XSI Softimage - Photorealistic Rendering Tutorial.pdf - XSI Softimage - p.pablo

Q & A

SOLUTIONS / FIXES / ADVICE

TH E M ON TH

S ubmit ted by Gerry, v ia ema il

“How do I set up FG for

photorealistic rendering?”

FAC TFILE

about photorealism because, apart from the spelling,

there isn’t really anything ‘real’ about it. What we

perceive as being ‘realistic’ in an image (or an

animation, for that matter) has more to do with whether it’s

consistent with our expectations of it, rather than whether it’s truly

representative of the real world. Over time, we get used to certain

motifs being rendered in a certain way, and any deviation from this

stands the risk of being regarded as strange or unnatural. As an

extreme example, imagine Godzilla painted in vibrant pink and yellow

stripes. No matter how good the paint job is, or how cute he looks,

the effect would still look slightly odd, to say the least.

Another important point to note is that whenever we look at a

photograph, we’re not actually looking at reality itself, but a

representation of it. Even though variables such as f/stop (focal

ratio, or depth of ﬁ eld), motion blur, camera lenses, different ﬁ lm

types and grain don’t exist in the world around us, we could

rationally claim that the image we’re scrutinising is unrealistic if any

of these variables are absent. In effect, we have to mimic not only

reality but the equipment used to ‘capture’ it.

However, unless you’re fortunate enough to have a huge render

farm tucked away in your cupboard, you’ll soon come to realise that

there’s no way you can enable all the bells and whistles in your 3D

application and still keep render times reasonable, particularly if

you’re generating multiple passes. This includes the feature referred

to in the question: ﬁ nal gathering (FG) – an advanced method of

indirect lighting that samples light reﬂ ected from surrounding

surfaces. And while some of its effects can be ‘faked’ in

compositing, this comes at the cost of quality – the results just

won’t look quite as good as their 3D counterparts.

Given that realism is about a whole lot more than just activating

the switch that simulates the light bouncing around in your scene,

the following walkthrough sets out a selection of techniques for an

effective ﬁ nal gathering workﬂ ow, including how to use FG in

combination with ambient occlusion. The procedures can be further

reﬁ ned, and can be used all at once or separately, depending on your

requirements. For now, pour yourself a new cup of coffee (purely as

reference material, of course) and let’s get started.

FOR

Softimage XSI

DIFFICULTY

Intermediate

TIME TAKEN

1-2 hours

ON THE CD

• Full-size screenshots

• Supporting scene ﬁ les

ALSO REQUIRED

N/A

3D WORLD June 2006

TDW78.qa_lead 072

6/4/06 4:17:47 pm

Q&A | Simulating water

QUE ST ION OF

SOFTIMAGE XSI

This issue’s solution is supplied by Ola Madsen, who works

as a 3D artist for Digital Context in Sweden. The image above

is a fairly accurate picture of his desk at the end of the day

T here’s a certain irony involved when people enquire

072 | 3D WORLD

Creating photorealistic images | Q&A

STAGE ONE | Shades of grey

Start by opening the ‘CoffeeCup’ scene on the CD.

The ﬁ rst thing you need to do, which you should do

with any scene, is turn off the ‘ambience’ light, as

this gives a ﬂ at and unrealistic illumination. Next, open an

Explorer (shortcut [8]) and delete the default light as well.

Your scene is now completely black, so any illumination will

have to be created and, more importantly, controlled by you.

The lighting is difﬁ cult enough without having all

the textures and materials complicating things

further. At this stage, you’re really only interested in

getting the illumination to play properly, so any redundant

information in the image may cause unnecessary distraction.

Nevertheless, a common ‘mistake’ made by a lot of 3D artists

is to set the lights up in a fully textured/shaded scene.

Seeing as the cups already have individual colours

applied to them, you don’t want to modify their

actual materials. Instead, you can temporarily

override their local materials by placing the cups in a new

Partition and by applying a shader to it. Select all of the

cups and press [3] to switch to the Render toolbar.

