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Topic: weird trig problem (Read 370 times) |
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wilson
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weird trig problem
« on: Apr 12th, 2004, 4:32am » |
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hi.
So I'm doing this bigger project, but am trying to test this bit of code to make sure it works before I move on. Basically, I want to have individual nodes reposition themselves according to the perspective of a single entity - in the project, this will be a user, in the test code, it's a rectangle. For example, there are a bunch of nodes positioned around a center point. When the user moves forward or backward, these nodes move closer / farther away along the y-axis. When the user turns (using the A or D keys, the nodes move closer / farther away from the user's vision -- which means they are basically just rotating around a circle with the center point the x,y of the user (which in this case happens to be the center of the stage).
But I'm having a slight problem making this work. A quick test of the below code should make clear what I want to have happen. And everything is working as long as the angle of the node is between 0-180 degrees. However, if you try and position a node in front of you (180-360 degrees, assuming 270 degrees is right in front of the you) by moving it along the y axis (with w and s keys), then it seems to flip the axis.
This seems like it should be a simple problem, but I'm having a really difficult go at it.
If there is someone out there who sees a solution, I'd be ever so appreciative of a reply.
Code is below. Thanks for any help in advance.
_Will
float i; //angle
color c; //color of circle
float x, y, track_x, track_y; // x, y are stage locations,
//track_x, track_y are locations relative to the center of the circle (the square)
boolean moveY; // true when user is moving along the y axis (using w and s keys)
float radius, convert; //radius is the radius between circle and the center square
String direction; // to tell showDirections which kind of arrow to draw
void setup()
{
size(400, 400);
i=90; //start w/ radius of 90 degrees
c=color(0, 0, 0);
background(255);
moveY = true; //assume we are moving forward initially
convert = radians(i); //convert i degrees to radians to work with
radius = 50; //initial radius of 50 px
track_x = radius * cos(convert); //grab x position from center
track_y = radius * sin(convert); //y position from center
x = width/2+track_x; //x position on stage
y = height/2+track_y; //y position on stage
}
void loop()
{
//static stuff
background(255);
stroke(0);
fill(0);
rectMode(CENTER_RADIUS);
ellipseMode(CENTER_RADIUS);
rect(200, 200, 5, 5); //draw out a rectangle at center -- represents the user
showDirection(direction); // pass direction to a function
// that draws out the appropriate arrow
//trig stuff
noStroke();
fill(c);
radius = dist(width/2, height/2, x, y); // calculate radius
if (moveY) { //if we are moving forwards or backwards along the y axis
//use cosA=b/c
float temp = track_x/radius;
//grab updated angle
i = degrees(float(Math.acos(temp)));
//update ellipse data in relation to center point
convert = radians(i);
track_x = radius * cos(convert);
track_y = radius * sin(convert);
//draw out ellipse
ellipse(x, y, 5, 5);
} else {
//reposition the ellipse according to the current radius and angle
convert = radians(i);
track_x = radius * cos(convert);
track_y = radius * sin(convert);
//update stage position
x = width/2+track_x;
y = height/2+track_y;
ellipse(x, y, 5, 5);
}
keyPressed();
}
void mousePressed() {
println(i);
}
//handles the key stuff, moving forward, backward, or turning left or right
void keyPressed()
{
if (key == 'a') {
if (i<=0) {
i=360;
} else {
i-=1;
}
direction="LEFT";
moveY=false;
}
if (key == 'd') {
if (i>=360) {
i=0;
} else {
i+=1;
}
direction="RIGHT";
moveY=false;
}
if (key == 'w') {
y+=2;
direction="FORWARDS";
moveY=true;
}
if (key == 's') {
y-=2;
direction="BACKWARDS";
moveY=true;
}
}
//draws out helper arrows that signal current direction / rotation
void showDirection(String dir)
{
if (dir=="NONE") {
rect(10, 10, 10, 10);
}
if (dir=="LEFT") {
line(10, 20, 50, 20);
line(40, 10, 50, 20);
line(40, 30, 50, 20);
}
if (dir=="RIGHT") {
line(10, 20, 50, 20);
line(20, 10, 10, 20);
line(20, 30, 10, 20);
}
if (dir=="FORWARDS") {
line(10, 20, 20, 10);
line(20, 10, 30, 20);
line(20, 10, 20, 50);
}
if (dir=="BACKWARDS") {
line(20, 10, 20, 50);
line(10, 40, 20, 50);
line(30, 40, 20, 50);
}
}
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arielm
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Re: weird trig problem
« Reply #1 on: Apr 12th, 2004, 10:27am » |
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not tested, just an intuition: try to use atan2() instead of acos() or asin()
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Ariel Malka | www.chronotext.org
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wilson
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Re: weird trig problem
« Reply #2 on: Apr 12th, 2004, 4:56pm » |
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hm. atan2 doesn't seem to work, although I'm not sure exactly why.
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wilson
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Re: weird trig problem
« Reply #3 on: Apr 12th, 2004, 5:11pm » |
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also... the var track_x (which holds the x position of the ellipse relative to the center point of the stage) seems to be inconsistently positive or negative... not sure why that is, but it may be a problem -- and abs(track_x) is giving me a NaN at certain points...
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arielm
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Re: weird trig problem
« Reply #4 on: Apr 12th, 2004, 5:56pm » |
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i didn't meant to replace acos() as it is with atan2(),
rather:
you use the "cosA=b/c" pattern to find out an angle, so i meant that you have to adapt this pattern to "tanA=a/b" and then: A = atan2(b, a)
all this because acos() and asin() are well-known trouble makers (i.e. producing NaN, aka errors, at some angles), also atan2() is always returning a "full angle" (i.e. no headache with signs, directions, etc...)
another tip: you don't have to convert to degrees() and then back to radians()... just keep everything in radians...
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Ariel Malka | www.chronotext.org
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wilson
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Re: weird trig problem
« Reply #5 on: Apr 12th, 2004, 7:07pm » |
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thanks. I changed the angle calculation to:
i = atan2(y-(height/2), x-(width/2));
(substracting height/2, width/2 to position according to the 0, 0 position of the coordinate origin -- could also prob. just use translate)
so this works in most cases, but also, by what I can tell, yeilds random errors at some points...
thanks for all your help -- any other suggestions would be much appreciated.
code now looks like:
//not yeilding a negative track_y
if (moveY) { //if we are moving forwards or backwards along the y axis
//use tanA=a/b
//grab updated angle
i = atan2(y-(height/2), x-(width/2));
//update ellipse data in relation to center point
track_x = radius * cos(i);
track_y = -(radius * sin(i));
//draw out ellipse
ellipse(x, y, 5, 5);
} else {
//reposition the ellipse according to the current radius and angle
track_x = radius * cos(i);
track_y = radius * sin(i);
//update stage position
x = width/2+track_x;
y = height/2+track_y;
ellipse(x, y, 5, 5);
}
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wilson
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Re: weird trig problem
« Reply #6 on: Apr 12th, 2004, 7:15pm » |
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update: the above code seems to work, above 180 degrees, when the user is rotating clockwise... but counterclockwise rotation resets angle (i) to 0 degrees. ugh.
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