I am trying to develop basic enemy AI on a simple platformer game after following Shaun Spalding's gamemaker 2 platformer tutorials on youtube. My code is the exact same as his on the tutorial but for some reason, when my enemy detects collision with the wall he turns around as he is suppose to and then detects another collision where there is none, causing him to turn around again.
This is my code:
// Horizontal collision
if (place_meeting(x+hsp, y, oWall)) {
show_debug_message(hsp)
while (!place_meeting(x+sign(hsp), y, oWall)) {
x += sign(hsp); // slows down I think
}
hsp = -hsp;
}
x += hsp;
The -hsp part is where he turns around. Somehow, he is detecting another collision as soon as he does so, even though the value of hsp is inverted. Can anyone possibly point me in the direction of why this may be occuring?
(Value of hsp initialized at 3 and never changed only for inversion).
Is it turning back to the wall after a short while, or is it stuck and is flickering to left and right rapidly? Both could involve that the collision point isn't updating well.
When I face with collision problems, I'll use a crosshair sprite, and draw it at the same position as where it should be colliding. that way I've a visible view of the 'collision point'.
Another cause could be the sprite's origin point, that determines at which position the x and y appears, and that the sprite by turning collides with the wall itself. Keep in mind that the origin point is at the center of it's collision mask, to avoid been stuck in a wall.
EDIT: Another possibility: the collision point still checks inside the sprite.
For that, you could also try using an offset that keeps the collision point away from the sprite collision, but to let that work, you'll need to keep the inverse direction away from your horizontal speed. Something like this:
// Horizontal collision
_offset = 15; //moves the collision point away to check in front of the sprite. value depends on the size of the sprite.
_dir = 1; //the direction, should only be 1 or -1
//hsp should no longer be used to inverse, use a new variable (like _dir) instead
collisionPoint = (hsp + offset) * _dir;
if (place_meeting(x + collisionPoint , y, oWall)) {
show_debug_message(collisionPoint)
while (!place_meeting(x+sign(collisionPoint), y, oWall)) {
x += sign(collisionPoint); // slows down I think
}
_dir = -_dir
}
x += hsp * _dir;
Hi I have the following implementation of ball to ball collision detection in ruby, which works fine for most collisions. How ever there are some flaws when balls hit each other at certain angels.
I have put my implementation down here if you need more info tell me. But Im wondering more in general terms. What causes the balls to swirl around each other at certain angels of impact.
def ball_collider! ball
for ball2 in #balls do
next if ball.object_id == ball2.object_id
next unless box_overlap ball2.boundbox, ball.boundbox
next unless ball_overlap ball, ball2
dx = ball2.x - ball.x
dy = ball2.y - ball.y
dist=Math.sqrt(dx**2 + dy**2)
bonuce_point_x = ball2.x - (ball.radii + ball2.radii) * dx / dist
bonuce_point_y = ball2.y - (ball.radii + ball2.radii) * dy / dist
bounce_line = [[bonuce_point_x,bonuce_point_y],[bonuce_point_x-dy,bonuce_point_y+dx]]
ball2.bounce! bounce_line
ball.bounce! bounce_line
motion_left = ball.unmove! bounce_line, true
ball_controller! ball if motion_left > 0.1
end
end
def box_overlap box1, box2
return (box1[:width] + box2[:width] > (box1[:x] - box2[:x]).abs) && (box1[:width] + box2[:width] > (box1[:y] - box2[:y]).abs)
end
def ball_overlap ball1, ball2
dx = ball2.x - ball1.x
dy = ball2.y - ball1.y
return (dx**2 + dy**2) < (ball1.radii+ball2.radii)**2
end
In your physics model, you are presumably updating positions and doing collision checks at discrete time intervals. This means that when you detect a collision between a pair of balls, the actual collision would have taken place somewhat before that time, and your calculations will thus be based on the wrong movement vectors. You could do calculations to get the correct time of impact, but this could get messy due to other objects being involved in collisions with the two objects you're checking within the same time interval. Try increasing the frequency (ie. reducing your time intervals) to limit the problem.
As for why you get the "swirl" effect: the calculations will cause the balls to still be colliding with each other after your modify their vectors, even when they are now moving away from each other, which will again cause your next iteration to pull them towards each other's center again, and so on and so forth.
