2D box collision

[Nicky Peter Hollyoake]

Ok, I've done box collision hundreds of times and I may be over looking something. All the tiles around the border collision works fine, but the 2 blocks in the center it's having a problem with.

'map collision test
glMatrixMode(GL_PROJECTION)
glLoadidentity()
glOrtho(0, 6, 6, 0, 0, 1)
glMatrixMode(GL_MODELVIEW)
textmode(TEXT_OVERLAID)

data 1, 1, 1, 1, 1, 1
data 1, 0, 0, 0, 0, 1
data 1, 0, 1, 0, 0, 1
data 1, 0, 0, 1, 0, 1
data 1, 0, 0, 0, 0, 1
data 1, 1, 1, 1, 1, 1

dim map(5)(5)
dim p#(1)=vec2(1, 1), lp#(1)
dim x, y

for y = 0 to 5
   for x = 0 to 5
       read map(x)(y)
   next
next

while TRUE
   glClear(GL_COLOR_BUFFER_BIT)
   cls

   lp# = p# 'last position before we move
   if scankeydown (VK_DOWN) then
       p#(1) = p#(1) + 0.1
   elseif scankeydown (VK_UP) then
       p#(1) = p#(1) - 0.1
   elseif scankeydown (VK_LEFT) then
       p#(0) = p#(0) - 0.1
   elseif scankeydown (VK_RIGHT) then
       p#(0) = p#(0) + 0.1
   endif

   for y = 0 to 5
       for x = 0 to 5
           if Map(x, y) = 1 then
               'collsion
               if (p#(0) + 1) > x and p#(0) < (x + 1) and (p#(1) + 1) > y and p#(1) < (y + 1) then
                   'p#=lp# 'go back on our last position
                   locate 8, 20: print"collision!"
                   drawtext()
                   glColor3f(1, 0, 0)
               else
                   glColor3f(0, 1, 1)
               endif

               glPushMatrix()
                   glTranslatef(x, y, 0)
                   glBegin(GL_QUADS)
                       glVertex2i(0, 0)
                       glVertex2i(0, 1)
                       glVertex2i(1, 1)
                       glVertex2i(1, 0)
                   glEnd()
               glPopMatrix()
           endif
       next
   next
   
   glPushMatrix()
       glTranslatef(p#(0), p#(1), 0)
       glBegin(GL_QUADS)
           glColor3f(0, 0, 1)
           glVertex2i(0, 0)
           glVertex2i(0, 1)
           glVertex2i(1, 1)
           glVertex2i(1, 0)
       glEnd()
   glPopMatrix()
   
   sleep(75)
   swapbuffers()
wend

if I changed this line

dim p#(1)=vec2(1, 1)

to

dim p#(1)=vec2(2, 3)

no collision, it's fine. I think I may of found a bug? Let me know if it's happening to you.

If someone can help me find the problem I'd appreciate it & also I'm creating a 2D mini game with my friend who is a graphics designer.

- Nicky

[shadow008]

I think I speak for everyone when I say we were all holding our breath waiting for someone to comment. Thank you.

Anyhow...

Something has to be up with the precision of the variables in the background.

Added in a line right after the line locate 8, 20: printr"collision!"
(note "print" got changed to "printr"):
printr p#(1) + " < " + (y + 1.0) + " = " (p#(1) < (y + 1.0))

And it's coming up with stuff like "4 < 4 = -1(true)"

Also, you need to put "drawtext()" right before "swapbuffers()" for the text to show up properly over the boxes.

The best I can come up with is to go straight floating point variables and see if the problem persists. Controlled, simple tests such as "if 4.0 > 4 then..." work out fine though.

[Nicky Peter Hollyoake]

dim a# = 0
dim x = 2

do
   a# = a# + 0.1
loop until a#>2.9

print (a# < 3)
drawtext()

On my computer that returns true.

But:

dim a# = 3

print (a# < 3)
drawtext()

returns false.

I also have to put drawtext at the end of my text for it to show...But that one isn't bugging me as much as this. It seem it only fails when you increment the float variable, weird.

Hopefully someone has the answer to this.

- Nicky

[Nicky Peter Hollyoake]

I was hoping to avoid using this way, but until I can figure out what's going on this is a work around.