From the Pass > Partition menu, choose New

Partition. With the partition still selected, click Get >

Material and pick Lambert. Set the RGB values for

Diffuse and Ambient to 0.5 to ensure a neutral colour. By

limiting the objects that are to be lit in your scene to shades

of grey, it will be a lot easier for you to see the effect of the

colour and the intensity of each light.

Apart from certain situations, the purpose of

lighting is to make your objects visible in an

otherwise completely black environment. However,

a well-lit scene does more than illuminate your objects – it

deﬁ nes them. While you’ll most likely strive to keep your

models smooth in your ﬁ nal image, you will actually beneﬁ t

doing the opposite as you’re setting up the lighting.

With the partition still selected, click Get > Property

> Geometry Approximation and switch to the

Polygon Mesh tab. Each of the coffee cups is made

up of a number of polygons, which will be rendered as either

smooth or faceted depending on your Discontinuity settings.

Set the Angle to 0 (zero) to ensure that the polygons will be

rendered completely ﬂ at (you’ll see why in the next stage).

STAGE TWO | Let there be light

The idea is to start from a three-point lighting set-up

and reﬁ ne it as you advance. But instead of standard

lights, you’ll use 3D geometry with different

coloured constant materials applied to them. As you progress

through the steps, make sure to frequently draw a render

region to see the effect each adjustment has on the scene.

You do need at least one light for the ﬁ nal gathering

(FG) to work, so start by creating a standard

spotlight from the Get > Primitive > Light menu.

Position the spotlight approximately at X:6, Y:3, Z:0, set

Colors to RGB 0.83, 0.88, 0.93 and the Intensity to about 0.4.

Open the Render Options PPG (Render > Region > Options…).

Switch to the FG tab and tick Final Gathering to enable it.

Increase the number of Bounces to 2. You can leave

the rest of the settings for now. When it’s time to

render the ﬁ nal image, increase Accuracy to 200

(or more), set the Min Radius to 0.2 and the Max Radius to 1.

Now let’s start creating the lights. From the Get > Primitive

menu, choose Sphere and change the settings to match

those shown in the screenshot above (included on the CD).

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Q&A | Creating photorealistic images

STAGE TWO (Continued) | Let there be light

Rotate and position the sphere behind the spotlight.

Apply a Constant shader and set RGB to 2.0, 2.2,

2.5. Duplicate it and make it slightly smaller (see

next screenshot for how to position the spheres). Click Get >

Material > MakeLocal Material, and change RGB to 1.99, 2.0,

1.57. Duplicate the sphere again and make it larger – click

Material > MakeLocal Material. Set RGB to 3.5, 3.5, 3.5.

With the lights in place, it’s time to ﬁ ne-tune their

position and intensity, and this is where the ﬂ at

polygons will come in handy. To get the most out of

your lighting set-up, aim for each of the cups’ polygons to

have a slightly different shade to the adjacent polys. While

it’s good to work with one light at a time, the fact that you’re

using FG means each modiﬁ cation affects the entire scene.

For this reason, you should work by going back and

forth, making small changes each time. Note that

the Accuracy and Radius settings in the Render PPG

also affect the illumination of the scene and not just the

quality. Once you’re satisﬁ ed with the lighting, either delete

the partition (holding the overrides) or create a new Pass, to

revert to the cups’ original shading and materials.

STAGE THREE | Ambient occlusion and adding mattes

Even though you’re using FG to illuminate your

scene, combining it with a subtle effect of ambient

occlusion will place the cups on ﬁ rm ground, and add

to the overall depth and richness of the image. From the Pass

> Edit > New Pass menu, choose Empty. Select all the cups

and the ground object and, from the Pass > Partition menu,

choose New Partition.

With the partition selected, apply any shader from

the Get > Material menu and press [7] to open a

Render Tree. Click Nodes > Illumination > Ambient

Occlusion (AO) and connect the node to the Surface input of

the Material node. Open its PPG and change the settings

(refer to screenshot on the CD). Open the Background_Objects_

Partition’s PPG and set the Render Visibility to Hide Members.

