A Primer of Rendering Terms and Techniques· Radiosity, global illumination and HDRI. Radiosity and global illumination are terms that are often used interchangeably. In essence, they involve calculating the physical light bounces within a scene, taking into account the energy of the materials in use. The biggest benefit is the ability to create soft shadows and diffuse lighting. The biggest downside is that render time can be very long. (Note that some new render engines use brute force, at the expense of accuracy, to generate some fairly nice renders in reasonable times.) HDRI (High Dynamic Range Image) is a popular technique for use with radiosity. HDRI contains illumination information that goes outside the typical 255 RGB scale and is exceptional for creating realistic reflections. In combination, using radiosity, HDRI, and standard rendering techniques can generate excellent imagery. Couple these with Photoshop or your favorite image editor and you have the tools for achieving exceptional realism. · Alphas and compositing layers. Alpha masks are grey-scale images that tell the image editing software which areas are “masked” and which are not. This technique is useful for isolating areas of your render for later tweaking. For example, I use alpha masks on areas where I want to achieve effects like blur or hue/saturation edits (excellent for exploring color changes). Compositing is the art of taking images and blending them together so that they seem as one. For example, I may chose to place the flashlight we produce in this exercise on a wood table. I would then render a table and composite the flashlight onto the table using an application like Adobe Photoshop or Eyeon DFX+/Fusion. · Material channels. Some material properties that are common with most rendering software include: - Color. Color of the object. - Diffuse. Level of “diffusion” the material exhibits when lit. Some software approximates this attribute with a property called “ambient” or the color the material exhibits when not lit. (Imagine a red ball in a dark room.) - Luminance. If an object emits its own light (e.g., a light bulb or neon tube), you would set a luminance value of 100% to eliminate effects on the material from lights in the scene. - Transparency. Opaqueness of the object. - Reflection. Ability to reflect objects within the scene. - Bump. Way to simulate subtle texture on a surface (such as machining lines) without deforming the object. - Specular. Simulates the hot spot reflections on an object. (Note: For the utmost realism, use an HDRI for reflection. If supplementing the true reflection, set the specular value low with a wide setting.)
Here are a few guidelines for using the material channels.
- Typically low color values - Typically low diffuse values - High transparency values - Refractive (index of refraction) properties - Diffuse+reflection+transparency = 100% is a good starting point - Use a fresnel fall-off shader to enhance transparency and reflective properties
- Diffuse + reflection = 100% - Color can be controlled by reflection or by using color and low diffuse values - High and narrow specular highlights. (Because metals typically reflect their inherent color, set specular color to 50% grey. Plastics can be near white.)
- Phong is excellent for plastics - Lambert is good for materials that have little or no specular, like wood and fabric - Oren-Nayar attempts to use the material’s roughness to simulate its specular and is good for metals - Blinn is an enhancement over Phong and is good for plastics or metals
Note: Not all applications support these rendering types exclusively. You may need to add shader types or set illumination types. |