Tutorial 6: Specular Bloom

Ever seen a picture of a car in a showroom or on display at a car show where the chrome and other really shiny parts were reflecting lights that were so bright, they had a halo or glow around them? This is the type of effect "Bloom" performs. Bloom is independent of Gaffer and is applied as an "Image Filter." It gives you the option of adding this halo to the bright parts of your image, or even limiting the effect to just the surfaces you specify.

Load "bloom_urn.lws", our classic urn scene. The urn surface is a bit different, with more of a chrome look than previous tutorials, but the lighting setup has been unchanged. Render a frame to see how this looks.

Now we'll start playing with those bright highlights. Start by dragging out a limited region box that fits the urn's bounding box. This will give us a closer look at the urn surface in Bloom's preview, and it will cut test render times if any are needed. Open the effects panel and go into "image processing". Under "image filter plug-ins" you will find Bloom. Add it to your scene, and render.

Render a frame. Oddly, you'll see no difference in your image! Bloom is a bit strange in that its default behavior is to do nothing. This is because its interface gives you a preview of your image, which requires a previous rendering. Usually you'll add Bloom, render, and only then go into Bloom's options to set up the effect you want.

Return to Bloom's interface. Deselect the "Disabled" button to activate the plugin, and select the"Full Preview" button. This gives us a view of what the plugin does to our image. This makes it extremely easy to design your effects!

Start by experimenting with the three RGB cutoff controls, concentrating on the lower, "Source Preview." This preview shows you what sections of the image are going to have the Bloom halo around them, while the "Full Preview" shows what the final output will look like. For this example, cutoff values of 75% selects areas that are from the specular highlights.

The bloom shape and size is controlled by other parameters, especially the "Bloom Radius" and "Falloff Shape" controls. The upper preview shows the effect of these controls as you experiment. For this example we want to slightly exaggerate the bloom effect, so let's set "Bloom Radius" to 100%, "Fall Off Shape and Brightness" to 50%, and "Red-Green-Blue Cutoff" to 75%-75%-65%. Leave "Bloom Intensity" at 100% and "Gamma" at 50%. Take a look at the preview. Not too bad! Again, with the preview, it's easy to see the effects of the controls.

One problem that shows up is the ground. It's fairly bright, but we don't really want it to be blooming. We can restrict the blooming to stay only on the urn by using LightWave's "Special Buffers" to flag only those surfaces that we want to bloom. Click on the "Use Special Buffer" button, and go back to LightWave. We need to flag the surfaces we want to bloom, so visit the "Urn" surface. Click on the "Special Buffers" button at the bottom, and you'll be presented with a simple control that says simply "Bloom". Enter a value of 255. (These values are always between 0 and 255, with 255 being "full strength"). Re-render the frame.

Now back in the Bloom panel, you can see that only the urn has bloom effects applied to it. You can even toggle the "Use Special Buffer" button to see the difference. If you ever change the value of a surface in LightWave, you'll need to re-render to "tell" Bloom about the value, however.

As you can see, Bloom is not difficult to use, mostly because of its built in preview. There are a lot of sneaky tricks you can experiment with, especially when you use cutoff values of 0%, which turns the Bloom effect into a blur tool that affects all of your image.