Monthly Archives: February 2010

Linear Curve Adjustments and Histograms

In the previous article on the topic of Photo Editing with Histograms, I discussed the six basic adjustments found in Adobe’s raw processing engine and how those settings affect the image and the image histogram. Then I posted a poll asking your experience level with curves, which also offered up some basic concepts and links for further reading. I also posted a few videos having to do with curves adjustments. So if you’ve been following along, you should have a decent grasp of how the tool works.

In this article, I’ll be discussing how various curve adjustments affect the image and the image histogram — but only in the realm of linear curves. So… linear… curves? Yup, you can do straight lines in the curves adjustment and they have their applications. So let’s look at a few extremes, then I’ll show you how to apply the linear adjustment to an image in need. After this, we’ll dive into the curvy stuff. And after that… I’m sure I’ll have something else to ramble on about.

LINEAR MANIPULATIONS

Let’s just start with some basic straight-line adjustments. Wait… isn’t this supposed to be about curves? Have no fear, we’ll get there — but these basic concepts are essential for the advanced concepts. Keep in mind throughout these examples that the input scale is along the bottom of the curve dialog and the output scale is along the left side of the dialog.

Each of the following examples shows the curve dialog overlaid with the base histogram, the adjusted histogram, and the base image from bottom to top. The original image is shown in example 3.

What I’m showing above is a spectrum of linear manipulations within the curves dialog. We start at full contrast and work our way to zero contrast, followed by negative contrast. Let’s go through them one by one…

The first image shows what would happen if your curve was pushed vertical. What I’m doing is taking all the pixels with tones to the left of the line and forcing them to be black. I’m also forcing all pixels with a tonal value to the right of the line and forcing them to be white. Everything between the set black and white points is scaled according to the diagonal line, thus we see a higher contrast (remember what I said about slope a few days ago?).

The second image shows a high contrast linear curve. This is very similar to the previous example, but I’m allowing midtones to remain somewhere between black and white. I’m still forcing black and white pixels, but not to the extreme as with the vertical line adjustment.

This third image shows absolutely nothing — it’s what we started with. A perfect diagonal line from the upper right to the lower left corner represents a perfectly untouched input/output relationship.

The fourth image shows the opposite of the second image. Here, I’ve moved the black and white points along the edge of the box, but in the other direction. This tells the software to convert all perfect black pixels to some level of dark gray, and all perfect white pixels to some level of light gray. Everything between black and white is scaled accordingly, thus we see a lower contrast.

The fifth image shows a perfectly horizontal line — exactly opposite from the first image of absolute contrast. What I’ve done is mapped every single input tone to output a single tone value. All blacks, grays, and whites are now a single tone value.

And our final image shows the opposite of image three. I’ve turned the curve upside down, inverting all the tones. Blacks map to whites, whites map to blacks.

APPLYING LINEAR CURVE ADJUSTMENTS

Those of you acquainted with curves and levels will say “linear curves can be accomplished with levels!” True. I’m not disputing that fact, I’m just showing how to use curves in a linear fashion. For those using Lightroom or ACR, they don’t have access to levels within the raw processor interface.

At any rate, here’s how I would apply a curve adjustment to the base image/histogram.

So what did I do here? If you look close, you’ll see that I basically did a version of #2 shown above. I set my black and white points so that my histogram spans the entire tonal range. This gives me a true black and white tonal value in the image. Of course, it’s not perfect at this point, but it’s a hell of a lot better than it was!

This is the most used real-world application of the linear curve adjustment — setting your black and white points. There are other uses depending on your artistic vision, but you’ll have to explore those yourself.

So now that we’ve covered the linear adjustments, let’s move on to the non-linear stuff! Any questions on the linear adjustments?

Photoshop Curves Video Tutorials

I’m still working on putting together my article on using curves and histograms to edit photos, so in the meantime here are some good video tutorials on the subject. These tutorials are easier to understand because they show how the curve adjustments affect the image in a more dynamic fashion.

This first tutorial is fairly comprehensive for the grayscale and combined RGB channels, covering the basics of contrast and brightness adjustments while also hitting on a lot of little tips and tricks. The creator of this video also jumps into some of the things you should not be doing with curves so that you can avoid these situations.

This second video is a little more basic than the first, but it presents some of the material in a slightly different manner. If you watched the first and you don’t have a good grasp on the curves dialog, watch this one and see if it helps.

And if you have already watched the first two videos and you still don’t have a handle on things, this last one from our friend Donnie might help. He also gets into color channel curve adjustments, so this is helpful for the more advanced users.

Tone Up Your Curves Skills

Yesterday, I posted a poll asking “How Well Do You Know Your Curves?” and I’m seeing a slightly skewed response toward the “less experienced” side of things. That’s totally cool, and I’m glad so many of you chimed in to let me know!

As I gear up to post my next article on “processing via histograms” I’m coming to the conclusion that I should put up a bit of background info on the curve adjustment tool. This tool is deserving of a book just because of the flexibility and complexity that it encompasses… but I’m not going to write a book on this stuff. Instead, I’ve put together a few thoughts and screenshots followed by links to articles far more comprehensive than my own.

