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# Radiometry: Why are solid angles used?

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Hi! [smile] I'm trying to learn more about lighting in graphics, and it seems that radiometry is the vocabulary used for advanced stuff. I'm having a hard time understanding many of the very basic radiometry concepts, though - some of it is very confusing to me and has been bugging me for a very long time. This is basically my question:
• Why do we need to use a solid angle in the various radiometry equations, instead of directions? Let's say i want to know the light (power) which hits a point x on a surface from a certain direction d (i.e. i want to know the incident radiance). The way i would do it would be to just look at the incoming light from the given direction d, but in radiometry they look at the incoming light from a solid angle instead. It seems to me that a solid angle is not just one direction, but many directions (those directions along all the vectors from x and to the every points on the solid angle on the hemisphere). So when using a solid angle, i'm not just getting the light from direction d, but also from a lot of other directions, which i were not interested in...
My question rephrased as 3 questions: (basically the same question 3 times, but they may get you an idea of where i stand)
• Why are solid angles needed instead of normal directions (like, vectors or spherical coords)?
• If it is true that a solid angle represents several directions, why is it used instead of a single direction?
• Normally in computer graphics, we use rays for simulating how light travels (afaik), but in radiometry they use solid angles. Why? (isn't it extremely costly to calculate this??)
I would be very grateful for any help or pointers! [smile] I haven't found anywhere which explains basic stuff like this is explained - all the places i've seen, they skip over why it is done the way it is done and jump directly into how it's done. :(

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I don't have an answer for you as rendering isn't my main area of interest, but you'd have much better luck posting in the graphics programming and theory, or math and physics forums than you would in For Beginners.

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Ok, thanx. I thought it was too basic to post there. [smile] Is there a way to move it or is cross-post allowed in this case?

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A moderator can move it, but as long as you don't flood the forum, I'd imagine one crosspost in one other forum is perfectly fine.

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Ok, i'll take the chance :) Thanx!

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Quote:
 Original post by dashurcA moderator can move it, but as long as you don't flood the forum, I'd imagine one crosspost in one other forum is perfectly fine.
Uh...no. Closed.

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