etinSeven.com presents the ultimate checklist to get rid of unwanted spots, blotches and splotches when using VRay's renowned irradiance map global illumination method. Newcomers to planet VRay will keep feeling the panic at their first encounter of the blotch kind and the same questions will continue to be asked. We provide the definitive survey of solutions ...

The splotches can more or less be divided into three categories:

1: General smooth artifacts in indirectly lit areas of a render.

This phenomenon can roughly be compared to the noise that occurs in areas of a digital photograph where not enough light was present to capture a clear snapshot of that area. In other words: it is simply a shortage of light information, resulting in image coarseness in the areas that are insufficiently lit.

Solutions:

• In the VRay render settings turn up the amount of hemispheric subdivisions (GI sampling detail), lower the color treshold values and normal treshold values, for a more detailed evaluation of bounced light in the scene. If necessary raise the interpolation samples for more smoothness. But don't raise the interpolation samples too much or you will lose detail in your GI.

• In the VRay render settings adjust the QMC (Quasi Monte Carlo) sampler parameters: lower the noise threshold and the early termination amount and raise the minimum samples.

• If you can afford to, add some subtle extra light to the scene, reaching the darkest problem areas.

• Try to avoid using the native 3ds Max Metal shader. It can cause VRay splotches as well as increased rendertimes.

• If you're creating animation, the incorrect GI mode for animation can cause noisiness and blotchiness in the GI solution, usually due to changing elements in a scene. Use the fly-through mode only if your camera moves through a non-changing environment.

• Purchase a so-called denoiser (usually affordable) and use it to effectively filter low frequency noise (subtle large blotches), medium frequency noise and/or high frequency noise (fine grainy noise) from your imagery. Photoshop CS 2 and subsequent Photoshop versions offer a native noise reduction filter, but the best denoiser I know is Neat Image - www.neatimage.com. It comes with a Photoshop plug-in, so you can access Neat Image as a Photoshop filter. Experiment with the various settings until you've got an effective noise reduction result that does not sacrifice image details. Utilizing this approach you can considerably reduce rendering times using lower render settings and then relatively easily post-process the image to smoothness using a qualified denoiser. It might save your ass when the deadline monster is heavily breathing in your neck and it works great for denoising your digital photographs as well.

2: Smooth artifacts in problem areas of a render (hard edges etc.).

Solutions:

• Check if every element that's present in the scene has at least some thickness. Objects without any thickness can be a cause of the smooth bright 'light leaks'.

• Try to avoid using the native 3ds Max Metal shader. It can cause VRay splotches as well as increased rendertimes.

• In the VRay render settings turn on the "Check sample visibility" checkbox in the irradiance map's advanced parameters rollout. This can especially be a solution if the irradiance map has a low resolution. It works by checking if the sample points in the irradiance map are actually visible from each point before using them in the interpolated lighting solution.

• In the VRay render settings turn up the amount of hemispheric subdivisions (GI sampling detail), lower the color treshold values and normal treshold values, for a more detailed evaluation of bounced light in the scene. If necessary raise the interpolation samples for more smoothness. But don't raise the interpolation samples too much or you will lose detail in your GI.

• In the VRay render settings adjust the QMC (Quasi Monte Carlo) sampler parameters: lower the noise threshold and the early termination amount and raise the minimum samples.

• Under the VRay render dialog System rollout, lower the "Face/level coef." value. This will improve the GI analysis in problem areas such as corners etcetera. There will be a slight rendering time increase as you lower the value.

• If you can afford to, add some subtle extra light to the scene, reaching the darkest problem areas.

3: Harsh artifacts.

Can occur when highlight levels are being pushed too far beyond white and/or when certain mangled UVW coordinates of bitmapped objects cause bouncing light rays to become 'confused'. Another cause for these kind of artifacts to become visible can be tinted glass.

These kind of blotches do not necessarily occur where the highlights are, but can occur at seemingly random positions in the render and can also be regularly shaped.

Solutions: here we go (please keep in mind that the following solutions might also help to get rid of the previous splotch categories) ...

• Activate all "Apply" checkboxes in the Cropping/Placement section of your bitmaps (no need to actually specify a cropping region), or run the free MaxScript that automates this, available at www.varcht.com/tools.htm, or even better: install the special recompile of the Max version 5.x bitmap map, that can be downloaded from the registered users area at www.vrayrender.com.

• Don't use the default Max Raytrace material or map (use a VRay material or map in stead).

• Try to avoid using the native 3ds Max Metal shader. It can cause VRay splotches as well as increased rendertimes.

• Don't use the default Max Reflect/Refract map (use a VRay map or material in stead).

• Be careful using the free plug-in map ColorCorrect by Cuneyt Ozdas. Although ColorCorrect is an extremely useful map, when used with VRay it might cause an output surplus of your input color or input map, resulting in the return of the dreaded artifacts.

• Avoid overbright highlights: use a shader like Phong whenever possible and/or try to limit your shader's specular and glossiness values.

• Try turning off "Reflective GI caustics" in the VRay renderer settings (it's off by default). It can cause overbright highlights, especially when using a bright HDRI map for your lighting.

• Limit your map output (do not raise your map's output level to higher than 1.0 and do not raise the map value in front of the map slots to higher than 100). Be especially careful with bright HDRI maps. These can be a considerable cause of the dreaded blotches.

• Be careful using self-illuminated materials, especially if their output is multiplied.

• Make sure that any object with a material using explicit mapping contains proper mapping coordinates. If any meshes are imported from another program with mapping coordinates, try applying UVW coordinates within Max, by using the UVW Map or UVW Unwrap modifier.

• If you have got colored glass in your scene, try hiding the objects that contain the tinted glass material. The involved materials can be the cause for colored harsh spots in your scene. If this turns out to be the cause, try changing the settings of your tinted glass (preferably use VRay materials for everything and in stead of using the diffuse color use a subtle fog color to tint your glass, and set the diffuse color to black).

• Make sure that you are not reusing the irradiance map that was calculated for your previous rendering(s). Uncheck the "Don't delete" checkbox in the Advanced Irradiance Map settings. Also make sure that the single frame GI calculation method is checked, so the irradiance map is calculated from scratch with each rendering.

• In the "Object settings" submenu of the VRay renderer's System rollout, turn down the "Receive GI" value of the object(s) showing the harsh white blotch(es) and if necessary the "Send GI" value of the object(s) that are very shiny and/or reflective in the scene.

• If you can afford to, try changing the angle, distance and/or strength of the lights in your scene. This also goes for the offset of environment-based lighting, especially when using HDRI maps. A slight difference in the lighting situation can prove the difference between splotches and no splotches, as the bouncing light rays will travel along a different path before they reach your camera.

• Try to trap the problem cause of the artifacts, by hiding suspected problem-causing elements from your scene and rerendering until the artifacts have disappeared.

• If all this still doesn't work, then you can try using a different GI method than the irradiance map.

Hope the above helps you to lead a healthy, prosperous, blotchless life of rendering with VRay. If not, you can always go and create an evil splotch character within the Max environment, animate its visibility track to 0 and render it with VRay to exorcize the evil artifact ghosts out of your renders. Let's call that voodoo debugging.

Cheers,

Metin Seven

www.metinseven.com
www.sevensheaven.nl
www.figurefarm.com