When you’re happy with the AO shader, hit the

render button. As you’ll be doing a fair amount of

adjustments to the images, render them in 16-bit

depths (using the SGI ﬁ le) instead of the default 8-bit. This

will ensure that quality is maintained throughout the project.

You should also render the images at a larger resolution, as

the edges in the ﬁ nal image will need to be cropped.

While the need for mattes may not be critical in this

scenario, it can be the difference between a couple

of minutes’ work in compositing or having to re-

render an entire sequence. From the Pass > Edit > New Pass

menu, choose Empty and name it ‘mattes’. From the Pass >

Partition menu, create three New Partitions and name them

‘red_matte’, ‘green_matte’ and ‘blue_matte’.

Select the partition named ‘red_matte’ and apply

a Constant shader. Set the RGB to 1, 0, 0. Apply a

Constant shader to ‘green_matte’ and ‘blue_matte’,

too, and set the RGB to 0, 1, 0 and 0, 0, 1. Select the blue

cups and, in Explorer, right-click on the ‘red_matte’ portion

and choose Add Selected Objects. Next, add the violet cups

to ‘green_matte’ and the turquoise cups to ‘blue_matte’.

Open the Background_Objects_Partition PPG and set

Render Visibility to Hide Members. Set the rendering

output and format to the same as in the two

previous passes and hit Render. Once completed, go to the

View > Layouts menu and switch to Compositing. (If you’re

using XSI Foundation , you can use Photoshop to composite

and edit the images instead.)

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074 | 3D WORLD

Creating photorealistic images | Q&A

STAGE FOUR | Compositing using the FX Tree

In the FX Tree menu, choose Passes > Get All Passes

to automatically import the rendered images into

the FX Tree. Add and connect the operators to match

the accompanying screenshot. Please refer to the

annotated full-size screenshot on the CD for further

explanation of the operators’ different settings.

When compositing the shadows or ambient

occlusion, people often add the images and set the

blending mode (Math Op in the FX Tree) to multiply.

However, shadows aren’t black at all – it’s a case of there

being a selective absence of light. For a subtle and realistic

result, you should add a colour corrector to the image and use

the shadow/AO image as a matte to limit the effect.

As discussed in the introduction, effects such as

depth of ﬁ eld can bring your computer to its knees if

applied in the previous stage. However, there’s

another way: open the FileOutput operator and render the

composite. Once saved to disk, you can project the same

image back into your 3D scene. From the View > Layouts

menu, switch back to Default layout.

STAGE FIVE | Depth of ﬁ eld

From Pass > Edit > New Pass, choose Empty. Name

it ‘Depth_of_ﬁ eld’, open an Explorer and set the scope

to Passes. Select the Background_Objects_Partition

in your new pass and apply a Constant shader. Click on the

Connection icon and choose Image. Click the New button and

pick the image you just rendered from the FX Tree. In

Texture Projection, click New and select Camera Projection.

In the Explorer, pick the Camera. Select it and press

[Enter] to open the Camera PPG. Switch to the Lens

Shaders tab, press the Add button and pick the

Depth_of_ﬁ eld shader. Select the same in the shader stack

(the list) and press the Inspect button. Determine which

value to enter to get the focus where you want it. It can be

tricky to animate by hand, so let’s use an expression instead.

From the Get > Primitive menu, pick Null and

name it ‘dof_focus’. Return to the depth of ﬁ eld

PPG and right-click on the animation icon next to

‘Focal Distance in inches’ and choose Expression Editor… from

the menu. In the Editing Pane (the white area), enter ‘ctr_

dist_cam(dof_focus.)’ and press the Apply button. The

expression ensures that the focus will always be on the

null’s location. You can now tinker with the null’s position

and the ‘Depth of Field Strength’ parameter in the Depth _

of_ﬁ eld PPG until you’re completely satisﬁ ed with the result.

Render the ﬁ nal image, sit back and pop the kettle

on. You’ve just earned yourself another (real) cup of that

sweet, delicious, irresistible coffee. Make sure you enjoy

every drop! ●

June 2006 3D WORLD

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XSI Softimage - Photorealistic Rendering Tutorial.pdf

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