gazebosim/gz-common

Collada <transparency> setting

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#229 opened on Jul 12, 2021

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bughelp wanted

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Description

Environment

  • OS Version: Ubuntu 18.04
  • Source or binary build? both - all versions

Description

The transparency result is different between Ignition ColladaLoader and threejs's CollaLoader if <transparency> tag is present, and <transparent> tag is missing, e.g. the follow tag produces an opaque mesh in Ignition but a transparent mesh in threejs:

            <transparency>
              <float sid="transparency">0.000000</float>
            </transparency>

Looking at the collada spec (pdf) spec, the default mode should be A_ONE. The current code sets transparency in the following way:

      dstFactor = 1.0 - color.A() * transparency;
      srcFactor = color.A() * transparency;
      _mat->SetTransparency(dstFactor);

This should produce a transparency value of 1.0 which means fully transparent.

Other notes

_mat->SetTransparency(dstFactor); is a bit confusing since srcFactor is the one that corresponds to the object's material opacity. However, the math works out to be correct because:

transparency = 1.0 - opacity
             = 1.0 - srcFactor
             = dstFactor

Contributor guide

Research direction
Inspect the ColladaLoader code at ign common/graphics/src/ColladaLoader.cc around line 2751-2753. Understand the transparency calculation and compare with the Collada spec (default transparent mode should be A ONE). Also check the threejs ColladaLoader implementation for reference. Fix the calculation to set srcFactor = color.A() * transparency and dstFactor = 1.0 srcFactor, and ensure SetTransparency uses the correct factor.
Tech stack
cpp
Domain
computer graphics
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C++Git
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60