Why does image-based lighting (IBL) like HDR textures do not function as shadow-casting lights?

Questions
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1

as donmccurdy answered at Generate shadow effect by using HDR file:

image-based lighting (IBL) like HDR textures do not function as shadow-casting lights.

I’m curious about the reason behind this.

  • Is it possible to cast shadow by using HDR as environment texture?
  • Is it too expensive to implement this?

It seems that Babylon.js also does not support this： Lighting and Shadows from HDR Texture

2

The most common shadow technique “shadow mapping” does not work with environmental lighting since there is no position in 3D space from where you can generate a shadow map. That is however possible with punctual lights like spot, directional and point lights.

3

I understand that we can not generate shadow by using “shadow mapping” for IBL.

However, it is said that Blender can generate shadow when using IBL. How does Blender accomplish it?

4

Where have you read that?

5

I imagine this would work with a path-tracer like Blender’s Cycles renderer. I’m not sure how it could be done in a realtime rasterizer like Blender’s Eevee, or THREE.WebGLRenderer.

6

that isn’t accurate