Finding points on a rectangle at a given angle

Question

I'm trying to draw a gradient in a rectangle object, with a given angle (Theta), where the ends of the gradient are touching the perimeter of the rectangle.

I thought that using tangent would work, but I'm having trouble getting the kinks out. Is there an easy algorithm that I am just missing?

End Result

So, this is going to be a function of (angle, RectX1, RectX2, RectY1, RectY2). I want it returned in the form of [x1, x2, y1, y2], so that the gradient will draw across the square. In my problem, if the origin is 0, then x2 = -x1 and y2 = -y1. But it's not always going to be on the origin.

Answer 1

Let's call a and b your rectangle sides, and (x0,y0) the coordinates of your rectangle center.

You have four regions to consider:

    Region    from               to                 Where
    ====================================================================
       1      -arctan(b/a)       +arctan(b/a)       Right green triangle
       2      +arctan(b/a)        π-arctan(b/a)     Upper yellow triangle
       3       π-arctan(b/a)      π+arctan(b/a)     Left green triangle
       4       π+arctan(b/a)     -arctan(b/a)       Lower yellow triangle

With a little of trigonometry-fu, we can get the coordinates for your desired intersection in each region.

So Z0 is the expression for the intersection point for regions 1 and 3
And Z1 is the expression for the intersection point for regions 2 and 4

The desired lines pass from (X0,Y0) to Z0 or Z1 depending the region. So remembering that Tan(φ)=Sin(φ)/Cos(φ)

    Lines in regions      Start                   End
    ======================================================================
       1 and 3           (X0,Y0)      (X0 + a/2 , (a/2 * Tan(φ))+ Y0
       2 and 4           (X0,Y0)      (X0 + b/(2* Tan(φ)) , b/2 + Y0)

Just be aware of the signs of Tan(φ) in each quadrant, and that the angle is always measured from THE POSITIVE x axis ANTICLOCKWISE.

HTH!

Answer 2

Ok, whew!, I finally got this one.

NOTE: I based this off of belisarius's awesome answer. If you like this, please like his, too. All I did was turn what he said into code.

Here's what it looks like in Objective-C. It should be simple enough to convert to whatever your favorite language is.

+ (CGPoint) edgeOfView: (UIView*) view atAngle: (float) theta
{
    // Move theta to range -M_PI .. M_PI
    const double twoPI = M_PI * 2.;
    while (theta < -M_PI)
    {
        theta += twoPI;
    }

    while (theta > M_PI)
    {
        theta -= twoPI;
    }

    // find edge ofview
    // Ref: http://stackoverflow.com/questions/4061576/finding-points-on-a-rectangle-at-a-given-angle
    float aa = view.bounds.size.width;                                          // "a" in the diagram
    float bb = view.bounds.size.height;                                         // "b"

    // Find our region (diagram)
    float rectAtan = atan2f(bb, aa);
    float tanTheta = tan(theta);

    int region;
    if ((theta > -rectAtan)
    &&  (theta <= rectAtan) )
    {
        region = 1;
    }
    else if ((theta >  rectAtan)
    &&       (theta <= (M_PI - rectAtan)) )
    {
        region = 2;
    }
    else if ((theta >   (M_PI - rectAtan))
    ||       (theta <= -(M_PI - rectAtan)) )
    {
        region = 3;
    }
    else
    {
        region = 4;
    }

    CGPoint edgePoint = view.center;
    float xFactor = 1;
    float yFactor = 1;

    switch (region)
    {
        case 1: yFactor = -1;       break;
        case 2: yFactor = -1;       break;
        case 3: xFactor = -1;       break;
        case 4: xFactor = -1;       break;
    }

    if ((region == 1)
    ||  (region == 3) )
    {
        edgePoint.x += xFactor * (aa / 2.);                                     // "Z0"
        edgePoint.y += yFactor * (aa / 2.) * tanTheta;
    }
    else                                                                        // region 2 or 4
    {
        edgePoint.x += xFactor * (bb / (2. * tanTheta));                        // "Z1"
        edgePoint.y += yFactor * (bb /  2.);
    }

    return edgePoint;
}

In addition, here's a little test-view I created to verify that it works. Create this view and put it somewhere, it will make another little view scoot around the edge.

