I'm not sure exactly where to post this, so if someone has a better place then just let me know and I'll move it accordingly.
For those who have seen Scott Robertson's "How to Render" DVD series, there's a point in the 1st DVD (rendering planar objects) where he uses a particular construction to determine at what point a light source will hit a plane in perspective. For visual reference, here's a link to show what I'm trying to explain:
I don't really understand how he goes about constructing this, so if anyone knows a solid method to do so, I'd appreciate it. Any help would be great. Thanks!
From what I can see from the picture, the numbers represent a value for each face he has selected. The lines with angle radius's that are attached to the faces represent the direction of light. The other lines represent the slope of the face. The way you determine what the light source angle is, is by matching an arbitrary point of the object, to its corresponding shadow. For instance, on the nearest part of the edge facing the viewer, the shadow of that edge 'kinks'. This kink correlates to the corner of that nearest facing side. Draw a non-permanent line connecting those two points. That line is now the literal representation of the direction of the light source. This angle then can be translated to the other faces by placing it on the line that represents the slope of the face. To determine how illuminated a particular face should be, you make a value scale. A common scale is to use 5 values ranging from your lightest light, to darkest dark. The face that has the lightest value will be the one that has the most direct light aimed at it (in the picture he erased the line of that face, but it still can be seen). For the lightest face in the picture, he assigned a 0 to it. The reason he did this, is because that face was perpendicular to the light source which meant it received the most direct light. The angles on the other faces gave him an idea of how dark or light that face should be. As the angles decline, the intensity of light declines as well unless there is another light source such as bounced light. So in the picture, you see how he wrote "2 to 3" when pointing to the left side of the face of the object, and for the ground he writes "2 to 3 ish"? This is him assigning of value which is based on entirely on what you think is right. The other numbers on top of the picture say 2 for both those faces, which is kind of confusing since they obviously are both different in value. I'm not exactly sure why he did that, but my guess it it is because he has a 4-5 point light scale, and he is thinking of them as a heavy 2, and a light 2.
If you still need help I can do a paint-over if you like.
Update: I just took a look at your sketchbook, and I noticed that most of what I just talked about you already seem to know about. So I guess I'm not sure what you are asking. BTW, you have some attractive perspective studies. Kudos on your progress.
Last edited by RyanWC; February 17th, 2013 at 10:16 PM.
Moved from tutorials.
There's no construction involved, he's just postulating a side light that hits the inclined plane at 90 degrees and the ground plane at 45 degrees.