I think swept means determining if objects will collide at some point, not just whether they are currently colliding, but if I'm wrong tell me.
I have objects with bounded boxes that are aligned on an axis. The boxes of objects can be different sizes, but they are always rectangular.
I've tried and tried to figure out an algorithm to determine if two moving AABB objects will collide at some point, but I am having a really hard time. I read a question on here about determining the time intervals when the two objects will pass at some point, and I didn't have a problem visualizing it, but implementing it was another story. It seems like there are too many exceptions, and it doesn't seem like I am doing it correctly.
The objects are only able to move in straight lines (though obviously they can change direction, e.g. turn around, but they are always on the axis. If they try to turn off the axis then it just doesn't work), and are bound to the axis. Their bounded boxes don't rotate or do anything like that. Velocity can change, but it doesn't matter since the point of the method is to determine whether, given the objects' current state, they are on a "collision course". If you need any more information let me know.
If someone could provide some pseudocode (or real code) that would be great. I read a document called Intersection of Convex Objects: The Method of Separating Axes but I didn't understand some of the pseudocode in it (what does Union mean)?
Any help is appreciated, thanks.
When a collision occurs, the boxes will touch on one side. You could check whether they would be touching for pairs of sides (LR, RL, UD, DU).
If it would simplify the problem, you could translate the boxes so the first box is at the origin and is not moving.
Something like the following code:
dLR = B.L - A.R;
dRL = A.L - B.R;
dUD = B.U - A.D;
dDU = A.U - B.D;
vX = A.xV - B.xV;
vY = A.yV - B.yV;
tLR = dLR / vX;
tRL =-dRL / vX;
tUD = dUD / vY;
tDU =-dDU / vY;
hY = dUD + dDU; //combined height
hX = dLR + dRL;
if((tLR > 0) && (abs(dDU + vY*tLR) < hY)) return true;
if((tRL > 0) && (abs(dUD - vY*tRL) < hY)) return true;
if((tUD > 0) && (abs(dRL + vX*tUD) < hX)) return true;
if((tDU > 0) && (abs(dLR - vX*tDU) < hX)) return true;
return false;
I'm working on tile collision. Currently, I just draw the tile map the normal way (two for loops) and there is no scrolling. Right now, to check if my player is over a tile, I use tileX = (int)person1v.X / 16;
tileY = (int)person1v.Y / 16;
However, I want to detect collision before I hit the tile so it could act as a wall. How do I detect collision before even making the move?
If the player moves 3 pixels at a time then check for:
leftTile = (int)(person1v.x - 3) / 16;
And for the tile to the right:
rightTile = (int)(person1v.x + 3 + 16) / 16;
well instead of moving him first and checking for collision afterwards, check for collision for the future position of the character and if there is no collision then change the character's position.
I'm writing a little tile-based game, for which I'd like to support light sources. But my algorithm-fu is too weak, hence I come to you for help.
The situation is like this: There is a tile-based map (held as a 2D array), containing a single light source and several items standing around. I want to calculate which tiles are lit up by the light source, and which are in shadow.
A visual aid of what it would look like, approximately. The L is the light source, the Xs are items blocking the light, the 0s are lit tiles, and the -s are tiles in shadow.
0 0 0 0 0 0 - - 0
0 0 0 0 0 0 - 0 0
0 0 0 0 0 X 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 L 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 X X X X 0 0
0 0 0 - - - - - 0
0 0 - - - - - - -
A fractional system would be even better, of course, where a tile can be in half-shadow due to being partially obscured. The algorithm wouldn't have to be perfect - just not obviously wrong and reasonably fast.
(Of course, there would be multiple light sources, but that's just a loop.)
Any takers?
The roguelike development community has a bit of an obsession with line-of-sight, field-of-view algorithms.
Here's a link to a roguelike wiki article on the subject:
http://roguebasin.roguelikedevelopment.org/index.php?title=Field_of_Vision
For my roguelike game, I implemented a shadow casting algorithm (http://roguebasin.roguelikedevelopment.org/index.php?title=Shadow_casting) in Python. It was a bit complicated to put together, but ran reasonably efficiently (even in pure Python) and generated nice results.
The "Permissive Field of View" seems to be gaining popularity as well:
http://roguebasin.roguelikedevelopment.org/index.php?title=Permissive_Field_of_View
You can get into all sorts of complexities with calculating occlusion etc, or you can go for the simple brute force method: For every cell, use a line drawing algorithm such as the Bresenham Line Algorithm to examine every cell between the current one and the light source. If any are filled cells or (if you have only one light source) cells that have already been tested and found to be in shadow, your cell is in shadow. If you encounter a cell known to be lit, your cell will likewise be lit. An easy optimisation to this is to set the state of any cells you encounter along the line to whatever the final outcome is.
This is more or less what I used in my 2004 IOCCC winning entry. Obviously that doesn't make good example code, though. ;)
Edit: As loren points out, with these optimisations, you only need to pick the pixels along the edge of the map to trace from.
The algorithms being presented here seem to me to be doing more calculations than I think are needed. I have not tested this but I think it would work:
Initially, mark all pixels as lit.
For every pixel on the edge of the map: As Arachnid suggested, use Bresenham to trace a line from the pixel to the light. If that line strikes an obstruction then mark all pixels from the edge to just beyond the obstruction as being in shadow.
Quick and dirty:
(Depending on how big the array is)
Loop through each tile
draw a line to the Light
If any pary of the line hits an X, then it is in shadow
(Optional): calculate the amount of X the line passes through and do fancy maths to determint the proportion of the tile in shadow. NB: This could be done by anti-aliasing the line between the tile and the Light (therefore looking at other tiles along the route back to the light source) during the thresholding procedure these will appear as small anomolies. Depending on the logic used you could potentially determine how much (if at all) the tile is in shadow.
You could also keep a track of which pixels have been tested, therefore optimize the solution a little and not re-test pixels twice.
This could be dome pretty well by using image manipulation and drawing straight lines between pixles (tiles) If the lines are semi transparent and the X blocks are semi-transparent again. You can threshold the image to determine if the line has intersected an 'X'
If you have an option to use a 3rd party tool, then Id probably take it. In the long run it might turn out to be quicker, but you'd understand less about your game.
This is just for fun:
You can replicate the Doom 3 approach in 2D if you first do a step to convert your tiles into lines. For instance,
- - - - -
- X X X -
- X X - -
- X - - -
- - - - L
...would be reduced into three lines connecting the corners of the solid object in a triangle.
Then, do what the Doom 3 engine does: From the perspective of the light source, consider each "wall" that faces the light. (In this scene, only the diagonal line would be considered.) For each such line, project it into a trapezoid whose front edge is the original line, whose sides lie on lines from the light source through each end point, and whose back is far away, past the whole scene. So, it's a trapezoid that "points at" the light. It contains all the space that the wall casts its shadow on. Fill every tile in this trapezoid with darkness.
Proceed through all such lines and you will end up with a "stencil" that includes all the tiles visible from the light source. Fill these tiles with the light color. You may wish to light the tile a little less as you get away from the source ("attenuation") or do other fancy stuff.
Repeat for every light source in your scene.
To check if a tile is in shadow you need to draw a straight line back to the light source. If the line intersects another tile that's occupied, then the tile you were testing is in shadow. Raytracing algorithms do this for every object (in your case tile) in the view.
The Raytracing article on Wikipedia has pseudocode.
Here is a very simple but fairly effective approach that uses linear time in the number of tiles on screen. Each tile is either opaque or transparent (that's given to us), and each can be visible or shaded (that's what we're trying to compute).
We start by marking the avatar itself as "visible".
We then apply this recursive rule to determine the visibility of the remaining tiles.
If the tile is on the same row or column as the avatar, then it is only visible if the adjacent tile nearer to the avatar is visible and transparent.
If the tile is on a 45 degree diagonal from the avatar, then it is only visible if the neighboring diagonal tile (towards the avatar) is visible and transparent.
In all other cases, consider the three neighboring tiles that are closer to the avatar than the tile in question. For example, if this tile is at (x,y) and is above and to the right of the avatar, then the three tiles to consider are (x-1, y), (x, y-1) and (x-1, y-1). The tile in question is visible if any of those three tiles are visible and transparent.
In order to make this work, the tiles must be inspected in a specific order to ensure that the recursive cases are already computed. Here is an example of a working ordering, starting from 0 (which is the avatar itself) and counting up:
9876789
8543458
7421247
6310136
7421247
8543458
9876789
Tiles with the same number can be inspected in any order amongst themselves.
The result is not beautiful shadow-casting, but computes believable tile visibility.
I know this is years old question, but for anyone searching for this style of stuff I'd like to offer a solution I used once for a roguelike of my own; manually "precalculated" FOV. If you field of view of light source has a maximum outer distance it's really not very much effort to hand draw the shadows created by blocking objects. You only need to draw 1/8 th of the circle (plus the straight and diagonal directions); you can use symmerty for the other eigths. You'll have as many shadowmaps as you have squares in that 1/8th of a circle. Then just OR them together according to objects.
The three major pros for this are:
1. It's very quick if implemented right
2. You get to decide how the shadow should be cast, no comparing which algorith handles which situation the best
3. No weird algorith induced edge cases which you have to somehow fix
The con is you don't really get to implement a fun algorithm.
TK's solution is the one that you would generally use for this sort of thing.
For the partial lighting scenario, you could have it so that if a tile results in being in shadow, that tile is then split up into 4 tiles and each one of those is tested. You could then split that up as much as you wanted?
Edit:
You can also optimise it out a bit by not testing any of the tiles adjacent to a light - this would be more important to do when you have multiple light sources, I guess...
I've actually just recently wrote this functionality into one of my projects.
void Battle::CheckSensorRange(Unit* unit,bool fog){
int sensorRange = 0;
for(int i=0; i < unit->GetSensorSlots(); i++){
if(unit->GetSensorSlot(i)->GetSlotEmpty() == false){
sensorRange += unit->GetSensorSlot(i)->GetSensor()->GetRange()+1;
}
}
int originX = unit->GetUnitX();
int originY = unit->GetUnitY();
float lineLength;
vector <Place> maxCircle;
//get a circle around the unit
for(int i = originX - sensorRange; i < originX + sensorRange; i++){
if(i < 0){
continue;
}
for(int j = originY - sensorRange; j < originY + sensorRange; j++){
if(j < 0){
continue;
}
lineLength = sqrt( (float)((originX - i)*(originX - i)) + (float)((originY - j)*(originY - j)));
if(lineLength < (float)sensorRange){
Place tmp;
tmp.x = i;
tmp.y = j;
maxCircle.push_back(tmp);
}
}
}
//if we're supposed to fog everything we don't have to do any fancy calculations
if(fog){
for(int circleI = 0; circleI < (int) maxCircle.size(); circleI++){
Map->GetGrid(maxCircle[circleI].x,maxCircle[circleI].y)->SetFog(fog);
}
}else{
bool LOSCheck = true;
vector <bool> placeCheck;
//have to check all of the tiles to begin with
for(int circleI = 0; circleI < (int) maxCircle.size(); circleI++){
placeCheck.push_back(true);
}
//for all tiles in the circle, check LOS
for(int circleI = 0; circleI < (int) maxCircle.size(); circleI++){
vector<Place> lineTiles;
lineTiles = line(originX, originY, maxCircle[circleI].x, maxCircle[circleI].y);
//check each tile in the line for LOS
for(int lineI = 0; lineI < (int) lineTiles.size(); lineI++){
if(false == CheckPlaceLOS(lineTiles[lineI], unit)){
LOSCheck = false;
//mark this tile not to be checked again
placeCheck[circleI] = false;
}
if(false == LOSCheck){
break;
}
}
if(LOSCheck){
Map->GetGrid(maxCircle[circleI].x,maxCircle[circleI].y)->SetFog(fog);
}else{
LOSCheck = true;
}
}
}
}
There's some extra stuff in there that you wouldn't need if you're adapting it for your own use. The type Place is just defined as an x and y position for conveniences sake.
The line function is taken from Wikipedia with very small modifications. Instead of printing out x y coordinates I changed it to return a place vector with all the points in the line. The CheckPlaceLOS function just returns true or false based on if the tile has an object on it. There's some more optimizations that could be done with this but this is fine for my needs.
i have implemented tilebased field of view in a single C function. here it is:
https://gist.github.com/zloedi/9551625
If you don't want to spend the time to reinvent/re-implement this, there are plenty of game engines out there. Ogre3D is an open source game engine that fully supports lighting, as well as sound and game controls.