Often, you will simply be told to color-correct your underwater footage you shot with your GoPro HD Hero 3. Now, since we've shot in Protune and with the automatic white balancing switched off there shouldn't be any incorrect colors...?
Well, sort of. The white balancing is switched off so it can't ruin your footage anymore. However, the overal color impression of your footage may still not be too convincing. Of course, your audience's expectations also play an important role. Not least, we humans are all but objective in our vision: we all have adaptive color grading built-in, which has been quite useful all these past millions of years. But as our brain plays some tricks on the images it sees, we often need to grade video footage in order to achieve certain impressions.
Somehow, image colors are like election pledges: brilliant colors, the more saturated, the better. But never show the unmasked (or ungraded) truth: your audience may not like to see it.
In the end, we often don't really need to correct the colors in our footage, as there is no real error to cure. Often, we need to grade colors. There are specialists that focus solely on this task and these specialists not only work on film, but also TV material more and more get color-graded too. A really prominent example is the U.S. production CSI:Miami that likes to drown in yellow-red Kitsch colors.
But back to our footage fresh from a GoPro HD Hero 3. Since Protune and WB raw touch the sensor data as few as possible it gives us room in post processing to achieve widely ranging color-grading according to our taste and needs.
Effect Group: Water Protune
In my previous article I showed you the basics of post processing Protune footage. So far, we worked on the three primary color channels red, green, and blue simulateneously. Most of the time this is sufficient for footage shot above the waterline. And it saves us unnecessary work.
This situation changes quite a bit when shooting below the waterline: one reason is the medium water that filters light very differently depending on its wave length (color). In addition, we have many small particles floating in the water that scatter light quite differently. And then our light from above also changes throughout the day quite considerably, giving us changing colors. Finally, don't forget your dive light that also adds its very specific light properties. In the end, we have to grade our colors individually with respect to the channels for red, blue, and green.
In order to save us some work, we first create a new Kdenlive effect: the Protune RGB effect. As our starting point we reuse the basic Bézier courve I introduced in the previous article. So, get yourself some video clip shot with a GoPro HD Hero 3; any small part suffices. Throw it onto the timeline in Kdenlive. Then select it in the timeline.
|New group for our Protune RGB effect.|
However, this time we need to change the channel from RGB to Red. In consequence, this curve will only affect the red channel.
Next, create a group for this effect. In the menu of the effect click on «Create group».
Then add two more instances of the Protune effect to this group. You do so by first adding the effects to your clip and then dragging them into the group. Set the channels of both to Green and Blue respectively.
|Effect group containing separate R, G and B curves.|
Not Pea Soup: Mountain Lake Lies
|Standard Protune color grading.|
When you apply the basic Protune RGB effect we just created then we will get the following result, as shown to the right. This example also has undergone the usual process of increasing color saturation and sharpening.
Well, okay, but still needs more tweaking. While the colors are more or less the correct colors, I'm not going to pay that amount of money for the diving license when all I get is this green tint. The weather this year wasn't exactly the usual and left it traces in the lakes. And as ex-chancellor Gerhard Schröder dyes his hair, so can we.
Simply grab the left handle of the top right curve endpoint with your mouse and drag it down a little bit. This will lower the whole curve, but still keep the endpoint intact.
There is no single, correct setting in this business for grading your underwater footage. Instead, you need to carefully grade each scene individually. Be careful to not reduce the green to much, otherwise you end up with a purple tint.
|Clear, blue mountain lakes.|
Not looking like a high alpine pea soup anymore.
Pike Fiction: Straussee I
The Short Programme First...
Well, not exactly too impressive.
Technically spoken, this ain't wrong colors. Believe me. This lake is what you see: green. In this situation we may just raise our shoulders and mumble something about documentary stuff. Or we might declare it GoPro HD Hero or Hero 2 footage. But instead of blaming something else let's see what we can get from this green footage.
|Adjusting green and blue carefully.|
There isn't barely any red color in the image. It's better to not increase red levels or otherwise we will get a brownish soup. Just leave the red curve as is.
