With kdenlive 0.7.8 I added some color scopes, used for displaying color information. In this section I will give a brief overviews over scopes in general and explain the most basic scope, the histogram, in detail.
Color correction. This is a really important topic in video editing. It starts with simple stretching of the tonal range if the brightness is not ideal, goes on with white balance to ensure that white remains white and not blue, and finally ends with creating looks which make your video look unique. (Remember the blueish Minority Report? The contrasty The Departed?)
For color correction we basically need two things, Effects for changing the colors and Scopes for monitoring the changes. The first scope I'm showing now is, as already mentioned, the histogram:
Read more in the full article …
Let’s first take a look at the basic options available in all scopes.
During the process of color correction you’ll want to keep this option enabled. When not color correcting, it should be disabled as it usually heavily impacts the performance of playback. (There is a lot of calculations going on in the scopes.)
Note that you can always update a scope by clicking on it.
So far about scopes in general. Now let’s take a closer look at the Histogram.
When the Histogram receives an updated image from one of the monitors, each of these pixels consist of a Red, Green, and Blue component. Each of these values lies within a range of 0 and 255, which are the numbers you can represent with one Byte. 0 means that the component is not shining at all (i.e. it is black), 255 means that it is shining as bright as possible.
The Histogram is merely statistics; it shows how often a component of a certain brightness occurs. So what the Histogram then does is actually quite simple:
Example: If the red value is 0, increase the height of the bar at position 0 (that is at the very left) of the histogram by 1. If it is 42, increase bar 42 by 1. And so on.
The Histogram only shows the distribution of the luminance of the selected components – nothing more, nothing less. Also when looking at the RGB channels separately, instead of at the calculated Luma component only. You cannot really guess the colors in the image.
Really? Yes. Take a look at these two images.
Exactly the same Histogram. Totally different colors. (What you can do is guessing the color tone; see below.) But what is the histogram good for now?
To answer this question, I would like to point an article from the «Cambridge in Colour»: Understanding Digital Camera Histograms: Tones and Contrast and the second part Luminance & Color. Although written for digital photo cameras, exactly the same applies for digital video cameras. Both articles are easy to read and understand (and may also be of interest for experienced users).
Two special things about this histogram.
Nevertheless, the blue component does reach 0. This means that the darkest pixels are still slightly orange and didn’t lose all color information yet.
In this case the candles cause the clipping. (Not too bad here, because the lost detail isn’t important for the image.)
The RGB components also show very well that the shadows are not neutral grey but orange, otherwise the color heaps on the left would, as in the gradient histogram above, have their center at the same position. There isn’t a lot to correct here, what could be done is raising the shadows with a Curves effect, but this is a matter of taste and the intended mood for the final movie.
We immediately notice two things:
Note that the Histogram is not very accurate for white balance. Later I will introduce a much more accurate scope.
Monitoring correct exposure is the Histogram’s strength! The exposure can be corrected with curves as well, but this time I will use the Levels effect.
I’ve lowered the Input white level of the Luma channel until one of the RGB components reached 255. Lowering the input white level further would cause clipping on the wall and lost image information. (Which may be desired in certain circumstances!)
This process is called Stretching of the tonal range.
In kdenlive 0.7.8 the histogram can be adjusted as follows:
Y' = 0.299 R' + 0.587 G' + 0.114 B'
Y' = 0.2126 R' + 0.7152 G' + 0.0722 B'
Most of the time you will want to use Rec. 709 which is, as far as I know, mostly used in digital video today.
The sample files used above can be downloaded here:
The Histogram is a great tool for exposure correction, together with the Curves and the Levels effects. It helps to avoid clipping (burned out areas) and crushed blacks (the opposite) when applying effects.
Thanks for reading! Continue with the Waveform and the RGB Parade.
Please drop your comments below.
Simon A. Eugster (Granjow)