Why? - Normal issue...
when I bake a high poly mesh to a low poly mesh, the flat parts look like in the images. These are two examples of two different models. The highlighted parts are totally flat parts, but instead of looking flat, it generates those weird normals. At first I let it pass, but now it is unacceptable, it makes the model in path tracers look bad.
Replies
These gradients are essential to compensate for the smoothing of your low poly mesh. You may be able to reduce gradation by using hard edges/smoothing groups. More here: https://polycount.com/discussion/107196/making-sense-of-hard-edges-uvs-normal-maps-and-vertex-counts/p1?new=1
If you're having issues with other renderers, it may be a tangent space synchronization issue, I would recommend baking with the same tangent space that the target renderer uses, if possible.
Thanks, weighted normals do the job.
And "To avoid artifacts, you must split your uvs where you use hard edges" also fix other detail I was having
A simple box bake. Another thing compared to the first image which is also a simple box bake.
What shading method do you use in your path tracer? Normals are usually baked at a smooth shaded mesh. And so you should use smooth shading in your engine too. Then the normals should fit.