|Rendering Mattes |
Software: Autodesk 3ds Max
Author: Matt Richardson & Cary Ng
Mattes are used to aid in the isolation and selection of specific areas of an image for post-processing in a paint or compositing package. The term matte describes a template one uses to delineate an area, similar to how artists use molds to mark an impression to be manipulated. In Photoshop, any grayscale image loaded as an alpha channel or a clipping mask can be used as a matte, ready to be color-corrected. 3ds Max offers the flexibility to create mattes associated with 3D, such as Alpha, Object & Material ID, and Z-depth. It also offers the ability to create more specialized mattes such as RGB mattes, Normals, RawTotalLighting, and other Custom mattes that target specific areas to be adjusted in post-production.
Alpha / RGBA Mattes:
The simplest matte is the Alpha, which is usually the negative space around the 3D scene. In this living room rendering, the alpha matte is the area outside of the windows where the background view, a 2d image in this example, is dropped in.
Living room rendering
Area outside of the windows is the alpha matte
Creating RGBA mattes, or renders comprised of the primary red, blue and green colors, allows one to render three or four mattes at a time. We created RGB mattes for our Living Room scene to isolate the floor, rug, ceiling and walls - all within a single rendering.
In 3ds Max, simply create standard materials with primary colors in the self-illuminated slot.
These colors are used as mattes in Photoshop by loading the color channel as a selection.
Render Passes as Mattes:
To enhance the airy feeling of light as it recedes in depth, or atmospheric perspective, the final Living Room rendering utilized several additional render passes as mattes.
The Z-Depth pass was used as a selection to color-correct the black and white values in the scene from the front of the space near the camera to the rear where light is streaming in through the windows.
Similarly, selections created by targeting specific color ranges in the Normals pass give an artist the flexibility to color-correct based on facing angles. In the Living Room the normals pass was used to highlight the sofa area. The faces facing upwards were isolated and color-corrected to enhance the sheen on the fabric.
The RawTotalLighting pass is used in a similar way, for instance, to tint lighting effects from light sources or to enhance / dull their effect based on the falloff of the 3d light source.
Since there are countless ways to create selections in Photoshop, custom mattes can be created to target specific areas. Every grayscale or color rendering contains information from which to create mattes. The task of targeted color-correcting begins by thinking about how it is best to isolate that area to be corrected, whether it is based on its geometry, facing angle, proximity to light, color, or other parameters.
Custom rug as main feature
In our last example, a simple hotel corridor has a custom rug as a main feature, and the colors needed to match the real-world sample. Due to the color effects from global illumination, the rendering did not match and color-correction was needed. To make matters more difficult, the carpet pattern was irregularly shaped and the three colors within the pattern needed to be corrected separately. The colors couldn't be isolated easily in Photoshop in the 3D rendering because there were differences in light and shade as the carpet passed through several areas with different illumination and color bleed. A custom matte was created to help isolate the colors by rendering the diffuse map in the self-illuminated slot.
The flat evenness of this utility rendering allowed colors to be selected separately based on the color range, such as the aqua colored inner border.
For a recent project, we had a match moved shot where we filmed on green screen a woman walking up a set of stairs with our camera on a crane following her up. We then wanted to composite her into a scene where she would be walking up a 3d staircase instead.
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Before rushing in and starting to render out pieces that will be used as mattes, let’s take a step back and assess the problem at hand. Brute force tells us that whatever 3d objects are between the camera and where we want to place our woman will need to be rendered as mattes.
But what if there are certain objects that exist both in front of our subject and behind them? Now we will need to go into the actual geometry and start separating faces, which is not really a great approach. Also, the actual stair thread will need to be treated in this same way where the bottom and side faces will be turned white to occlude our subject while the top of the thread would remain black. Depending on how complex your scene is, these steps can be quite time consuming. Only if there was another way…
By taking a different approach, the same end result can be achieved with much less effort. Instead of turning some objects white and some objects black to be able to render out mattes, what if we turned all of the geometry in our scene black and then put in a white card that exists only where we want our woman to go?
With this, we don’t have to break up any geometry. It also allows us to be able to reposition our subject to a different part of our scene without having to reassess which objects are occluding them.
While a more traditional approach would use objects that are rendered white to occlude something, this approach uses the white area of a render to include something. By thinking from this point of view, you can sometimes save yourself a great deal of time.
Download Scene File:
Scene File [4.78MB]