'map collision test
glMatrixMode(GL_PROJECTION)
glLoadidentity()
glOrtho(0, 6, 6, 0, 0, 1)
glMatrixMode(GL_MODELVIEW)
textmode(TEXT_OVERLAID)

data 1, 1, 1, 1, 1, 1
data 1, 0, 0, 0, 0, 1
data 1, 0, 1, 0, 0, 1
data 1, 0, 0, 1, 0, 1
data 1, 0, 0, 0, 0, 1
data 1, 1, 1, 1, 1, 1

dim map(5)(5)
dim p(1), lp(1)
dim x, y

p(0) = 10
p(1) = 10

for y = 0 to 5
   for x = 0 to 5
       read map(x)(y)
   next
next

while TRUE
   glClear(GL_COLOR_BUFFER_BIT)
   cls
   
   lp = p 'last position before we move
   if scankeydown (VK_DOWN) then
       p(1) = p(1) + 1
   elseif scankeydown (VK_UP) then
       p(1) = p(1) - 1
   elseif scankeydown (VK_LEFT) then
       p(0) = p(0) - 1
   elseif scankeydown (VK_RIGHT) then
       p(0) = p(0) + 1
   endif
   
   for y = 0 to 5
       for x = 0 to 5
           if Map(x, y) = 1 then
           'collsion
           if ((P(0)/10.0) + 1) > x and (P(0)/10.0) < (x + 1) and ((P(1)/10.0) + 1) > y and (P(1)/10.0) < (y + 1) then
               P=lP 'go back on our last position
           endif
               
               glPushMatrix()
                   glTranslatef(x, y, 0)
                   glBegin(GL_QUADS)
                       glColor3f(0, 1, 1)
                       glVertex2i(0, 0)
                       glVertex2i(0, 1)
                       glVertex2i(1, 1)
                       glVertex2i(1, 0)
                   glEnd()
               glPopMatrix()
           endif
       next
   next
   
   glPushMatrix()
       glTranslatef((P(0)/10.0), (P(1)/10.0), 0)
       glBegin(GL_QUADS)
           glColor3f(0, 0, 1)
           glVertex2i(0, 0)
           glVertex2i(0, 1)
           glVertex2i(1, 1)
           glVertex2i(1, 0)
       glEnd()
   glPopMatrix()
   
   sleep(75)
   swapbuffers()
wend

I times everything by ten in a integer and I divide it by 10 when I need to. If anyone has got any better solutions, please share.

- Nicky

[shadow008]

This actually looks like a real bug.

Quick screenshot of the debugger after running:



As you can see, map(3)/map(p#(0)) should print out:
{1,0,0,1,0,1}
but instead, it prints the value of map(2):
{1,0,1,0,0,1}

This is definitely irrational behavior.  Nothing really I can help.

However, I was going to suggest you do your collision detection in an easier way, while taking advantage of the uniform grid pattern:

Something like:


if map(p#(0))(p#(1)) = 1 then
Which, technically, would be faster because it's a lookup and not a comparison.

But the above bug throws that one out the window so... I dunno what to say.

Good luck!

[matthew]

Hi Nicky, is there anything you've done differently in this program that you haven't done in the past?

As your other collision detection routines seemed to work fine.

[Nicky Peter Hollyoake]

Aug 13, 2014 9:40:37 GMT -5 matthew said:

Hi Nicky, is there anything you've done differently in this program that you haven't done in the past?

As your other collision detection routines seemed to work fine.

I've not done anything differently as you can see from my previous post it works fine when I work with integers but doesn't work with floating variables. I've checked other demos around and everything I play is off by 0.1 and then I made this check:

dim a# = 0
dim x = 2

do
a# = a# + 0.1
loop until a#>2.9

printr a#

a# = int(a#)

printr a#

a# = 3.0

printr int(a#)
drawtext()

The results for me is...

3
2
3

That's weird. Maybe it's my computer? I mean I ain't having no issues/viruses, etc. Could it be a compatibility issue? I'm running Windows 7 Ultimate 64bit.

Thank you Shadow I am aware of that routine (map(p(0), p(1))) and I'm using something similar in my main project.

- Nicky

edit: don't ask why I put val, it's gone now, but it doesn't change anything.

[matthew]

I don't think that the problem you're encountering is due to your computer or the limitations of Basic4GL. It's probably got more to do with the inaccuracies that arise when you use floating-point numbers. There's a good article about the problem on this Wikipedia page.

