Lacq’ing in Depth

Building a simple highlight glow effect is very easy using Conduit nodes:

First the highlights are isolated with Levels, then blurred, and finally composited on top of the original image using Screen blending. Screen is a familiar operation to all Photoshop users: it’s similar to Add, but whereas Add easily results in oversaturated colors and clipped whites, Screen effectively cuts the top layer’s opacity as its color values approach white, thus resulting in a more pleasing appearance.

This is what it looks like:

Well, it’s a regular glow, not too terrible for an effect that can be created with three nodes and easily reused whenever you’re in the mood to add some ’70s styling for your productions. Now, you might recall that Conduit is a cutting-edge floating point HDR compositor. Perhaps you’re asking yourself: “What if we used linear light for this effect, like all the cool people are doing nowadays?”

“Linear light” essentially means working with your images in a colorspace that closely resembles the original light intensities seen by the camera. Many people think that compositing with a linear light workflow results in more organic imagery that’s closer to traditional photographic processes, as opposed to the “digital video” look.

So let’s put it to a test by converting the glow effect to linear light. At the top and bottom of the conduit, we now have the crucial Video->Linear and Linear->Video nodes that handle the colorspace conversion between regular “video” color and linear light. The Levels and Gaussian Blur are the same, with one important difference: the Levels node is no longer marked in red, which means that its output is no longer clipped into the standard 0-1 range, but instead it outputs color values that go below zero and above one — that’s possible with floating-point color.

After the Gaussian Blur, there’s now a Clip node which clips those floating-point values back into standard range before they’re composited over the original image. Otherwise the negative color values will have some unexpected results (which I’ll show later).

The last change done is to replace the Screen node with Add. The Screen algorithm assumes that color values are strictly within the standard 0-1 range, so we can’t use it in a HDR effect. Instead, we’ll just use Add — in fact, Screen was originally introduced in Photoshop with the aim of simulating the look of images added in linear light, so in this case Add is truly “the real thing”.

Ok… So how does it look? Roll over with mouse on the image below to see the linear light version.

You don’t necessarily have to be impressed with the linear light version — there’s no right or wrong answer here, they are just different looks… Also, this example was deliberately subtle. Let’s try it with another image. Original:

With glow (again, rollover with mouse to see linear light version):

I find the linear light version nicer because it preserves the highlight detail, making the sky look more realistic and overall it has less of a video filter appearance. But sometimes a strong visible glow may be just what you want… You can download the .conduit file and try it yourself.

Oh, what about the linear light version if Blur’s output wasn’t clipped, which I mentioned above? Here’s what it looks like:

Without clipping, the negative color values created by Levels are propagated down to the Add node, resulting in these deep dark areas. (Obviously, when negative values are fed into Add, it becomes Subtract.) So watch out for those rogue negative HDR values!