Preventing Blotches and Artifacts in your Renders
Blotches and artifacts in an SU Podium render can generally be attributed to issues in two broad categories:
Sampling errors related to render engine settings:
- Blotchy lighting due to preset choice, artificial lights, or Podium settings.
- Unexpected artifacts from incorrect Podium settings - Activating "caustics" when it is unnecessary, for example.
- Jagged edges due to low anti-aliasing settings.
- Incorrect shadows (in vegetation especially) due to insufficient "light bounces."
Issues due to modeling:
- SketchUp faces are reversed, i.e. the backside is facing the camera instead of the front side.
- Walls, ceiling, or floor were modeled without any thickness.
Please read below for descriptions and examples of each of these issues:
SU Podium uses a biased rendering algorithm. This means that every attribute (light bounces, shadows, reflection, anti-aliasing, etc.) is configured with a minimum and maximum number of "samples" for each given preset. This ensures a specific quality level, and at the same time keeps render times within a reasonable range.
Blotches occur when the max light bounce samples in the selected preset are not sufficient to render a specific scene. For example, this might occur if you use an exterior preset to render an interior scene. This is because exterior scenes require fewer light bounces than interiors, and have a lower maximum value for that attribute.
Blotches can also occur when creating a render with the "preview" or "default" presets. Switching from "interior default" to "interior high" will eliminate blotches in the majority of cases, at the expense of increased render times.
(Talk about QMC here, noise, and resolution here.)
Faces in SketchUp have a "front side" and a "back side," with the back-side typicaly represented with a slightly darker bluish-gray color. As a general rule, the backside of a face shouldn't be visible in your render. So if you have an interior model, and three of the four walls are oriented properly, but the fourth is reversed, you will see something like this:
And this can cause all kinds of sampling errors, because it means the "surface normal" of that wall is facing the out when it should be facing in. So we get results like this, with blotches and what we call "light-leaks."
If we right click on that face and choose "reverse face" in the contextual menu, the sampling errors should immediately be resolved, as seen here:
Modeling walls without any volume/thickness can cause very similar issues. A face in SketchUp is simply a planar surface that connects four vertices, and mathmatically it has no thickness. This causes problems during raytracing, and often results in errors in the final rendered image.
A wall in the real world, even a thin one, has some volume. To prevent sampling errors, model your walls with some thickness. Something on the order of 20-30 centimeters is generally good practice.
If we re-render this scene, the blotches and light leaks should disappear: