Use curves in Snapseed

- Hi I'm Sean, and this week on Mobile Photography Weekly, we're going to take a look at a custom editing feature in one of the most useful apps that you can have on your mobile device. I'm referring to Google Snapseed, which is available for both Android and iOS. There are a lot of really useful ways that you can edit your images in this app, and one of them is to use a feature called curves. Let's take a look. So, curves is not a feature in a lot of mobile apps, but it's becoming more common in some of the more full-featured, photo-editing apps.

If you're already familiar with Photoshop, Lightroom, or similar programs, then you may already be aware of how you can use curves to edit your images. So in Snapseed, I'm going to tap on the pencil to get my tools, and I'll just choose curves. So, by default, it starts out and applies a curve preset for hard contrast. And you can see that down there at the bottom in the thumbnails. There's actually quite a lot of presets that are available there, and one of the ways to get a sense of how the curve is affecting the image, if you're new to working with curves, is just to try out some of these presets and see how the image is changing based on how the shape of the curved line is being adjusted.

So for instance, if I tap on neutral, that's no change at all. Here's soft contrast. Hard contrast is a little bit more pronounced. There's brighten, here's darken ... There's faded, et cetera. So, if you go through and take some of these for a test drive, you'll get a sense of how the curve is affecting the image. I'm going to go back to the hard contrast just 'cause I think that's a good one to start out with. And I'm just going to tap on the little swatch booklet there to close the view of all the presets, so we can see the curve in its entirety here.

So, to this image of the sand dune, I have added a gradient from black to white and then a row of swatches from black to white. And this corresponds to the histogram that Snapseed is overlaying over the curve dialogue. So, on the far left side we have total black, and on the far white side we have total white. And in between is that gradual ramp from black to white. If you look at the points on the curve, they correspond to the area on the histogram if you draw a straight line immediately down from that point.

So if I tap on the highlighted point there, if you draw an imaginary line straight down, it corresponds to the area of the histogram that you see there on the right-hand side: the brighter parts of the image. Same thing with the shadow point there, if I tap on that, that corresponds to the darker parts of the histogram that you see on the left side. So with a hard contrast curve that we have here, the highlights are being raised up and made brighter, and the shadows are being lowered and made darker.

This curve is also often referred to as an S curve, simply because it recalls the shape of the letter S. So, in terms of contrast, you might be asking yourself: well what's the difference between adjusting contrast here in curves and using the contrast slider out in the tune image section? So let's go into that tune image section, and if a drag vertically on the screen, I can see my choices there, and there's contrast. And now if I drag over to the right, I can increase contrast, and if drag to the left I can decrease it.

So, this works good for minor contrast adjustments, but the advantage of working with curves is that it gives you a lot more control. So, let's just take a look at that here. So let's say I wanted to darken down the tones in the shadow areas of the dune. I could do that just by dragging down here on this point, but maybe I don't want the highlights to get so bright. So I can just pull those down and keep the upper end of the curve unchanging like that. So, here I am darkening the darker parts of the dune, but the highlights are staying the same.

So if you look up at the sky, there's really not a lot of change happening there. Now one thing I want to point here is that when you apply a fairly significant contrast curve, such as we have here, you'll notice a color change on the dune. That's because whenever you adjust the main RGB curve in an image, it oftentimes affects the colors in the image, and you get a boost in color saturation and sometimes even a slight color shift. I'm going to tap on the little swatch book there and reset that back to neutral.

And let's go and tap on the little color filters here because on the opposite end of these choices here is luminance, so if I tap on luminance, and let me apply that same type of curve there. I'm going to pull down on the darker parts and pull up on the brighter parts. Notice that the dune is not changing color as much as it was when we were adjusting the RGB curve. So the luminance setting there is affecting only the brightness or luminance values of the image.

That's a really useful setting there to be able to come in and apply custom adjustments. Again, I'm going to reset this back to neutral. We'll come in to our color filters here, and in addition to luminance and RGB, we can also adjust the individual color curves. So if we go under red, that's going to allow us to either add red or its opposite, or complimentary, color of cyan. And by the way, if you want to remove a point from the curve, just drag it out of the box.

In that way it behaves exactly the same way as Adobe Photoshop. I'm going to come over here to the filters and choose the green curve. That's going to allow us to add green or its compliment of magenta. And then finally in the blue, we can add blue or its compliment or opposite of yellow. So there's a lot of power there in terms of how you might choose to use the curve to apply edits to color balance or to correct a color caste that is unwanted in the image.

So for instance, in this image here, I could always add a little bit of yellow; putting a point there on the blue curve. And then come over here to red and add a little bit of red. So the combination of red plus yellow is going to give me a warming tone to that image and create a nice warm feel, which really matches the subject matter quite well. I'm going to go back to that swatch there and reset that back to neutral and just set that back to hard contrast.

Now, one thing to point out that you should be aware of are dangerous curves. A curve becomes dangerous when the movements or adjustments in the curve become too abrupt. So for instance, watch what happens to the gray ramp at the bottom of the image as I take this highlight point and I drag it up to the top. So it kind of hits the ceiling of that curve box. Notice how I'm losing highlight detail. Notice how the highlights there are getting blasted out to total white. So any tone that is to the right of where that curve line is flattening out on the ceiling is being blown out to total white, and I'm losing highlight detail.

Let me bring that back, and let's do something similar with the shadow point here. I'll bring that down, and look what happens to that gray ramp. See how I'm losing shadow detail? So any time you have the curve flattening out along the bottom there, and anything to the left of where it flattens out, is being forced to total black, and I'm losing detail. Another type of curve to be aware of is what I call a shelf, and a shelf is where you flatten out part of the curve. So, wherever it was a diagonal line means each of those tones along the line were different.

Where the line becomes more horizontal, it means all those tones that used to be different are now the same, and this is why we're getting this ugly, flat gray in there. So that also is a dangerous curve. Generally you want to make sure that the movements of the curve are gradual and gentle and not too abrupt. Alright, let's go and apply this to a real image and not this demo image. I'll just tap cancel there. And let's go out and get another photo to work with.

So I'm going to go into the curve there. Right off the bat we have that nice hard contrast curve, and we can play around with that and see what that looks like. But what we're getting here is a little bit of a color shift in the curved side of the building, right behind the staircase there. So, one way we might fix that is to apply a luminance curve. So let me set that back to neutral. I'll tap on the color filter, and I'll choose luminance. And then I'm going to apply the same type of curve.

And here you can see I'm getting a much more neutral effect of how that curve is affecting the polished-steel surfaces of the concert hall. So that looks a lot nicer, a lot more neutral. Plus, if I didn't want the highlights on the top of the building to be as bright, I can always pull those down so that they're not being washed out quite so much. We'll put another point there and maybe bring up the midtones a little bit.

As you can see, the curves interface is not necessarily easy to decipher if you're new to it, but it does provide much more control over contrast than the basic swipe left or right method in Snapseed's tune image controls. And curves really excels if you want to apply custom color styling and effects or separate contrast and luminance adjustments from affecting the colors in the image.