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Canon Camera    says:

paulssons has kindly granted permission for me to have a hack at their photo for this discussion.

-- from paulssons - (?)

Crap Camera says:
Despite the riot of colour, this still looks a little washed out - rainy day I know. But I think a little post processing along the lines of Margulis LAB technique, steepening the A & B curves by 10% to would punch up the colours.

spikeblacklab Pro User says:
Very nice flowers - beautiful coloring! I agree with Crap Camera, unfortunately the image is a bit washed out... That is very easy to fix, though. Almost every online camera developing service (Kodak, Clark Color Labs, etc.) allows you to create an account and upload photos, whether or not you order prints. They all have software to correct exposure and color, and then you can use that image. I don't really know what Crap Camera is referring to, but those should work if you don't have your own software... And of course, if you have it, there's always Photoshop. :)

OK, here is what I was referring to, and yes I’m using Photoshop. It may be possible to do the same or similar with GIMP or other tools.

The Histogram graphically shows the Contrast Range. The horizontal X scale is from black to white, and the vertical Y axis is the count of how many pixels are at each brightness point. You can see the bulk of the pixels are in a reasonably narrow range. If you took the mid point of the graph, the bulk fall towards the right side, so there is not much that is dark.


I plan to shift the graph to the left, darkening the image in selective places, and spread the graph out to get a greater dynamic range. I’m going to do it in LAB with Curves.

What is LAB?, it is an acronym for Lightness, A Chanel and B Chanel. It is a Colour Space, another way of describing colour. You are probably familiar with the RGB Colour Space (Red, Green, Blue), and your printer probably uses CMYK Colour Space (Cyan, Magenta, Yellow, blacK).

The huge advantage of LAB is the separate Lightness channel. We can adjust the contrast range with no risk of affecting colour.

1. Converting the image to LAB:
Image | Mode | Lab Colour

2. Finding the Darkest & Lightest spots.
Sometimes this is obvious, others not so easy. A useful tool is Image | Adjustments | Threshold. Slide the slider down to the left, the last point of the picture to turn to white is the darkest part of the photo. With this point still visible, press Enter, then use Ctrl-Z to toggle back and forth with your photo.

3. Marking the Darkest & Lightest spots
Use the Colour Sample Tool, and click on the darkest point of the photo to mark it. Now when you use the Colour Sample Tool or Eyedropper Tool, you should change the default Sample Size from Point Sample to 3 by 3 Average in the Tool Bar :


It is very hard to get an accurate reading from a single point, you may sample a random bit of noise.

Do another Image | Adjustments | Threshold. Slide the slider to the right this time. The last point of the picture to turn black is the lightest part of the photo. With this point still visible, press Enter. Ctrl-Z to toggle back & forth again, and use the Colour Sample Tool to mark this lightest point on the photo.
Have a look at the Info panel:

It shows our first point #1 with a value L:3 and our #2 L:99
L values range from 0=black to 100=white, so we do in fact have pretty much the full range already, it’s just there are very few pixels in these areas, and most are in the mid tones as seen in the Histogram above. This is why an Auto correct on this photo will not work.

4. Contrast adjustment with Curves.
It is best to do this on a separate Layer, so we can compare our adjustments back with the original layer.
Layer | New Adjustment Layer | Curves

The defaults are fine, click OK

Curves takes a bit of explaining. The graduated bars on mine are showing Light to Dark. If I click on the bar, it switches the graph to Dark to Light. It really doesn’t matter which way you have them, unless you are comparing with someone else’s. Make sure that Channel: is set to Lightness.

The Curve is designed to show Changes. While it is that straight diagonal, nothing has been changed. Every pixel in your photo will fall somewhere on that line, depending on how light or dark it is. In fact if you click and drag on your photo, it indicates on the Graph where that point is – this becomes very useful when you know what you are doing. Clicking on the Curve line creates a Point which you can drag up or down, shifting all the pixels that match that point, lighter or darker. You can create lots of Points on the adjustment Curve.
A few configuration changes first. The default grid is a crude 25%, Alt-Click on the grid and it becomes a more accurate 10%.


