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Global Illumination can be a little confusing at first, but really helps in creating realistic, nice looking scenes. This Cinema4D tutorial by Cgtuts+ regular Konstantin Muromtsev will help you better understand Global Illumination by discussing various lighting techniques, as well as explaining some of the render settings needed to achieve a good result.
We'll first create a mobius strip ring and use it as an object to light. We'll also create materials for our scene. We'll be rendering our scene several times using various objects as light sources to see the difference between setups.
Starting section – creation of the main object: the mobius strip ring. Create a tube object (Objects > Primitive > Tube.)
Increase the inner radius of the tube and decrease the height.
Make the rotation segments exactly 6.
Check fillet and decrease it's radius to 1 cm (if cm is your default measure units like in my case).
Make the tube editable (C on keyboard or Functions > Make Editable) and select half of the object as shown. Remember that you need to select all the polygons in that half, so don't miss any of them.
Split the geometry of the tube using the split function (Functions > Split) and press "Delete" to remove duplicate polygons from the original object. Use optimize (Functions > Optimize) to remove any unused points.
Select all the points along the cut on one half of the tube, and rotate them 90 degrees.
Select the corresponding points on the other half of the tube and rotate them -90 degrees.
Create a Connect object (Objects > Modeling > Connect.) And add both parts of the tube to the object.
Create a HyperNURBS object and add Connect the object to it. Our mobius strip ring is now ready.
Creating a basic background (ramp): Using Create Spline, draw a similar spline as shown below (use linear, then modify it using Chamfer).
Duplicate the spline and move it to one side, create a Loft NURBS object (Objects > NURBS > Loft NURBS) and add both splines to it.
Before we start to use GI, let's create some materials for our scene. Create a new material (by double clicking in material browser). Change the color to white, and uncheck Specular. We'll be using this material on the background.
Create another new material. Change the color to yellow, use a orange to dark violet Gradient as the texture.
Check Transparency. Change the color to orange (255 202 118), set refraction to 1.4, use a grey to white Fresnel as the texture. Set Mix Mode to Multiply. Use a less saturated color for the Absorption Color (252 208 170.)
Check Reflection. Change the brightness to 80%, check Additive, use a grey to white fresnel as the texture. Set Mix Mode to Multiply and uncheck Specular. This is the material we'll use for the ring.
Create a new material. Uncheck Color and Specular, check Luminance and change the Brightness to 200%. This material will be used for all the lights in our scene.
Add the materials to the background and to the ring. Also place the ring on the ramp, so we get a basic studio setup.
Choose a good angle and add a camera (Objects > Scene > Camera.)
Open the render settings (ctrl+B), add Global Illumination from the "Effect..." button. Under GI settings, Irradiance cache, change the Stochastic Samples and Record Density to Low (we don't need sky high quality). In the Anti-Aliasing settings, change Anti-Aliasing to Best, Filter to Sinc (or Still Image).
Let's start experimenting with GI. Most basic GI light is just a Sky object without a complex texture and nothing more. Create a Sky object (Objects > Scene > Sky), apply a light material and render (ctrl+R.) Now we have an over lit scene, because of the 200% we added into the luminance of our material. Over or under lighting is a common mistake in GI.
Decrease the brightness in the light material to 125%. Now it's looking quite good. Increase the brightness back.
Any object (or objects) can light the scene, so never underestimate primitives. Create a sphere, make it big and move it to one side. Apply the light material to it.
Turn off the sky and render the scene. Now we have interesting light reflections in an overall dark scene.
A Plane with luminance materials applied can emulate windows. Create a Plane object (Objects > Primitive), make it big and move to the side, apply the light material again. Also Turn off the sphere object.
Render the scene. We have now achieved a different look, just by changing the light source from one primitive to another.
Rotate the plane and place it above the scene.
Render the scene again. This render looks similar to the one we created with the sky object, but here we have more expressed light reflections and shadows. Also overall the scene doesn't look as blown out.
Let's try another primitive. Create a tube object, scale it up and place it above the ring. Apply the light material to it and turn off the plane object.
Render the scene. We've got good light reflections (again) and the background doesn't look as flat as it did with the plane.
Moving on to more complex objects. Create a large spline in the form of rough arc.
Create a big line spline.
Create a Sweep NURBS object (Objects > NURBS > Sweep NURBS) and add both splines in there. Now apply the light material to the Sweep NURBS object.
Turn off the tube object and render the scene again. The render looks similar to the previous ones, but now has different details. Though the background has became flat again.
Now let's try to light the scene from more than one spot. Create two disk objects, scale them up and place them on each side. Apply the light material and turn off the Sweep NURBS object.
Render the scene. Now we have the same nice reflections and the background isn't flat. Also this render has more complex shadows than the others, except for the scene using the tube light (though it's hard to say that tube is one spot light).
Move the left disk in front of the main object (our precious mobius ring.)
When we render the scene, we'll see that our ramp background is flat yet again (only the left side is looking better), but we've now got very cool looking highlights on the front of the ring.
Sometimes it seems that by adding more light emitters we can improve the final image, but that's not quite right. Turn on the Sweep NURBS light, and decrease the brightness of the luminance in the light material to 120%.
Render the scene. Though the reflections look quite good, overall the scene is slightly dull. Turn off the Sweep NURBS and change the brightness of the light material back.
Lighting is very important for setting the mood of the final image. It's all about the right colors in the right places. Duplicate the light material two times. Make one color a Luminance orange (almost red: 255 84 51) and use a blue color (57 75 255) for the another. Apply the newly created materials to the disks.
Render the image again. Now we have a bright front highlight (like in the render using the two white disks) and a very nice "atmosphere" in the scene, very colorful. Also the background isn't so dull, because it now has a nice gradient color.
Now it's time to improve the quality of the final image. Open the render settings. In the Global Illumination settings, change Stochastic samples and Record Density to at least medium. The Stochastic Samples (Accuracy) option defines a certain amount of samples for each shading point (details, complex shadows, etc.). Record Density records and optimizes stochastic samples (values under this option are already very optimized for most cases, and shouldn't really be changed). These changes will improve the quality of our GI, but also increase the render time.
Render the scene one final time. This image has more expressed shadows and details, we can also see the caustics more clearly. Also the highlights and shadows are much smoother than before. GI can produce very interesting, good looking results. So keep on experimenting!