[Nicky Peter Hollyoake]

Oh, I kinda get it, and using "str$" gives me a better understanding:

dim a# = 0

do
a# = a# + 0.1
loop until a#>2.9

printr str$(a#)
drawtext()

How would I get a more accurate collision if I can't determine what my real digits are? For now my project is still under going using my other routine but I'd like a better solution if you got anything you suggest.

- Nicky

[matthew]

Well I found a large thread on the forum here from back in 2009. It deals with a program that SMC wrote for detecting collisions, you'll notice that it appears to use the trick that you used of multiplying everything by Ten then dividing it when you need to calculate the position. Based on articles I've seen on collision-detection; creating a Constant and then multiplying/dividing by it looks like the accepted way of doing things, sometimes programmers will use 100 instead of 10 for even more accuracy.

[DJLinux]

Hello Nicky your player has a center position so 0.5,0.5 is 0,0 in the map you know ?

Another point are your map x,y coords are integers% and the player position are single#.
In scope of rounding errors it's a good practice to subtract a short value of the player size for collision detection.
This is why i user mx=px#-.49 : my=py#-.49 and not .5

DJ
'map collision test

data 1, 1, 1, 1, 1, 1
data 1, 0, 0, 0, 0, 1
data 1, 0, 1, 0, 0, 1
data 1, 0, 0, 1, 0, 1
data 1, 0, 0, 0, 0, 1
data 1, 1, 1, 1, 1, 1

dim map(5,5)

function CollisionTest(px#,py#) as integer
 dim mx,my ' map x,y
 mx=px#-.49 : my=py#-.49 ' left top corner
 if map(mx,my) then return true : end if
              my=py#+.49 ' left bottom corner
 if map(mx,my) then return true : end if
 mx=px#+.49             ' right bootom corner
 if map(mx,my) then return true : end if
              my=py#-.49 ' right top corner
 if map(mx,my) then return true : end if
 ' no collision
 return false
end function



glMatrixMode(GL_PROJECTION)
glLoadidentity()
glOrtho(0,6,6,0,-1,1)
glMatrixMode(GL_MODELVIEW)
textmode(TEXT_OVERLAID)

dim px#=2.5
dim py#=3.5
dim lpx#
dim lpy#
dim x,y

for y = 0 to 5
 for x = 0 to 5
   read map(x,y)
 next
next

while TRUE
 glClear(GL_COLOR_BUFFER_BIT) : cls
 lpx# = px# ' last x position before we move
 lpy# = py# ' last y position before we move
 if scankeydown (VK_UP) then
   py# = py# - .1  : if py#<=0.5 then py#=0.5 : end if
 elseif scankeydown (VK_DOWN) then
   py# = py# + .1  : if py#>=5.5 then py#=5.5 : end if
 elseif scankeydown (VK_LEFT) then
   px# = px# - .1  : if px#<=0.5 then px#=0.5 : end if
 elseif scankeydown (VK_RIGHT) then
   px# = px# + .1  : if px#>=5.5 then px#=5.5 : end if
 endif
 for y = 0 to 5
   for x = 0 to 5
     if Map(x,y) = 1 then
       glPushMatrix()
         glColor3f(0,1,1)
         glTranslatef(x, y, 0)
         glBegin(GL_QUADS)
           glVertex2f(0,0)
           glVertex2f(0,1)
           glVertex2f(1,1)
           glVertex2f(1,0)
         glEnd()
       glPopMatrix()
     endif
   next
 next
 ' collsion ?
 if CollisionTest(px#,py#)=true then
   locate 0, 10: print"collision!"
   glColor3f(1, 0, 0)
   px#=lpx#
   py#=lpy#
 else
   glColor3f(0, 0, 1)
 endif
 glPushMatrix()
   glTranslatef(px#, py#, 0)
   glBegin(GL_QUADS)
     glVertex2f(-.5,-.5)
     glVertex2f(-.5, .5)
     glVertex2f( .5, .5)
     glVertex2f( .5,-.5)
   glEnd()
 glPopMatrix()
 locate 0,8 : print "player " + px# +":"+ py#  
 DrawText()
 SwapBuffers()
 Sleep(50)
wend

[Nicky Peter Hollyoake]

Ahh, I see, thank you! That actually helped a lot, appreciate it.

- Nicky