Next, those Eye Droppers. Double click the left one “Set Black Point”, this opens the Colour Picker. It will default to absolute black (L:0 A:0 B:0 or R:0 G:0 B:0), which is rather brutal, as it contains no detail. The Set White Point will default to absolute white (L:100 A:0 B:0 or R:255 G:255 B:255). These endpoints are used when you press Auto, or use those Image | Adjustments | Auto Levels, Auto Colours, Auto Contrast tools. Set to the defaults they could blow out your highlights and plug your shadows. It is better to err on the side of caution, and allow you to fine tune things later by resetting these to Black Point L:10, White Point L:95.

Now you can use the Set Black Point Tool, and click on the #1 darkest point of your image you marked previously, and then use the Set White Point Tool and click on the #2 lightest point you marked previously. Unfortunately for this particular image it is not going to achieve a lot since the endpoints where already roughly right, but I wanted you to learn the principle. When you use these Set Black Point and Set White Point tools in other colour spaces (RGB), you have to be very careful that you click on a neutral colour, otherwise it will introduce a colour cast to the whole photo. Using the LAB Lightness Channel completely solves this problem.

For this image I wanted to shift the histogram graph to the left, darkening the image, and spread the graph out to get a greater dynamic range. My final Lightness Channel Adjustment Curve looks like this:

I first moved the Dark end in by 20%, then added 4 roughly equidistant and shifted each one while observing the result on the photo, until I got what I wanted. The Lightness Histogram shows I have achieved what I wanted, a much more centred and better distributed Contrast Range.


5. Boosting Colour in LAB
We are not finished with our Adjustment Curves yet!. Now I want to add a little impact to the colours. Change the Channel Drop box to “a”, and drag the top and bottom endpoints of the curve in by 10% (one graph line).

Select Channel “b” and do exactly the same.
This greatly improves colour. How steep you make it determines how fluorescent it goes. 20% is usually too far, 5% pretty negligible, 10% is often about right.

Finally you can click on the OK button to close the curves dialog. Now you can marvel at your handiwork. Since you used a separate Adjustment Layer you can toggle it in the Layers Pallet by clicking on the Eye next to the Curves1 Layers, showing the untouched original, and the new improved.

You can double click on the graph to reopen your Curves if you want to fiddle further. Once we are happy with it, Layer | Flatten Image.
While we are still in LAB, there are another couple of tricks it is really good at, reducing noise and sharpening.

6. Reduce Noise
Not a problem in this image, but useful to know about.
Noise can be reduced by blurring the A and B Channels.
Noise is nearly always mostly in the B Channel.
Use the Channels Pallet, select the b Channel.

If you have CS2, Filter | Blur | Surface Blur, radius:10 threshold:10 on A & B channels
If not, Filter | Blur | Gaussian Blur, radius:3-10
If it is really bad, Filter | Noise | Dust and Scratches, radius:1 threshold:0
Noise Ninja or similar tools do a better job.

7. Sharpening
Again this is best done in LAB on the Lightness Channel as it avoids introducing colour aberrations caused by sharpening. Use the Channels Pallet, select the Lightness Channel.


Filter | Sharpen | Unsharp Mask.
Try setting the Amount to 500 and the Radius to .1 and Threshold to 0. You can sharpen a lot more in the LAB Lightness channel, than you can in RGB.

8. Saving
You need to convert it back to RGB to save it as a JPG.
Image | Mode | RGB Colour
File | Save As, jpg format, as a copy, quality 10


This of course is all a matter of personal preference if you think the modified version is better than the original. But this is a powerful, & relatively easy process that can be very useful on a number of images.

References
1. Dan Margulis' new book, Photoshop LAB Color : The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace.

2. Curves by Ron Bigwlow
Originally posted at 3:52PM, 19 May 2007 PST ( permalink )
Canon Camera edited this topic 32 months ago.