Mirroring an object along Z-Axis results in improper lighting

Question

Let me start off by saying, I am not certain if this is a Blender problem, or a problem with Roblox rendering. They're having no luck finding the problem there, so I'm coming here to see if it's something Blender-related.

I have a model which loads up in reverse of how I need it to be. The left side is on the right. So, in Blender, I have mirrored it, and flipped the normals.

The grey brick is a light source in these two screenshots.

I've tried extruding, I've tried using -1 Z size instead of the mirror function, I've tried flipping and recalculating normals every way I can. But the lighting only looks right when I don't flip the Z axis at all, like so:

Is there a function in Blender that can fix this, or is it not a problem with the model, but the rendering program? (Blender's render shows no difference in lighting.)

Answer 1

I found the solution. Applying Scale resulted in the model exporting with proper lighting. After applying -1 Z to the scale to mirror the model, I used Apply Scale, and the lighting was as it should be when imported to Roblox.

Answer 2

I use Blender to create content for a game which makes use of a game engine which has a fundamental incompatibility issue with Blender. Where Blender defines the Z axis to be vertical, the game engine assigns th Vertical to the Y axis. It is worth checking to see if a similar incompatibility is involved here. Perhaps the render engine has reversed one axis (or more) so that where Blender interprets a positive value as meaning one direction, the render engine inerprets that same direction as a negative value.

I suspect that in your case if you orient your model so that the front of the car is at positive X, the Y axis in the render engine is oriented 180 degrees from the orientation in Blender. If this is the case, you may need to revise your workflow.