The technique is mine and perheaps it is not written in the reference books but it works well. I assume that this method is not applicable for clothes or other stuff which are not metallic.
The Iray texture generated is forseen to be used with a "PBR Metallicity/Roughness" base mixing.
For such a texture, Substance painter generates several maps and I point these four out :
The 3Dlight texture model is often a "Plastic" or a "Glossy Metallic". This one has the following interresting fields :
With a simple image editor (I use Photoshop but it can be another simplier program), I create a new Glossy map by inverting the roughness from Substance painter. This is ok as the roughness is the opposite of the glossy.
I create also from the Metallic map a diffuse strength map by inverting the Substance painter map and decreasing the middle tones to 15% (Black is 0%, white is 100%). As far as the metals shader should be near black to get this miror effect we like.
I want to make a minimal 3DLight texture to get a nice and quicker rendering.
I Choose the "Metallic" Lighting model in most case because th reflections are more realistic with this model.
Then, I load :
And that is to be done for each surface of the object. When it is completed, I don't forget to save the new material preset and test it.
To make my test, I just take a DS Default Light Preset (dzDistantLight). I tune the rotation of the light and don't forget to activate the shadow-raytrace.
It is now also the time to play a little. In this one I add 2 point-lights (30%) for the main lights of the little car.
Tuning the parameters is according our feelings but this must be a good start to get an object renderable in 3DLight if we have only an Iray texture available for it.