Lacq’ing in Depth

Some people think that video color correction should be exactly what the name says, strictly limited to correcting problems that couldn’t be fixed while shooting: matching color temperature between shots, adding a bit of gain to lighten harsh daylight shadows, and so on.

This seems to be an ascetic school of thought: rather than indulging themselves at the smörgåsbord of digital color manipulation, they refuse it completely and denounce it as destructive to the pure spirit of the transparent cinematic image, or something along those lines. Yet there’s a solid reason why so many directors, colorists and video artists go for those extreme color looks — it helps them deliver the story or emotion better.

The image below is from a DV video shoot. This shot ought to have some kind of emotional impact, but instead it’s severely hampered by its technical shortcomings. The only story it’s telling me is: “Two people are pretending to have sex on a theatre stage… And is that a bluescreen visible behind the velvet curtain?”

I didn’t have much of a vision as to what should be done with this shot, so I decided to try tossing around some “extreme” nodes in Conduit to see what comes out:

High contrast overexposure, grain, and a golden yellow tint — a bit like a Tony Scott movie set in France, perhaps.

Looking at the nodes on the right, you can see that I’m “misusing” the Cineon to Linear node for color correction purposes. Although it’s a tool intended for a very specific colorspace conversion, we can take advantage of its interesting logarithmic tone curve to pop this image’s highlights into High Dynamic Range superbright stratosphere.

The rest of the nodes are easy to explain: Channel Mixer node does the yellow tint, and Over combines the blurred noise with the HDR image. Because the result is HDR, at the end of this conduit, a Linear to Video node is used to apply display gamma to the image.

Next, I wanted to try mixing a different color into the highlights, to make this image less uniform in color. What I got is basically an “inverse glow” — the image is blurred and tinted, and the result is subtracted from the original image. The resulting juxtaposition of deep blue and yellow/orange creates a nice contrast in my eyes (although it might look quite different on your monitor, the blue is frail):

In order to knock down those overexposed highlights, I added a Highlight Knee node. This node’s sole function is to round off superbright HDR highlights in a pleasing way — here is what it looks like when the knee value is set to about 0.8:

Yeah, I think I prefer a more peaceful look to the previous overexposed mayhem. Let’s take it further and use a Saturation node to desaturate the whole image… With the added trick of using the original image’s luminance to drive the saturation amount, so we get non-uniform desaturation:

See how the Saturation parameter is driven by an image value, rather than a plain number value? This effectively gives pixel-level control over the effect, and can be really useful for building more complex effects. Almost all nodes in Conduit allow their parameters to be controlled by images in this fashion — some accept color input, some only scalar (i.e. greyscale) like Saturation here.

I applied this trick a second time, to drive the Levels node by a previous Gaussian Blur. I also played with the blur size a bit, to see if this desaturated look could be better with a larger blur. This is the final image (roll over with mouse cursor to see the original):

Do you like it, or is it too extreme? To me, this has a “Paris 1968″ kind of mood. (But then again, I was born in 1980, so what would I know about the Sixties…)

It’s important to note that this look depends entirely on Conduit’s pervasive High Dynamic Range support. It uses both superbright values — the Cineon to Linear node is generating values that go over 10.0 — and negative values (in the subtracted blur). For comparison’s sake, below the same image without HDR values at the start, that is, the Cineon to Linear node is set to clip to standard 0-1 range… Roll over to see the clipped version:

(The conduit for this effect is available for download here. If you just want to have a look at the nodes, here’s a picture — I’m not putting it inline because it’s quite large.)

If you’ve used Avid editing systems, you’re probably familiar with the “film dissolve” transition effect that is offered alongside regular video dissolve. Film dissolve is supposed to morely closely approximate the look of a traditional optical dissolve effect created by printing film frames on top of each other at varying exposures.

I don’t claim to know the algorithm actually used by Avid, and it’s been a while since I’ve used their systems… But logically something similar should be achieved by blending the images in linear light colorspace, as done for the glow in this previous post.

Because Conduit can be used as a transition filter in FCP, it’s easy to roll our own dissolve effect. Let’s try it with these two still images first:

Here’s a frame from the middle of the dissolve. The first is the video dissolve — just a Blend node at 0.5 bias. The second is the same thing with linear light conversions applied:

The difference is fairly clear: in the video version, the station image is obviously cut with black, and the hands are much more pronounced with a dull blue tint to them. Overall the linear version looks substantially better to me — the first image kind of screams “digital layering” (and that’s just so ’90s, you know).

As a little detour, this is a good opportunity to show what the Field mode in Conduit’s plot window is good for. Unlike Curve and Cube plots, the Field plot is only useful for the specific purpose of examining a transition effect. Here’s the Field plots for the video and linear dissolves:

The difference between the two maybe looks more enormous than it actually is. You have to remember that this plot is HDR; it is drawn in the range of -0.5 to 1.5 (as shown by the input boxes in the corners). Only the area fenced by the grey lines in the middle shows us the standard color range (0 to 1), the sides and corners are actually HDR blending. So the bottom-left corner shows the result of blending two negative input colors. The result is something negative, which is of course displayed in black. Likewise, the top-right corner shows the result when both colors are above 1, which displays as white.

Because we’re not dealing with HDR images here, we can just focus on the standard range in the middle of the field. In video mode, it looks just like we’d expect: a linear gradient from black to white. The linear version is more like a radial gradient with a rounded shape. The most important difference is seen if you look in the top-left and bottom-right corners of the standard range. At these points, the other image is full white (1) and the other image is full black (0). In linear light colorspace, this results in a much brighter grey than in video colorspace.

Well, enough with the Field plot… But please comment if you’d like to have a more detailed explanation of its workings, I can make a separate post about it.

The linear light blend is often going to have even more of an advantage if we do any color correction on the input images. On the right are the two blending paths used for the previous example, but with an extra Exposure node tossed into the mix on both sides. The Exposure node brightens the hands image so that its highlights are close to 1, and adds a bit of yellow tint.

The result images are below. This time the transition is at opacity 0.65. Rollover with mouse to see the linear version:

Click here to download the .conduit file for this effect

I know it’s not easy to get an idea of how the entire transition looks from just a few stills. I’d like to show a video comparison with some moving footage, but I’m running out of time now… So perhaps that will come later.