Final Gathering 101 In Softimage|XSI

By: Ted Daly

theodoredaly@gmail.com – http://tbd371.aisites.com

For this tutorial I will go over the basics of the Final Gathering lighting effect in XSI and Mental Ray.

Why Learn Final Gathering?

One of my philosophies when it comes to 3D animation has always been. You can make an immaculately modeled scene or object, but if you don’t know how to light it right it will just look average. On the other hand if you have a scene that might be lacking in the modeling department you can more than make up for it with a good lighting job.

What Is Final Gathering?

Final Gathering simply put is a simulation of the way light and color bounces off of objects and onto others.

How it works:

1) The camera emits photons out in all of the viewable area.

2) Photons travel through the air fading in intensity as they get further away form the source (camera).

3) The photons then bounce off of object and then start a new path absorbing the color of the first surface and “carries” it to the next.

4) Step 3 continues, dissipating in intensity after each bounce and distance traveled.

**Each type of material reflects and absorbs light differently. For example a shiny phong shader with some reflections will reflect more of the light and color energy onto the next object, (think a mirror) while absorbing much less of the incoming color than say a flat Lambert shader would.**

This is the same thing you see when you hold a bright green/red/yellow/blue piece of paper over a table top.

Final Gathering can be used in numerous ways. For this tutorial I will be explaining its basic features and operation, by showing how it can be used as the sole “light source” in the scene. Another very common use for Final Gathering is in conjunction with The Global Illumination feature in XSI. GI works almost exactly the same as FG except for instead of casting the photons out of the camera they are cast from the light source its self. FG comes into play by controlling the overall amount of color and light transfer that the GI is producing. When used correctly Final Gathering and Global Illumination can create visually stunning scenes.

I also have a few practical use examples of the Final Gathering effect at the end of this tutorial.

1) Open and get some good music playing…believe me it’s a necessity…

For the sake of time I am just creating a simple scene to show you the different effects that Final Gathering (FG) will give you and to save time on rendering.

Since each scene is different there is no one set way to set up the FG for all scenes. This includes the settings as well. The best advise I can give you to become good at rendering with FG is just practice and experiment with different scenes, sizes and complexities and the rest will come with time.

2) With the scene created it is time to get started with the fun stuff.

Picture1

3) First things first you have to go to the render options window in XSI. You can do this by pressing 3 on your keyboard.

4) Once there go to Render>Region and make sure Auto Refresh is turned *OFF* (no check mark next to it…see picture 2)

FG is a very memory intensive process and disabling the auto refresh will keep it re rendering each time you make a change in a window.

Picture2

5) Next go to the region options window…Render>Region>Options Then to the Final Gathering Tab in the options window.

This is where you will be spending the bulk of you time with FG.

Picture3

6) In this window you will want to enable Final Gathering by clicking the box circled in Picture3.

7) Now since FG is the result of the computer doing calculations between different surfaces ambient light by casting photons there needs to be a photon emitter i.e. a light source in the scene…but you do not want the light to interfere with the overall look in most cases. By default there is a hidden infinite light in every scene in XSI this will do just fine as the photon emitter.

You can access this light by going to Scene>and double-clicking on the Light circled in picture4.

**Since I am running XSI on windows XP Media Center my computer is in capable of running it in small font mode hence why the scene tab is covered in the picture.**

Picture4

8) This will bring up this options window (picture5). You will want to turn the Intensity of the light all the way down to 0 as to not add any additional light into the scene…we are now done with this window.

Picture5

9) You can now do a quick render of the scene by pressing the “Q” key on your keyboard and clicking and dragging the box around the desired area…you should get something that is pretty much black with a little light…not very interesting right?

Picture6

10) In order to fix this lack of light we need to add a bright light source in order to give the photons something to bounce off of and add some light.

We are going to do this by going to Get>Primitive>Polygon Mesh>Grid

You will then position/scale/rotate the grid over the scene in order to light the scene. (Picture7) note that this may take some tweaking to get it right for your scene.

**The height of the grid off the ground will affect the brightness of the final effect as the light fades with distance.**

Picture7

11) You can re-render the scene now (by right clicking with the quick render “Q” hot key active, you can also keep the same region active but hide it by middle clicking on the mouse so you don’t have to re-draw the region each time) and it will be a little bit lighter but still not to the point where we want it at…to do this we need to change the material of the grid to a bright white as to act like the sun. With the grid selected go to Get>Material>Constant.

