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In This tutorial at 5:28 he selects his Hi-Res mesh and then selects his Lo-Res mesh, and then proceeds to bake. Why not just bake the Hi-Res mesh? Why do you need to also select a Lo-Res mesh?

Also, is it common practice to do your re-topology first and then do all the Baking?

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    $\begingroup$ Because he would like to bake the info from the hi-res to the lo-res (in this case normals). Could better you explain what you instead expect to achieve by using a single mesh only in this particular case? $\endgroup$ – Carlo Feb 7 '17 at 21:28
  • $\begingroup$ I guess I just dont understand how the UV's of the HiRes mesh match the UV's of the LoRes mesh? How does blender know the difference between the LoRes and the HiRes? he didnt mention in the video, but do both object have to be in the exact same spot? $\endgroup$ – icYou520 Feb 7 '17 at 21:33
  • $\begingroup$ Yes they do have to be. UVs aren't necessary for this to be done; i.e. high-res might not be unwrapped (and unwrapping it could be a pain sometimes). $\endgroup$ – Mr Zak Feb 7 '17 at 23:05
  • $\begingroup$ @MrZak Ok thanks for that answer, now 2 dumber questions 1. Why the need to select the low res mesh? is it so Blender knows the UV to bake it to? 2. Does camera view matter when baking? $\endgroup$ – icYou520 Feb 7 '17 at 23:13
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    $\begingroup$ For one so Blender to know where to project the normals. As normal map stores direction of how the surface of high-res is positioned regarding the surface of the low-poly at any given point in each pixel, Blender has to know which mesh is low-poly and which is high-poly. Useful reading - blender.stackexchange.com/questions/41184/…, especially from Polycount. As for the second, no, it doesn't. $\endgroup$ – Mr Zak Feb 7 '17 at 23:28
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Without watching the video tutorial, a high and low resolution model cannot share the same UV-map therefore geometric details from the higher resolved mesh are baked into a normal- (or other type) map which is unwrapped from the low res model.

enter image description here The generated map is used to create details (fake geometry) for lower rendering time which is very common in games.

enter image description here

The workflow in this normal map example :

  • Unwrap the low res model
  • add a matrial and texture
  • select high res and then low res model
  • bake from selected to active

enter image description here

enter image description here

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  • $\begingroup$ I guess where I am confused. In your example, when you bake the Displacement map from the Hi res mesh how does blender know how to lay it out in the UV so the LoRes model can use it? $\endgroup$ – icYou520 Feb 7 '17 at 21:54
  • $\begingroup$ You find the bake button in the render tab, check "Selected to Active (Object)" depending on whether you use Blender internal or Cycles you need different workflows. You find many posts here on this. $\endgroup$ – stacker Feb 7 '17 at 22:34
  • $\begingroup$ Do both objects have to be in the same exact location? If so, say you have some deep scratches on your HiRes mesh, if you have it in the same location as your LoRes mesh, wont the faces of the low res mesh block out the deep scratch from your HiRes mesh? Sorry if I am confusing, I am just trying to wrap my head around this. $\endgroup$ – icYou520 Feb 7 '17 at 22:47
  • $\begingroup$ The objects need to be aligned there are more sliders like 'distance' where you can adjust settings, just play around and go into the tutorials. I will upload this example to another question here: blender.stackexchange.com/questions/59107/… $\endgroup$ – stacker Feb 7 '17 at 22:49
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Just a hobbyist here, please correct me if I'm wrong, but I think I have a decent grasp of normal maps.

Basically, the baking process is just the hi-res model "firing" rays onto the low-res from it's geometry. Then when the ray hits the low-res it alters the normal on that part of the object. That alteration is reflected on the Normal Map that you unwrapped for. Normal map data is just telling the renderer how to work with light rays when the light rays hit each part of the low-res. I find these images very helpful :

enter image description here

enter image description here

There are many many articles about this and I suggest reading everything you can find.

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