sliders to the point where the graph begins to move upward). In the three images below, you'll see the original Red channel, the corrected Red channel, and what the image looks like after making the change.
With that done, select "Green" from the Channel popup menu and do the exact same thing. The three images below show the original green channel, the corrected one, and the result.
With that done, select "Blue" from the Channel popup menu and do the exact same thing, one last time. The three images below show the original blue channel, the corrected one, and the result.
That's really about it. Simple, wasn't it? Most applications worth their salt have some sort of auto-level tool and it basically tries to do the exact same thing I just showed you how to do. However, sometimes the "mathematical" results aren't so good, so you need to know how to do this yourself.
Lastly, let's see the original image and the "fixed" image back to back:
A note: if your image manipulation program doesn't let you modify each color channel, you'll have to follow the same above technique on the combined RGB channel histogram.
posted by Maury @ 1:17 PM Permalink | 4 comments so far 
4 Comments:
Is there a way to adjust the individual colors within iPhoto?
Not in iPhoto, unfortunately.
The iPhoto histogram shows all the channels so what you have to do is just find a happy medium/average for the white and black points, like so:
Let's say that the far right (white point) ends at 100. Your image's Red channels goes to only 85, your Green channel goes to 98, and your Blue channel goes to 93. That gives you a 13 degree spread between the max "white" on your Red and Green channels.
So, what you could try first would be to set the white to the average of that spread — around 91/92 — and see how that looks. You'd do the same for the Black point.
Now, these numbers I'm throwing out are totally arbitrary and purely for illustration — but I think you get the idea. Just default to setting the anchors to the average.
Now, if your image's colors are totally messed-up from the get-go — like your camera adding way too much red or doing major over or under saturation — this trick may not work very good. You'd then resort to messing with the image's Temperature, Tint, and Exposure to try and balance the histogram.
Maybe I should write-up an iPhoto tutorial for this, too. =)
That's pretty much what I've been doing. It's just hard to get the levels to look right when the original is so out of whack. There's a fine line between realistic and "artsy" that becomes easy to cross unintentionally.
Yeah. When the source is a big overblown with lows or highs, the best best is to just stick with the RGB rather use the individual levels.
An example would be some of the Jack shots you posted on your blog (like the standing Spider-man pic). There's a whole lot of dark being considered "black" as revealed by the left spike in the histogram — almost all the image data is way on that end, and hardly any is toward white. This is due to an exposure issue with the camera, which may or may not be overcome by making changes to the camera's settings.
The problem the above presents is that you don't have much data to manipulate because so much of it is concentrated to one part of the spectrum (as it were). Your only decent alternative is to tweak the saturation and the other color settings — or just make the pic black and white and be done with it. =)
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