Now change the color to a bright white. You can also change the Incandescence Intensity to 1 from 0 under the Indirect Illumination tab.

12) Now it is time for the Render Region Window again. Here you will want to click the Automatic Compute button for the FG. This will survey your scene and according to its size and distance it will automatically adjust the Min and Max Radius to your scene…this of course can only be so accurate and requires tweaking.

Picture8

You can re-render the scene now…notice how the colors from one object show up on the objects near them much like how light bounces in real life? That’s the FG your looking at. For my final render you can look at Picture11 at the end and it will have the settings that worked best for my scene.

13) Now in order to get your scene to look just right you need to know how to tweak all of the settings here they are and there effects:

Preview finalgathering: this will let you watch your computer calculate the photons being cast. See Picture9

Picture9

Accuracy: This slider lets you adjust the overall accuracy or quality of the final render. This will have the biggest overall effect on your final render. It will also take up the longest time and memory, so be careful. If you have a fairly big and complex scene you would not want to drag the accuracy over 300 or it will most likely freeze your computer up…if not its time for a coffee break. (Also see Pre-Sampling Density) I have a few examples below. This can be coupled with PSD so mix and match for your scene.

Low Accuracy (10) V High Accuracy (300) V

As you can see differences in the accuracy make a huge difference in the final effect. But it is not just the accuracy that makes it look nice. It is a combination of all the sliders and practice that will make your scene look good in the end.

Min/Max Radius: These sliders let you tell the computer what you want the minimum and maximum radius away from that object in (XSI units) into be computed in the photon effect.

Bounces: This slider will let you adjust the number of bounces the photons are allowed to well, bounce… it is defaulted to 1 bounce. This means the photon will come down bounce off of say, the ground and hit the ball then stop, if it was set to a higher value the photon would hit the ground, then ball, then ground again or another object. If this slider is set to “0” then it is like having no FG effect at all. This will get you a more colorful effect see Picture10.

Picture10

Pre-Sampling Density: (PSD) This adjusts the density of the pre sampling the computer will sample (think preview final gathering). A smaller Pre-Sampling Density will render faster, but since less samples per XSI unit are being sampled the final render will not have the same quality. Most of the time leaving this at the default “1” will suffice just fine. Below

Low density V High Density V

Multiplier: Here you can adjust the different colors of the effect to say give your scene an overall green look to it.

Picture11

Final Gathering Maps

This section is all about the Final Gathering Maps and how it can save your life on big projects.

We will be using the same scene that we created before.

1) The Final Gathering Map section can be found the bottom of the Final Gathering Tab of the View Render Region Options Window. See Picture3

2) Under this bottom section there is two parts.

Map File Usage there are 3 different selections you can make here.

Overwrite file with generated FG points: This is the default usage type in which each time you render the computer will overwrite the previous information generated with a new set of points. In English this means it re-renders the entire scene each time; which is fine but can be a waste of time if you have the same objects with no objects moving.

See it does exactly what you would expect it to do.

Append generated FG points to file: This is where the fun begins…

1) Render the scene with the ball on the ground first with the Overwrite file with generated FG points selected (lower left)

2) Now with append generated FG points to file selected, re-render the scene.

Ahhhh! Much faster right?

What just happened is that XSI saved the information for the FG points from the first render and instead of computing a whole new set of them it used the old set for this new render.

This way of rendering is a much more efficient than re-computing the FG points each time you do a new render.

3) Now take the hide the render (Q and middle click) then translate the ball in the Y axis like pictured above.

4) Ok, now re-render the scene.

Not so good huh?

This is because XSI is still using the same set of FG points you generated from the first render back in step 1.

Also think back to the beginning of the tutorial when I went over how FG works. Since the FG rays are being cast from the camera, everything looks fine from this first render…but if you rotate to the back side of the object where the camera could not cast the photon rays to it’s a different story.

This is the one major draw back to the Append style of rendering with FG. But say if you are making a car and you are going to do multiple renders of it with different amounts of reflection in the paint, you would use this mode. It will save you from having to wait though the entire render for the FG while still getting the same effect.

Generate FG points only if file doesn’t exist: This mode is more of less a hybrid of the fist two we went over already.

The basic idea behind this mode is that XSI will allow you to compile multiple renders of the same object/scene from different areas in order to build a more or less “master file” of the FG points. This would be useful if you were doing a 360degree rotating view of an object.

Examples of Final Gathering Uses

Global Illumination/ Final Gathering Combinations