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ynm

Why is ray tracing slow?

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Hi,

I am writing a simple ray tracer and I heard that ray tracing in general is very slow. I don't know advance ray tracing techniques, so I don't know what makes it slow? E.g for full HD just need 1920 x 1080 = 2.073.600 rays.

In my rough guess, is the collision detection between rays and objects the most heavy work in ray tracing?

Regards
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Correct me if I'm wrong, but isn't it hundreds of times faster to do raytracing operations through the GPU? I haven't gone that low level yet with graphics stuff, but from other stuff I've seen that utilizes GPU, it seems like it'd be the optimal choice.
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[quote name='Magdev' timestamp='1355786661' post='5011852']
Correct me if I'm wrong, but isn't it hundreds of times faster to do raytracing operations through the GPU? I haven't gone that low level yet with graphics stuff, but from other stuff I've seen that utilizes GPU, it seems like it'd be the optimal choice.
[/quote]

This is true however even GPU raytraycing is too slow for realtime. I have seen raytracers that can run on the gpu fast enough for games but to get it to run this fast its nerfed to a point where you can easily get better looking graphics using conventional rendering. Edited by ic0de
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[quote name='Álvaro' timestamp='1355783551' post='5011838']
Doing almost anything 2 million times takes a lot of time. If you want to get improvements like anti-aliasing, motion blur and field-of-depth effects, you'll have to launch many (100?) rays per pixel.[/quote]Yeah, pretty much every feature for which people use ray tracing in the first place will multiply the work. Just doing simple shadows generates one shadow ray per ordinary ray for each light in the scene, so if you have 10 lights, it'll be almost 10x the work. (Shadow rays are not as expensive as normal rays, but still.) Want soft shadows? More work, say 3x. Want reflection and refraction with a moderate 3 bounces? If there's a lot of reflective and refractive objects in the scene, that might quadruple the entire workload again. Now we are already at 120x the original workload. If we then add AA, motion blur, DOF etc., for a difficult scene we'll be doing hundreds of rays per pixel, billions of rays for the entire image.
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