518 AutoCAD and Its Applications—Advanced Copyright Goodheart-Willcox Co., Inc. Another way to create a custom render preset is to select a standard preset in the Current Preset: drop-down list and then change one of the settings in the Render Duration or Lights and Materials area, as previously discussed. To delete a custom render preset, select the preset in the Current Preset: drop-down list and pick the Delete button next to the Create Copy button. The standard render presets cannot be deleted. Rendering Fundamentals Rendering Fundamentals Rendering requires signifi cant computer processing resources and much is happening when your computer is processing the scene. As previously discussed, rendering times increase at higher resolution and accuracy settings. The following sections will help you understand the process and troubleshoot problems when they arise. Sampling Sampling is a process that tests the scene color at each pixel and then determines what the fi nal color should be. This is most important in transition areas, such as edges of objects or shadows. Increasing the sampling will smooth out the jagged edges and incorrect coloring, but increase rendering time. Increasing the number of samples computed at each pixel means that the color displayed by each pixel will be more accu- rate. You may also notice thicker lines in the fi nal rendering. Higher render accuracy settings will have higher sampling rates. Raytracing Raytracing is a method of calculating refl ections, refractions, and shadows by tracing the path of the light rays from the light sources. The render accuracy settings previously described specify the maximum number of times that a ray can be refl ected or refracted. See Figure 19-5. Try to imagine a light ray bouncing off refl ective objects or traveling through transparent objects in your scene. Count how many surfaces the object must contact before it comes to rest. This can help you determine if you will need to increase the render accuracy settings. Indirect Illumination Indirect illumination is an AutoCAD mechanism that simulates natural, bounced light. If indirect illumination is not calculated and light does not directly strike an object, the object is black. When rendering without indirect illumination, other lights must be added to the scene to simulate indirect illumination. If you have placed lights in your scene and you render the scene using the Low render preset, you will notice that only the objects directly in the light’s path are illuminated. See Figure 19-6. This is not how lights work in the real world. Rendering the scene at a higher level of quality will allow AutoCAD to calculate more accurate indirect illumination, resulting in a more realistic scene. Global Illumination Global illumination (GI) is an indirect illumination process. Bounced light is simulated by generating photon maps on surfaces in the scene. These maps are created by tracing photons from the light source. Photons bounce around the scene from one object to the next until they fi nally strike a diffuse surface. When a photon strikes a surface, it is stored in the photon map. This map is calculated for each scene you render. The lights you create and the materials added to the objects in your scene determine the complexity of this map.