Question about casting a shadow on a slanted surface

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  1. #1
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    Question about casting a shadow on a slanted surface

    I've been trying to figure this out for more than a day now and I think I've found a method. But I'd like to check it with other artists and see if I made any mistakes. Someone might also know a faster way, which would also be interesting.

    Now, I've tried to use natural (sun) front lighting on my house in 2-point perspective. However, I don't know if this is the proper way to locate shadow points on a slanted surface (the roof, see image 1).

    Image 2 contains my complete cast shadow rendering. Image 3 was done to practise back lighting.

    Pointers and help would be greatly appreciated, thanks.

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    Steps for figuring out the shadow:
    1) Place the light source (L) and mark it's projection on the XY plane (Lxy)
    2) Mark the projection of the point you want to figure out (P) on the XY plane (Pxy)
    3) Draw the light ray passing through (L) and (P) and it's projection on XY passing through (Lxy) and (Pxy)
    4) The intersection of the two lines in step 3 is the shadow placement of P.
    5) Repeat the steps with (L) and (Lxy) in the same position.

    Notes:
    When placing a distance light source (sun) there are roughly 3 situations:
    1) The sun is behind the object, like in the 3rd drawing. (Lxy) will be approximately on the horizon.
    2) The sun is to the side of the object and not present on the image. The light rays will be drawn parallel to each other as they are also parallel to the image plane, so will be their projection on XY.
    3) The sun is in front of the object, like in the 1st drawing. Then you place the upside-down sun as you've done and point (Lxy) should be again on the horizon.

    XY plane can be replaced with any plane you want the shadow to form on.
    You better verify my information as I might be missing something but it makes sense to me.

    Regarding your first drawing, you are missing the projection of your point on XY (Pxy).
    If this point will be on the continuation of the right side of the blue line, then you got it right. There is no reason to use the vanishing points of the building to setermine the shadow (what you did on the left side of the blue line).
    Hope it helps =]

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  5. #3
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    I believe the shadow vanishing point of the inclined plane of the roof has to be on the vertical line of the sun, equal to the height of the inclined plane vanishing point. If that is the case then the shadows would be a different shape than the ones in your picture. Also the vanishing point of your building looks off on the left side.

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    Eray: Thanks for the explanation mate. That's roughly the same theory as I've learned from "Creative perspective for artists and illustrators" and the Scott Robertson DvDs.

    dpaint: I don't think they would, I've just found some resources from the Art Center College of Design and I see their method is the same as the one I'm using (see attachment 1 and 2). Taken from here and here. Also, my left vanishing point might be distorted a little, but then that's because I placed the picture pane to close to the "viewer" in my floor plan (see attachment 4). But since the building falls within the 60 degree FoV on the picture plane I don't really know how it could be distorted. It looks natural to me.

    I tried to clarify my own method a little in attachment 3. Thanks for the help so far guys. Let's hear some more opinions on this!

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  8. #5
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    From Perspective for Artists

    With the sun behind us we transpose it as in the last figure ; fix the " shadow" V.P. on a level with the " uphill " V.P. (as before), and place it immediately above the transposed sun.

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  9. #6
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    Just a quick question regarding angle of rays. Shouldn't sun rays be drawn parallel to each other, and local light rays radial?

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    Quote Originally Posted by Krilce! View Post
    Just a quick question regarding angle of rays. Shouldn't sun rays be drawn parallel to each other, and local light rays radial?
    parallel rays are affected by perspective just like the tracks of a railroad are.

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  11. #8
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    Quote Originally Posted by Krilce! View Post
    Just a quick question regarding angle of rays. Shouldn't sun rays be drawn parallel to each other, and local light rays radial?
    Like dpaint said, so the only time you would draw the sun rays parallel would be when they are coming from the sides. Like in this example.

    @dpraint: I see what you mean now and found out my shadow lines are indeed converging to a point on the vanishing trace of the "psuedo" sun. See attachment.

    However the different shadow lines vanish to different point (on the same trace though).

    I might have to build a model and try this out with real sun light.

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  12. #9
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    Alright, thanks for clearing that up for me.

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