Emissive map no longer works in ign-rendering6 (ogre2)

[iche033]

Environment

• OS Version: Ubuntu 18.04
• Source or binary build?
  source build: ign-rendering6 branch

Description

• Expected behavior: objects are rendered with emissive map
• Actual behavior: emissive map has no effect

I think the feature is broken when we ported from ogre 2.1 to 2.2.

Steps to reproduce

1. Test with ignition gazebo's fuel_textured_mesh.sdf world

ign gazebo -v 4 fuel_textured_mesh.sdf

2. The tubes in the Tube Light model do not appear to be lit when emissive map is used.

Here is a comparison of the Tube Light model in Ignition Fortress (top) and Dome (bottom)

Things I've tried

The call to set ogre2 emissive map is done here:
https://github.com/ignitionrobotics/ign-rendering/blob/ign-rendering6/ogre2/src/Ogre2Material.cc#L473

• I verified that this->ogreDatablock->getUseEmissiveAsLightmap() returns false and this->ogreDatablock->_hasEmissive() returns true
• setting emissive values (this->ogreDatablock->setEmissive) works
• I tried debugging the code in 800.PixelShader_piece_ps.any for applying emissive map and it looks like the emissive_map property is not set hence it does not affect the final color.

[iche033]

partial fix in #501. It's now rendering red color for grayscale emissive maps. We may need to look for a way to tell ogre that the emissive map is grayscale or convert the grayscale emissive texture to RGB.

[iche033]

fix for loading grayscale emissive map in #503