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I know how to create a cast shadow when the horizon is below the light source, but not when it's the other way around like in this image below.

enter image description here

How would I cast an accurate cast shadow?

  • if you're looking for realistic cast shadow, you'll need to use a lot of measures. See handprint.com/HP/WCL/perspect6.html#locallights on how to do it. Another link of interest (more specific to your problem, less maths) is learning-to-see.co.uk/value-cubes – Devin Aug 31 '17 at 21:45
  • @Devin I checked the links but they all show the horizon above the light source. I already know how to do all the measurements for those cases. My case is a bit different since the horizon is above the light source. – Desi Aug 31 '17 at 22:12
  • @Desi I think you are confusing the position of the light source with the position of the horizon as it affects perspective. The perspective determines the shape of the objects. The light source determines the position of the shadows irrespective of the position of the horizon. – Stan Sep 1 '17 at 5:30
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My result has no relationship to the horizon other than use it to determine the perspective of the box in a slightly skewed position with respect to the horizon as drawn. (We're looking down at it since it is below eye-level (horizon). I also chose to make a spotlight to sharpen the edges of the shadow for clarity. I show the box shadow as if it was opaque.

Cast shadow from source

  • I did not draw the third line n_n I'm lazy. – Rafael Sep 1 '17 at 5:40
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The process is exactly the same regardless where is the light source in the plane.

Define where the light source's(A) anchor point or "base" is on the plane(B)

Define where the base(D) of the vertex(C) you are working on is.

Draw a line from the light source to the vertex and cross it with a line drawn from the base of the light to the base of the vertex.

enter image description here

In the specific drawing you made the light looks a bit weird, but that is how you calculate it.

enter image description here

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