Transparency and sort-last (DB) compounds

Author: eilemann@gmail.com
State: Design

Overview

Transparency needs special attention when using a sort-last (DB) compound. Typically, all transparent objects have to be rendered back-to-front on top of the assembled opaque objects. This document explores what can and should be implemented in Equalizer to facilitate sort-last compounds with transparency.

Design

Serial Transparency Rendering

The simplest approach is to have one rendering pass for all transparent objects on the destination channel, after the assembly has happened. The order of operations would be:

    render opaque geometry in parallel
    assemble opaque geometry (in parallel)
    (gather assembled tiles)
    render transparent geometry on destination channel
  

This approach fits for most applications, which have to render small amounts of transparent geometry. The destination channel can be relieved of some rendering workload by using a smaller range, or load-balancing (once implemented). It does not work for applications where rendering the transparent object take a lot of time, for example volume rendering mixed with geometry.

Parallel Transparency Rendering

For applications rendering lots of transparent data, decomposing the transparent rendering is desirable. The order of operations could be:

    render opaque geometry in parallel
    assemble opaque geometry (in parallel)
    render transparent geometry (in parallel, 2D 'compound' on tile)
    (gather assembled tiles)
  

OR:

    render opaque geometry in parallel
    assemble opaque geometry (in parallel)
    save framebuffer [assembled result]
    clear framebuffer
    render transparent geometry in parallel
    save framebuffer [transparent geometry]
    restore framebuffer [assembled result]
    assemble transparent data w/ saved framebuffer [transparent geometry]
    (gather assembled tiles)
  

OR:

    render opaque and transparent geometry in parallel on different channels
    assemble (in parallel):
        z-assemble all opaque frames
        sorted-blend assemble all transparent frames
    (gather assembled tiles)
  

The last approach seems to be the best. The serial transparency rendering is a special case of this algorithm, where all transparent data is rendered on one channel only. This algorithm has the following preconditions:

• If the destination channel is also source, it has to render opaque geometry, or it has to readback its content for sorted assembly.
• Channels have to exclusively render opaque or transparent geometry. The application has to map the DB-range to that concrete boundary, or the server has to obey the boundary. Both approaches may need some changes in Equalizer to be implementable.
• A specialized eq::Channel::frameAssemble implementation using two steps to assemble, as outlined above. This could be implemented in Equalizer, and is useful for pure volume rendering as well.

The first approach is also nice, but uses for the transparent geometry an implicit 2D decomposition (parallel composition) or no decomposition (serial composition).

Implementation

TBD

API

TBD

File Format

  TBD

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