AppletTalk.com Java discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  problem in writing rendering engine          AppletTalk.com Forum Index -> 3D Graphics API's for Java View previous topic :: View next topic   Author Message CAFxX Guest Posted: Tue Mar 02, 2004 6:13 am    Post subject: problem in writing rendering engine i'm attempting to write a rendering engine but i'm stuck. what i can't figure out is how to compute where a ray hits the visual screen. for example: the finished image should be 800x400. that's 2:1. so somewhere in the 3d space there will be my rectangular visual screen (with proportions 2:1). when a light ray hits it I want to know which pixel (of the final image) I have to add the ray to. how can I do this? thank you in advanced for helping, CAFxX. p.s. the engine i'm writing is open source and available at http://sourceforge.net/projects/lightstorm Back to top Peter Ashford Guest Posted: Tue Mar 02, 2004 10:11 am    Post subject: Re: problem in writing rendering engine CAFxX wrote: Quote: i'm attempting to write a rendering engine but i'm stuck. what i can't figure out is how to compute where a ray hits the visual screen. for example: the finished image should be 800x400. that's 2:1. so somewhere in the 3d space there will be my rectangular visual screen (with proportions 2:1). when a light ray hits it I want to know which pixel (of the final image) I have to add the ray to. how can I do this? thank you in advanced for helping, CAFxX. p.s. the engine i'm writing is open source and available at http://sourceforge.net/projects/lightstorm "Rendering engine" sounds realtime, but casting rays sounds like Ray Tracing. Which are you doing? Back to top CAFxX Guest Posted: Tue Mar 02, 2004 8:59 pm    Post subject: Re: problem in writing rendering engine absolutely not real-time. it's a sort of radiosity algo. yeah i know. maybe i'm completely OT here. i just hoped someone could help. "Peter Ashford" <me (AT) here (DOT) there.com> ha scritto nel messaggio news:L5Z0c.7949$SZ.158041 (AT) news (DOT) xtra.co.nz... Quote: CAFxX wrote: i'm attempting to write a rendering engine but i'm stuck. what i can't figure out is how to compute where a ray hits the visual screen. for example: the finished image should be 800x400. that's 2:1. so somewhere in the 3d space there will be my rectangular visual screen (with proportions 2:1). when a light ray hits it I want to know which pixel (of the final image) I have to add the ray to. how can I do this? thank you in advanced for helping, CAFxX. p.s. the engine i'm writing is open source and available at http://sourceforge.net/projects/lightstorm "Rendering engine" sounds realtime, but casting rays sounds like Ray Tracing. Which are you doing? Back to top Peter Ashford Guest Posted: Tue Mar 02, 2004 9:58 pm    Post subject: Re: problem in writing rendering engine CAFxX wrote: Quote: absolutely not real-time. it's a sort of radiosity algo. yeah i know. maybe i'm completely OT here. i just hoped someone could help. Ive written a ray tracer in java, but nothing using radiosity. I would assume that the idea with radiosity is to use the radiosity passes to colour the objects in the world then use a simple raycast through the image plane to sample the viewed world. Are you trying to do something like that or something else? I might be able to help, but I'm not sure what exactly you're having problems with (and Ive never done radiosity, so I might not understand ) Peter Back to top CAFxX Guest Posted: Tue Mar 02, 2004 10:18 pm    Post subject: Re: problem in writing rendering engine well, radiosity is very similar to what you imagined. but what i'm actually trying to do is quite different. indeed, even in radiosity you need ray-tracing as the final step. i was trying to create an engine that won't need it at all. what i needed to understand was: "ok, i have a ray and a 3d plane (the screen). i also have the intersection of the ray and the plane. that's (x,y,z). now, which pixel do i have to color in the output bitmap?" "Peter Ashford" <me (AT) here (DOT) there.com> ha scritto nel messaggio news:ls71c.63$Nc3.1855 (AT) news (DOT) xtra.co.nz... Quote: CAFxX wrote: absolutely not real-time. it's a sort of radiosity algo. yeah i know. maybe i'm completely OT here. i just hoped someone could help. Ive written a ray tracer in java, but nothing using radiosity. I would assume that the idea with radiosity is to use the radiosity passes to colour the objects in the world then use a simple raycast through the image plane to sample the viewed world. Are you trying to do something like that or something else? I might be able to help, but I'm not sure what exactly you're having problems with (and Ive never done radiosity, so I might not understand ) Peter Back to top Peter Ashford Guest Posted: Tue Mar 02, 2004 11:26 pm    Post subject: Re: problem in writing rendering engine CAFxX wrote: Quote: well, radiosity is very similar to what you imagined. but what i'm actually trying to do is quite different. indeed, even in radiosity you need ray-tracing as the final step. i was trying to create an engine that won't need it at all. what i needed to understand was: "ok, i have a ray and a 3d plane (the screen). i also have the intersection of the ray and the plane. that's (x,y,z). now, which pixel do i have to color in the output bitmap?" Okay, well that's quite simple - you have the plane in 3D space, you need to translate back into pixels. You do that parametrically. You have the intersection (P) of the ray and the plane (VP) so convert the offset of P relative to VP into a pair of x,y values in the VP frame of reference (in the range 0-1) and multiply these values by the screen dimensions to get to pixels. To convert P,VP into an x,y cordinates: Your plane is (A,B,C,D) The point is P x,y are the unknowns A | B Quote: y --------P x D C The distance AP calculated using vector math (distance between A and P is the magnitude of (A-P)) The angle between AP and AB can be calculated using the dot product. The general rule is: r.q = |r| |q| cos(theta) so we can get the angle from the dot product by rearanging: cos(theta) = r.q / (|r| |q|) Once you have the angle of BAP and the length of AP you can find out the Y cordinate using trig: sin(theta) = y / |AP| ..: y = sin(theta) * |AP| You can calculate x in a similar way (angle PAD is 90 - BAP) so: sin(90-theta) = x / |AP| ..: x = sin(90-theta) * |AP| Thats how I figure it anyway, hopefully I haven't made any mistakes. Don't forget to normalise the vectors before using the dot product to get angles. HTH Peter. Back to top Peter Ashford Guest Posted: Tue Mar 02, 2004 11:42 pm    Post subject: Re: problem in writing rendering engine This might be obvious, but I forgot to mention: those x,y values relate to the VP frame, they need to be normalised to lie in [0..1] Back to top CAFxX Guest Posted: Sun Mar 07, 2004 7:52 pm    Post subject: Re: problem in writing rendering engine Quote: Your plane is (A,B,C,D) The point is P x,y are the unknowns A | B | y --------P x D C The distance AP calculated using vector math (distance between A and P is the magnitude of (A-P)) I really can't understand the drawing. Morover, in the "distance between A and P" i can't figure out what A is. The plane? Wasn't it VP? Something else? Back to top Peter Ashford Guest Posted: Sun Mar 07, 2004 10:45 pm    Post subject: Re: problem in writing rendering engine CAFxX wrote: Quote: Your plane is (A,B,C,D) The point is P x,y are the unknowns A | B | y --------P x D C The distance AP calculated using vector math (distance between A and P is the magnitude of (A-P)) I really can't understand the drawing. (A,B,C,D) is the plane VP. The formatting got stuffed up in posting, I'm afraid. Just use a proprtional font and push points B and C out to the right. There is a horizontal line to point P and a vertical line (using the | characters) from the top of the rectangle to P. Morover, in the "distance between A Quote: and P" i can't figure out what A is. The plane? Wasn't it VP? A is the top left coordinate of the plane. Points A,B,C,D are the four corners of the plane. Sorry its taken me so long to reply - I've been out of town on business. 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