@interface DebugEdgeView()
{
    int degrees;
    UIView *dotView;
    NSTimer *timer;
}

@end

@implementation DebugEdgeView

- (void) dealloc
{
    [timer invalidate];
}


- (id) initWithFrame: (CGRect) frame
{
    self = [super initWithFrame: frame];
    if (self)
    {
        self.backgroundColor = [[UIColor magentaColor] colorWithAlphaComponent: 0.25];
        degrees = 0;
        self.clipsToBounds = NO;

        // create subview dot
        CGRect dotRect = CGRectMake(frame.size.width / 2., frame.size.height / 2., 20, 20);
        dotView = [[DotView alloc] initWithFrame: dotRect];
        dotView.backgroundColor = [UIColor magentaColor];
        [self addSubview: dotView];

        // move it around our edges
        timer = [NSTimer scheduledTimerWithTimeInterval: (5. / 360.)
                                                 target: self
                                               selector: @selector(timerFired:)
                                               userInfo: nil
                                                repeats: YES];
    }

    return self;
}


- (void) timerFired: (NSTimer*) timer
{
    float radians = ++degrees * M_PI / 180.;
    if (degrees > 360)
    {
        degrees -= 360;
    }

    dispatch_async(dispatch_get_main_queue(), ^{
        CGPoint edgePoint = [MFUtils edgeOfView: self atAngle: radians];
        edgePoint.x += (self.bounds.size.width  / 2.) - self.center.x;
        edgePoint.y += (self.bounds.size.height / 2.) - self.center.y;
        dotView.center = edgePoint;
    });
}

@end

Answer 3

Javascript version:

function edgeOfView(rect, deg) {
  var twoPI = Math.PI*2;
  var theta = deg * Math.PI / 180;
  
  while (theta < -Math.PI) {
    theta += twoPI;
  }
  
  while (theta > Math.PI) {
    theta -= twoPI;
  }
  
  var rectAtan = Math.atan2(rect.height, rect.width);
  var tanTheta = Math.tan(theta);
  var region;
  
  if ((theta > -rectAtan) && (theta <= rectAtan)) {
      region = 1;
  } else if ((theta > rectAtan) && (theta <= (Math.PI - rectAtan))) {
      region = 2;
  } else if ((theta > (Math.PI - rectAtan)) || (theta <= -(Math.PI - rectAtan))) {
      region = 3;
  } else {
      region = 4;
  }
  
  var edgePoint = {x: rect.width/2, y: rect.height/2};
  var xFactor = 1;
  var yFactor = 1;
  
  switch (region) {
    case 1: yFactor = -1; break;
    case 2: yFactor = -1; break;
    case 3: xFactor = -1; break;
    case 4: xFactor = -1; break;
  }
  
  if ((region === 1) || (region === 3)) {
    edgePoint.x += xFactor * (rect.width / 2.);                                     // "Z0"
    edgePoint.y += yFactor * (rect.width / 2.) * tanTheta;
  } else {
    edgePoint.x += xFactor * (rect.height / (2. * tanTheta));                        // "Z1"
    edgePoint.y += yFactor * (rect.height /  2.);
  }
  
  return edgePoint;
};

Answer 4

Following your picture, I'm going to assume that the rectangle is centered at (0,0), and that the upper right corner is (w,h). Then the line connecting (0,0) to (w,h) forms an angle φ with the X axis, where tan(φ) = h/w.

Assuming that θ > φ, we are looking for the point (x,y) where the line that you have drawn intersects the top edge of the rectangle. Then y/x = tan(θ). We know that y=h so, solving for x, we get x = h/tan(θ).

