Faking light groups of LuxRender with YafaRay and GIMP

One of the most interesting features of LuxRender is the option to control light by groups. With this feature we can separate several light sources into groups, and change the intensity, temperature and turn them on and off during the render. It allows us to produce several different renderings from a single image, and save lots of render time. For instance, we can have in the same rendering lights representing daylight visualization and other lights for a night shot.

There are other tools that have similar options such as Maxwell Render and their Multilight feature, and even V-Ray and Mental Ray with the use of a MAXScript in 3ds Max. But, what if we could fake this effect using nothing more than GIMP or any other image editor? I was browsing through a SketchUp user forum this week when I found an interesting tutorial about the setup of a scene using SketchUp, Twilight Render and GIMP to fake this type of effect. Almost immediately I fired up Blender 3D and YafaRay to test it and the technique really works.

Here is a short video showing the results:

Faking light groups with YafaRay and GIMP from Allan Brito on Vimeo.

Visit vimeo to watch the video in HD.

As you can see the technique is quite easy to fake in GIMP. The secret to do that is a render for each light source present on the scene. After you have a different render for each light, organize them all in layers at GIMP and change the blending mode to addition.

Now, it's only a matter of changing the opacity of each layer and turn on and off them. As you can see on the video, the effect is quite similar to light groups, where the lights can be turned on and off, and the intensity controlled.

To complete the effect, we can use screen record software such as Camstudio, mentioned on the linked tutorial to create a video showing only the changes made to the lights. If you have to create different sets of lights for a visualization project, this technique will save a lot of render time. Just render one image for each light and do the composition in GIMP.