So let’s get started with curve adjustments, tones, ranges, slopes, color channels, and all the other associated fun stuff.

Keep in mind that this post is somewhat of a teaser intended to get you thinking about the topic at hand. Read it through, check out the images, and follow the links at the end. I’m hoping that you’ll have a better grasp of the curves tool by the time you’re finished.

WAIT… WHAT’S A CURVE?

If you’ve worked in Photoshop, The Gimp, Lightroom, Adobe Camera Raw, and many other pieces of photo editing software, you may have already used curves or at least seen them. It’s that box with a diagonal line through it, and you can usually manipulate that straight line into a curve through various methods.

A curve adjustment is a simple input-output tool that changes the tonal value of pixels by stretching or compressing portions of the histogram. So let’s say that you want all pixels with the tonal value of 190 to change to 200 (making the light tones lighter). The curve tool does this for you, but it also moves nearby tonal values to maintain a smooth appearance in the image.

Essentially, you need to know that as you move the curve down and to the right, tones will darken from their current values. Move the curve up and to the left, tones will lighten from their current values. A curve can have many bends and inflection points, so it is possible to apply different adjustments to different sections of the histogram.

THE INPUT/OUTPUT RELATIONSHIP

As I mentioned above, you can use the curve adjustment to designate tone transformations across the entire tonal range. If you want one section of tones to become brighter, you move the curve in one direction for that local area. If you want one section of tones to become darker, you move the curve in the other direction for that local area.

A side effect of curve adjustments is the increase and decrease of contrast for different tonal ranges in the image. The slope (or how steep the curve looks from left to right) determines how much contrast adjustment will be applied to that local area. A steep slope (closer to vertical than horizontal) will give you a higher contrast. A shallow slope (closer to horizontal than vertical) will give you a lower contrast. The interesting thing about the curve adjustment is that slopes changes will alway negate each other. So if you increase the slope in the midtones (thus increasing the contrast) with a traditional s-curve, you also decrease the slope in the highlights and shadows (thus decreasing the contrast).

Simple curve adjustments are applied to a combined rgb channel. Advanced curve adjustments can be applied to individual channels in any color space such as RGB, LAB, or CMYK. This type of thing gives you ultimate control of the tones for each color representation in your image across multiple color channels, but it can be difficult to visualize and control unless you have experience with the tool.

PRACTICE, PRACTICE, PRACTICE

To best understand curves, I would suggest starting out with grayscale images rather than color. Working with a single channel will be about three times more clear than working with three channels. This scenario will allow you to explore the relationship between input and output tones without having to worry about color effects.

If you have a good handle on how the curve tool works, try messing with the color channels in the RGB space to get a feel for how they work. It’s the same concept as with grayscale, but applied to each color (red, blue, green). You can also convert your image to LAB or CMYK color space and experiment with the curve adjustment.

FURTHER READING

This topic is huge just from a technical standpoint. So rather than regurgitate a bunch of stuff that’s already been said, be sure to check out these following articles. I’ve narrowed my choices down to four articles that I feel cover the main ideas.

Tonal Range and the Curves Tool
This link from Chromasia is actually an entire series of articles on the topic of curves adjustments and everything associated with them. If you have time to read through it, I would highly suggest doing so.

Using the Photoshop Curves Tool
While not as comprehensive as the first link, this article from Cambridge in Colour covers many of the basic lessons in curves adjustments. I like this one because of how concise it is with each topic.

Photoshop Curves: Stepping Up From Levels
This article from Earthbound Light is similar to the previous article, but it hits a few different points and presents the material in a slightly different manner. Both are worth reading.

Color Correction in Photoshop with the Curves Adjustment Tool
And finally, for those of you wanting to dive into color curves, this article from PSDtuts+ gives a good introduction. It doesn’t get terribly technical, but it should give you a good idea on how the color channels are affected by curves adjustments.

As I said, this article is just a precursor to my next article on the curve adjustment and how it affects the image and its histogram from a practical standpoint. So if you’re unfamiliar with curves, read these links and practice on some of your own photos to get familiar with the tool.

More to come later this week…

How Well Do You Know Your Curves?

I’m in the process of writing the 2nd part to a previous article on Photo Editing With Histograms and it basically centers around the curves adjustment tool found in many pieces of photo editing software. The problem I’m having is determining how deep to go into the topic — curves can be highly technical and complex if you start from ground zero.

So help me out with this poll and let me know your experience level with curves. I’m assuming that most of you know what it is and how to use it, but with an audience this size it’s hard to tell for sure. If I see a lot of people with absolutely no clue, I’ll try to include some of the basics in the article. But if the majority is already familiar with curves and how they work, I might trim that stuff down and just give a few links to other articles that cover the fundamentals.

{democracy:65}

And if you have any specific questions about curve adjustments or how they relate to the image/histogram, leave a comment and let me know! Hey, I can’t answer your questions if you don’t ask.

Link Roundup 01-31-2010