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im tryin to make a platformer base to use for future projects, but when travelling in positive directions, the collision is seemingly not detected until a full pixel further than it should be

            local xd=n.x+n.speed.x+n.w/2*sgn(n.speed.x)
            local yd=n.y+n.speed.y+n.h/2*sgn(n.speed.y)

            n.grounded=false
            n.walled=false

            if fget(mget(xd\8,(n.y+n.h/2)\8))==1 or fget(mget(xd\8,(n.y-n.h/2)\8))==1 then
                n.x=xd\8*8+(n.speed.x>0 and -1-n.w/2 or 8+n.w/2)
                n.walled=true
                n.speed.x=0
            end

            if fget(mget((n.x+n.w\2)\8,yd\8))==1 or fget(mget((n.x-n.w\2)\8,yd\8))==1 then
                n.y=yd\8*8+(n.speed.y>0 and -1-n.h/2 or 8+n.h/2)
                n.grounded=true
                n.speed.y=0
            end

            n.x+=n.speed.x
            n.y+=n.speed.y

negative velocity position readings(continuously accelerating by -0.005 pixels/second/second)
INFO: y=10
INFO: y=10
INFO: y=10
INFO: y=10
INFO: y=10
positive velocity position readings(continuously accelerating by 0.005 pixels/second/second)
INFO: y=29.7242
INFO: y=29.8109
INFO: y=29.9025
INFO: y=29.999
INFO: y=29

negative velocity position readings(given a constant velocity of -0.005 pixels/second while pressed against wall)
INFO: y=10
INFO: y=10
INFO: y=10
INFO: y=10
INFO: y=10
positive velocity position readings(given a constant velocity of 0.005 pixels/second while pressed against wall)
INFO: y=29.9844
INFO: y=29.9883
INFO: y=29.9922
INFO: y=29.9961
INFO: y=29

this happens at any speed/width/height combination, and the readings are from the last 5 frames before a collision is registered
i have a feeling this is because of flooring but i dont know how i would fix it :3

P#135108 2023-09-29 17:15 ( Edited 2023-09-29 17:16)

ok, x,y are the centre of the sprite/hitbox?

ok, when for example, the sprite is placed on 4x4 and your sprite has a with/height of 8x8, this means in your code the left xleft=x-w\2=0 and xright=x+w\2=8. which means, that the right side is outside of the box. When you paint a 8x8 sprite on 0x0, it will be drawn on 0x0 to 7x7.

I think, thats is your problem...

I would redesign the code complete by something like this:

check={
sprpos={x=-3,y=-3}, -- drawing position
left={{x=-3,y=-3},{x=-3,y=4}},
right={{x=4,y=-3},{x=3,y=4}},
top={{x=-3,y=-3},{x=4,y=-3}},
bottom={{x=-3,y=4},{x=4,y=4}}
}

And use this check-table to get the position of test and sprite drawing. Also you can alternate the test-points a little bit away from the corners, when your Sprite is not full squared.
use for the position-check something like

for pos in pairs(check.left) do
  if collision_on_pos then
     collisioncode
     break -- we don't need to check other points  
  end
end

so you can add more checkpoints, if needed (bigger sprite, unusual form).

P#135142 2023-09-30 06:34 ( Edited 2023-09-30 06:34)

oh and two important thinks. Your speed should not be greater than 8, otherwise it is possible to "skip" a block and jump through a barrier.

speed=mid(-8,speed,8)

should do the trick.

and you check if the sprite-flag 0 (value 0x01) is set and all others are not set.
When you later want to add a "ice"-block, you can use the sprite-flag1 here (value 0x02)
You must add then additional code to check.
But you can give the ice-block both flags 0&1 (0x01 | 0x02 = 0x03).

When you change your
mflag()==1
to
mflag()&1 == 1
you handle here your ice block and default block for collision. You check so only the flag 0 with the value (0x01)

P#135144 2023-09-30 06:44

x and y are the center on purpose, and i dont really want to have that many tables for each "class", as that would quickly inflate cartridge size

P#135162 2023-09-30 17:37

\(back slash) is rounded to an integer with flr().
When dealing with positive and negative values, the result will be biased toward lower values.

10.5 -> 10
-10.5 -> -11

Have you tried doing +0.5 before flr() or switching between flr() and ceil() depending on positive or negative?

P#135181 2023-09-30 23:29
1

oh shit i thought flr() was towards 0 not just down that'll probably fix it! thx!!

P#135329 2023-10-03 07:06

choose your poisen:

cls()
function dwn(i,d)
  return i>0 and flr(i/d) or ceil(i/d)
end
function up(i,d)
  return i<0 and flr(i/d) or ceil(i/d)
end

function l(i)
 b="   "..tostr(i)
 return sub(b,#b-2)
end
str=""
for i=-2,2,0x.4 do
 a=tostr(i).."      "
 str..=
    sub(a,1,5)..
    l(ceil(i))..
    l(flr(i))..
    l((i\1))..
    l(sgn(i)*ceil(abs(i)))..
    l(sgn(i)*flr(abs(i)))..
    l(dwn(i,1))..
    l(up(i,1))..
    "\n"

end
print(str)

printh(str,"@clip")
-2    -2 -2 -2 -2 -2 -2 -2
-1.75 -1 -2 -2 -2 -1 -1 -2
-1.5  -1 -2 -2 -2 -1 -1 -2
-1.25 -1 -2 -2 -2 -1 -1 -2
-1    -1 -1 -1 -1 -1 -1 -1
-0.75  0 -1 -1 -1  0  0 -1
-0.5   0 -1 -1 -1  0  0 -1
-0.25  0 -1 -1 -1  0  0 -1
0      0  0  0  0  0  0  0
0.25   1  0  0  1  0  0  1
0.5    1  0  0  1  0  0  1
0.75   1  0  0  1  0  0  1
1      1  1  1  1  1  1  1
1.25   2  1  1  2  1  1  2
1.5    2  1  1  2  1  1  2
1.75   2  1  1  2  1  1  2
2      2  2  2  2  2  2  2
P#135335 2023-10-03 10:16

thanks for the comprehensive list! can i ask why you put .25 in hex?

P#135347 2023-10-03 14:31

my first version was .2 - but thats a little problem. .2 is in hex 0x0.3333
in binar-system .2 is not a good value. The table would have rounding-errors in the first column. So i typed 0x0.2 to get a good value.

It is the same problem like 1/3 in our 10-system, it would be 0.3333333333333

P#135356 2023-10-03 17:55

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