Weighted Blended OIT Sample

Category: Performance Visuals

Min PC GPU: GeForce Fermi-class

Min Tegra Device: Tegra K1

@ GitHub: Weighted Blended OIT Sample Source Code

Description

Weighted Blended OIT Sample demonstrates how plausible OIT (Order Independent Transparency) can be rendered in a single geometry pass without any depth sorting, as described in McGuire and Bavoil 2013. The idea is to use additive blending to compute a weighted average of the translucent colors with a weighting function that falls off as the linear depth increases. This sample compares the quality and performance of Weighted Blended OIT with Depth Peeling.

APIs Used

OpenGL 4.0

Shared User Interface

The Graphics samples all share a common app framework and certain user interface elements, centered around the "Tweakbar" panel on the left side of the screen which lets you interactively control certain variables in each sample.

To show and hide the Tweakbar, simply click or touch the triangular button positioned in the top-left of the view.

Other controls are listed below.

Device	Input	Result
touch	1-Finger Drag	Orbit-rotate the camera
	2-Finger Drag	Move up/down/left/right
	2-Finger Pinch	Scale the view
mouse	Left-Button Drag	Orbit-rotate the camera
	Right-Button Drag	Move up/down/left/right
	Middle-Click Drag	Scale the view (up:out, down:in)
keyboard	Escape	Quit the application
	Tab	Toggle TweakBar visibility
gamepad	Start	Toggle TweakBar visibility
	Right ThumbStick	Orbit-rotate the camera
	Left ThumbStick	Move forward/backward, Slide left/right
	Left/Right Triggers	Move up/down
	A	Show TweakBar, Toggle Focused Item
	B	Close Focused UI, Hide TweakBar
	DPAD Up/Down	Move Focus to Prev/Next Item
	DPAD Left/Right	Decrease/Increase Focused Item

Technical Details

This sample is an implementation of the Weighted Blended OIT (Order Independent Transparency) algorithm from this research paper:
McGuire and Bavoil, Weighted Blended Order-Independent Transparency, Journal of Computer Graphics Techniques (JCGT), vol. 2, no. 2, 122--141, 2013.

Implementation

This SDK sample has two rendering methods for order-independent transparency:

Depth peeling is performing one geometry pass per peeled layer until there are no more fragments to be peeled. The layers are blended as they are peeled, in front-to-back order.
Weighted Blended OIT is a fast approximation of the depth peeling result, in a single geometry pass. In this geometry pass, the fragments are blended into a Frame Buffer Object with 2 draw buffers. In the first draw buffer (with format RGBA16F),
- RGB stores Sum[FragColor.rgb * FragColor.a * Weight(FragDepth)]
- Alpha stores Sum[FragColor.a * Weight(FragDepth)]
In the second draw buffer (with format R8),
- R stores Product[1.0 - FragColor.a]