Coswell Productions

LVPE: Color Correction in Cinelerra

(this post is part of the Linux Video Production Experience series, which chronicles my experiences with creating a high-quality home movie almost entirely in open source software.)

For my past videos in iMovie and Cinelerra, the only color correction I ever did was slap a Brightness/Contrast filter on a video track, adjust it until it "looked" right, and then made a DVD, wondering why my colors and lights/darks looked crappy. Then I did some research into how color correction in video is supposed to be done, and what specifications MPEG files have for color levels, and I started doing it right. This time, when I put in my final DVD into a DVD player and watched it on a TV, I was absolutely blown away at the clarity and quality of the video. I'll see if I can compile together enough of what I've learned about basic color correction into this article.

If you look up color correction articles on the Internet, you'll find lots of cool demos with Final Cut Pro's Color Corrector 3-Way filter. That filter and the Brightness/Contrast filters in FCP do a fantastic job. In Cinelerra, we can get pretty much everything you get in FCP for simple color correction, except for some of the abilities to adjust highlights and shadows separately (as far as I know).

Meet the Cinelerra VideoScope:

Cinelerra VideoScope

This will be your Weighted Companion Cube for this part of the editing process. The VideoScope will never threaten to stab you, and in the event that it speaks to you, disregard its advice. No, wait, I mean listen closely to the VideoScope, and it will tell you just how to fix your video. Place the VideoScope effect across the entire track of the footage you want to correct, and place it at the bottom. If you haven't done so already, place Brightness/Contrast and Hue/Saturation there too, above the VideoScope track. Also, turn off Background Rendering for this part of the process. It can mess things up.

Here's a bright, but somewhat muted scene that I want to make much more even levels-wise:

Cinelerra Color Correction - Before

In the VideoScope window are the two scopes: the waveform scope and the vectorscope. The waveform scope (left) shows me the light levels in each column of the video frame, with pure white at the top and pure black at the bottom. The vectorscope (right) shows me the colors in the frame, spread out around the color wheel. The closer to the outer edge, the more saturated the color.

With this scene, notice how, in the waveform scope, almost no pixels dip down below the 10% mark. I want to have all of the outliers at the bottom get clipped below 0%, and have the lowermost bumps in the solid areas just barely scrape the 0% mark. I'll also need to make sure the bright parts stay at the 100% mark. That will make the brightness and contrast in the scene relatively even.

Additionally, the colors aren't quite as bright and saturated as I'd like. In this production, I just bumped up the saturation on all of the movie footage by a small amount. I only did Brightness/Contrast adjustments on individual clips.

To see how Brightness/Contrast affects the waveform scope, open both and start adjusting. Contrast compress/stretches the values, and Brightness shifts them up/down. For Hue/Saturation and the vectorscope, Saturation compresses/stretches the dots around the center of the circle, while Hue rotates the dots around the circle.

After adjusting the Brightness/Contrast for this clip, and enabling the global Hue/Saturation adjustment, here's what the frame and scopes look like:

Cinelerra Color Correction - After

If you want certain effects, the scopes will help keep those consistent. For instance, for all outdoor winter scenes, I kept the data in the waveform scope between 10% and 90% instead of 0% and 100%, to give the outdoor scenes a much more muted, wintry look, and to make the indoor scenes bright by comparison:

Cinelerra Color Correction - Wintry Outdoors

Due to the way that Cinelerra handles effects keyframes, if you want to use a single effect across different clips, and apply different filter settings to each clip, you'll have to do this little dance:

  • Disable Keyframe Tweak Editing (the "key" icon in the main toolbar).
  • Move to a point in the middle of the clip and start editing.
  • Enable Keyframe Tweak Editing.
  • Put the playhead at the space between the clip you want to color correct and the next clip.
  • In the Effect setting, pick a slider and click the mousewheel up one, and down one. This sets a keyframe and that point.
  • Move back one frame (Keypad 1) and repeat the mousewheel action.
  • Your clip that you just corrected now has the same color information at the start and the end of the clip. If you need to fix the colors, you'll have to adjust both the keyframe at the beginning and the end of the clip. Fun. :/

(Note: A lot of things in Cinelerra need to be done this way, simply because you're modifying keyframes, not clip properties. Unfortunately for effects, you can't easily copy properties or affect two keyframes at once with your changes. You always change the keyframe to the playhead's immediate left, unless Keyframe Tweak Editing is enabled.)

Once all of your color correction is done, if you're exporting to a format other than DVD, you probably don't need to do anything extra. However, if you're exporting to a DVD that will play on a Standard Definition NTSC TV (SDTV), you'll want to drop one more filter at the bottom of your filter stack, above VideoScope, called RGB-601. Due to the way that analog NTSC TV handles color, you don't get to show all 256 RGB values on a TV, only 219 of them. When you're exporting your final files for encoding to DVD, you need to reduce the contrast using the RGB-601 filter, otherwise the brightest and darkest colors will get clipped. When the DVD is played back, the contrast will be restored, and everything will look crisp and bright.

See what else is here...