But the green and blue channel are much more promising; when you look at the screenshot to the right, you will notice that I've...
- lowered the green curve quite a bit.
- slightly increased the blue curve, but only very carefully. If you overdo it you end up with a purple tint. The reason for this to happen: you keep red, add slightly more blue, and decrease green. Virtually more red and blue means a purple tint.
Don't re-adjust the same scene for too long a time. It won't get any better after ten or twenty seconds, but often it will get worse. So don't spend too much time on the final adjustments. Changes are you are only ruining the first good impression.
This rule of thumb is also very helpful in working your way from scene to scene at a constant pace and without getting stuck at one particular scene.
You may have noticed in the image above that the overall image brightness has been lowered. This is caused by lowering the green curve. However, often we want to keep the image brightness even when grading colors. As we already saw in my previous article, we typically need to adjust each scene individually, not least to reduce too harsh jumps in brightness between scenes. Another reason may be to carefully control the brightness in order to achieve a particular scene impression.
|Adjust brightness and contrast.|
This time, we apply the curve to either all three channels RGB simulatenously or to the Luma. The results are slightly different, as Luma works on weighted brightness values calculated from RGB, while RGB directly affects all three channels simulatenously without any weightings.
First, we grab the left handle belonging to the top right endpoint of the curve. Drag it to the left so that your linear curve gets a hunch. This increases brightness mainly in the high range, that is, where the image already is bright. Grab the other handle belonging to the bottom left endpoint of the curve and, this time, drag it to the right, but very carefully. This will make the darker parts in your image even darker. The result is that the audience will perceive this as increased contrast. This actually works out quite nicely for this particular scene as the increased back light will make the pike's shape more prominent.
|RGB/luma adjustment: slightly increased brightness and contrast.|
Comparing our result to the original pea soup it may be considered lying, but leaves much better impression. Like politicians, me thinks...
True Lies: Straussee II
|Where are the fishes???|
In addition, this example clearly shows how fast your effect stack can grow just on a simple scene.
|Example effect stack for grading underwater footage.|
If you look closely at the screenshot to the right you may also notice another Bézier curve. We use it for brightness and contrast adjustment, just as before.
But there is another effect: tint. Eh, so this is color correction after all?! Not quite. I'm using this effect just for the final touch. At least in my experience this effect can't cure any real, intensive tint.
To slightly correct colors, click on the pipette next to Map white to. Then click on a pixel in the project preview that you would normally consider to be white. With diving footage, air bubbles are good candidates to pick. After picking you may need to further adjust the color you've picked manually. Typically you will need to increase the brightness of the color picked, otherwise the tint effect will lower the overall image brightness. We also need to change the tint setting to 250.
|Here is Howard!|
Finally, I want to show you a really useful Kdenlive feature: you can copy all the effects from one clip (scene) to another one in the timeline. This way, you don't need to laboriously recreate your effects stack anew with each scene. Instead, for scenes that are similar, simply copy over the effects. First mark the source clip in the timeline and press Ctrl+C (copy). Next, right click on the clip in the timeline to copy the effects to, then chose «Insert effects». Done. Now you can tweak the settings for the new scene.
Kdenlive allows you to easily and professionally work on your underwater footage from a GoPro HD Hero 3. Even as Kdenlive doesn't come with fancy color-grading special effects, it has all the necessary basic tools that allow you to work very efficiently on your footage. I've given you only a small glimpse on what Kdenlive can do ... also because I'm still learning more and more each time I'm using Kdenlive.
In this article, we've used these video effects for image grading:
- Group containing these effects:
- Bézier curve for red channel to umcompress the Protune tonal curve,
- Bézier curve for green channel, and
- Bézier curve for blue channel.
- as required, another Bézier curve applied to RGB simultaneously to adjust brightness and contrast.
- as required, a tint effect, for the final grading touch.
- Saturation (saturation: 200),
- Sharpen (amount: 700, size: 20).
As usual, useful feedback is appreciated.