Reflection in 2D

[mike_g]

If something moving at angle_a hits a flat surface that runs along angle_b how could I work out the angle at which the thing moving along angle_a bounces off the surface? I did trigonometry once but forgot it all

If someone could give me a little explanation, about how I could do this it would be very helpful

[Shockwave]

Well Mike, if it's just hitting the floor or wall and the surface that is being hit is not moving just reversing the direction of impact would work. However it depends what you're after, if the surface being hit is angled you'll have to use a little bit of math.

Probably best if you describe what you're trying to do in more detail

[mike_g]

Yeah maybe I'll draw a little diagram or something to try show what I mean.

[Shockwave]

Sure, do that and we'll fix it for you in no time

[Stonemonkey]

Crappy drawing time. Along with a little bit of FB code to show how reflections can be calculated. I think it can be done with trig but I much prefer this way. Maybe someone else who's done it with trig could post something too.

Code Select Expand


option explicit
option byval

'p1 and p2 are 2 points on the line to be reflected off
'vx,vy is the incoming vector to be reflected. The values in vx,vy are modified by this sub to contain the reflected vector.
sub vector_reflect(p1_x as single,p1_y as single,p2_x as single,p2_y as single,byref vx as single,byref vy as single)

'get the normal (line perpendicular to) of the line to be reflected off. Then normalise it.
    dim as single nx=p2_y-p1_y,ny=p1_x-p2_x
    dim as single d=1.0/sqr(nx*nx+ny*ny)
    nx*=d
    ny*=d
   
'the dot product gives the shortest distance from the end of the vector to the collision line (can be on the line beyond p1 or p2)
'multiply by 2.0 and will give the distance to the point on the opposite side of the line in the direction of the normal.
'The vector from that new point to the point of collision is the new reflected vector.
    d=(vx*nx+vy*ny)*2.0
vx-=nx*d
vy-=ny*d
End sub


sub main()
    screenres 640,480,32,2
    screenset 1,0
    randomize timer
    dim as integer mouse_x,mouse_y
    dim as single p1x=100+rnd*440,p1y=100+rnd*280
    dim as single p2x=100+rnd*440,p2y=100+rnd*280
    dim as single cx=(p1x+p2x)*.5,cy=(p1y+p2y)*.5,vx,vy
    do
        cls

'draw the wall
        color &hffffff
        line (p1x,p1y)-(p2x,p2y)

'draw the initial vector
        color &hff
        getmouse(mouse_x,mouse_y)
        line(mouse_x,mouse_y)-(cx,cy)

'calculate and draw the reflected vector
        color &hff0000
        vx=cx-mouse_x
        vy=cy-mouse_y
        vector_reflect(p1x,p1y,p2x,p2y,vx,vy)
        line(vx+cx,vy+cy)-(cx,cy)
        flip
       
    loop until inkey$<>""
end sub

main



[mike_g]

Nice one stonemonkey, I couldent have drawn a better diagram if I tried

This looks like what I was after. I'll have a good read through the code. Cheers.