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Global Illumination and Final Gathering in Softimage XSI 3.5.1 -part 2- |
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© 2004 Rob Wuijster Finetuning the rendering, the first steps: In the testrender from the renderregion we've seen adding GI to the scene will add additional light energy. So the risk to "washing" out your scene is almost inevitable. This is simply corrected by adjusting the exponent value of the spot. Open the spot's PPG and in the "Light>General" tab adjust the value from 2 to 2,5. In the renderregion you will see the GI effect being dimmed. This already looks a lot better. To show you how do exponent and energy values affect the scene, the following settings resulted in the the following renderregions:   As you can see adjusting the exponent value (this adjust the falloff of the light) will darken the scene. Adding more energy to the scene will slowly wash out the colors and shadows. What is the best setting would you ask? It is best to keep a balans between the amount of emitted photons and quality settings and the final render quality, because higher values will also create longer rendertimes. But it's best to start off a bit to the dark side (no pun intended), due the the "bounce effect" and the addition of Final Gatheringlater on, that will also add to the light calculation. For now we will work with an exponent value of 2.5, and keep the energy Intensity on the default value of 10000. Starting with too low a value is not a good idea, because when finetuning the effect, the overall effect of the photons will be less apparent, so you will need more photons to keep the effect visible.  The first setting doesn't have enough photons and gives a very "splotchy" effect, so this will not work later. The second and third setting gives a better result and will not render too long. The third setting will give the best quality so far. For testing purposes we will continue with a value of 50000. |
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Finetuning Photons in the render options:
What about getting rid of the "splotchy" effect?. XSI will calculate a default radius setting for the GI effect, but this is in most cases a too big a radius. Go into the Render Region Options and select the "Photon" tab. We will work with the Global Illumination "Accuracy" and "Radius" options and see how this does affect the render. What are these settings doing? Accuracy is a sampling parameter, which looks for a specified number of photons stored within the area set by the Radius parameter. It defines the resolution of the photon map. Higher values will give better results, but also give longer rendering times. The radius is measured in SOFTIMAGE units, and defines how far in units the renderer will search for photons in a scene. The smaller the value, the more detailed the render will look, but the more emitted photons you need and the longer it will take to render. Sof if you have a lot of small objects or parts of an object in your scene, you probably have to decrease the radius value and increase the accuracy value to get rid of some artifacts. Like the Energy and photons setting of the spot, a balance has been found for quality and speed. The default values for Accuracy and Radius are 30 and 0. Let's see what happens if we start changing these values:  When you increase the Accuracy value, more photons are collected and mixed together to get an average value. You can see that increasing the accuracy over 100 doesn't have too much effect for now, so we keep the accuracy at 100.  As for the radius, if we leave it at 0, XSI will automatically defaults to a value that it thinks is best. Most of the time this will not be acceptable, so we will have to set it to a different value. By increasing the radius level, the render starts to look better. An accuracy value of 100 and a radius value of 1 will do for now. It still has a lot of "noise", but every part of picture is illuminated well enough. |
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Finetuning Final Gathering in the Render Options: Now we add the Final Gathering to the scene to raise the quality of the rendering. Again go into the Render Region Options, select the "Final Gathering" tab and turn on Final Gathering. Also use the "Automatic Compute" button to set the "Sampling" Min and Max Radius. What are these settings doing? If you select "View Dependant" you can choose between the option of having the minimum and maximum radius values measured in scene units or in pixels. The effect of activating this option is to cast fewer final gathering samples for objects that are farther from the camera.We keep it off for now. The "Accuracy" value defines how many rays are fired from each pixel to calculate the indirect illumination. It's best to start with low values to tweak the effect (between 30-100) and increase this for the final render. Higher values will give better results but also longer rendertimes. The "Minimum and Maximum" Radius will identify the hemisphere above the pixels that are sampled for both direct and indirect illumination. The Photon Map contributes to the information Final Gathering "gathers" about the indirect illumination. Lower values (<1) will give better results but increase render times. "Filter Size" defines the range of the final gathering filter. The filter is designed to eliminate speckling by filtering out overly bright samples within the radius range. Increasing the filter size grows the range as follows: If the filter size is set to 1, a given sample point is filtered using all of its neighboring sample points. Increasing the filter size to 2 adds those sample points, and their neighboring sample points, to the filter, and so on. Higher values soften the contrast between neighboring final gathering samples and reduce speckling in the rendered frame. Setting the value to 0 disables filtering entirely, but may give better results when the final gathering "Accuracy" setting is relatively low. The "Automatic Compute" button will automatically computes minimum and maximum radius values based on the size of the scene's bounding box. So with bigger scenes you could build a cube around your scene to keep the FG rays "inside" the scene. Invert the normals and make it invisible to the renderer, this will help the FG calculation. So far for the theory, but turning on Final Gathering makes the render -again- way too bright. Go back into the Spot settings and adjust the "exponent" value to 3. The render will look a lot better with the decreased falloff.  Now it all comes down to find the best values for the photon and Final Gathering accuracy, to keep the rendering as fast as possible without losing quality in the render. |
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Final rendering: |
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That's all folks. |
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