If θ < φ, the line intersects with the right edge of the rectangle at (x,y). This time, we know that x=w, so y = tan(θ)*w.

Answer 5

There's a good (more programmatic iOS / Objective-C) answer to this question at Find the CGPoint on a UIView rectangle intersected by a straight line at a given angle from the center point involving the following steps:

1. Assume that the angle is greater than or equal to 0 and less than 2*π, going counterclockwise from 0 (East).
2. Get the y coordinate of the intersection with the right edge of the rectangle [tan(angle)*width/2].
3. Check whether this y coordinate is in the rectangle frame (absolute value less than or equal to half the height).
4. If the y intersection is in the rectangle, then if the angle is less than π/2 or greater than 3π/2 choose the right edge (width/2, -y coord). Otherwise choose the left edge (-width/2, y coord).
5. If the y coordinate of the right edge intersection was out-of-bounds, calculate the x coordinate of the intersection with the bottom edge [half the height/tan(angle)].
6. Next determine whether you want the top edge or the bottom edge. If the angle is less than π, we want the bottom edge (x, -half the height). Otherwise, we want the top edge (-x coord, half the height).
7. Then (if the center of the frame is not 0,0), offset the point by the actual center of the frame.

Answer 6

For Java, LibGDX. I've let the angle be a double to increase precision.

public static Vector2 projectToRectEdge(double angle, float width, float height, Vector2 out)
{
    return projectToRectEdgeRad(Math.toRadians(angle), width, height, out);
}

public static Vector2 projectToRectEdgeRad(double angle, float width, float height, Vector2 out)
{
    float theta = negMod((float)angle + MathUtils.PI, MathUtils.PI2) - MathUtils.PI;

    float diag = MathUtils.atan2(height, width);
    float tangent = (float)Math.tan(angle);

    if (theta > -diag && theta <= diag)
    {
        out.x = width / 2f;
        out.y = width / 2f * tangent;
    }
    else if(theta > diag && theta <= MathUtils.PI - diag)
    {
        out.x = height / 2f / tangent;
        out.y = height / 2f;
    }
    else if(theta > MathUtils.PI - diag && theta <= MathUtils.PI + diag)
    {
        out.x = -width / 2f;
        out.y = -width / 2f * tangent;
    }
    else
    {
        out.x = -height / 2f / tangent;
        out.y = -height / 2f;
    }

    return out;
}

Answer 7

Unreal Engine 4 (UE4) C++ Version.

Note: This is based off of Olie's Code. Based on Belisarius's Answer. Give those guys upvotes if this helps you.

Changes: Uses UE4 syntax and functions, and Angle is negated.

Header

UFUNCTION(BlueprintCallable, meta = (DisplayName = "Project To Rectangle Edge (Radians)"), Category = "Math|Geometry")
static void ProjectToRectangleEdgeRadians(FVector2D Extents, float Angle, FVector2D & EdgeLocation);

Code

void UFunctionLibrary::ProjectToRectangleEdgeRadians(FVector2D Extents, float Angle, FVector2D & EdgeLocation)
{
    // Move theta to range -M_PI .. M_PI. Also negate the angle to work as expected.
    float theta = FMath::UnwindRadians(-Angle);

    // Ref: http://stackoverflow.com/questions/4061576/finding-points-on-a-rectangle-at-a-given-angle
    float a = Extents.X; // "a" in the diagram | Width
    float b = Extents.Y; // "b"                | Height

    // Find our region (diagram)
    float rectAtan = FMath::Atan2(b, a);
    float tanTheta = FMath::Tan(theta);

    int region;
    if ((theta > -rectAtan) && (theta <= rectAtan))
    {
        region = 1;
    }
    else if ((theta > rectAtan) && (theta <= (PI - rectAtan)))
    {
        region = 2;
    }
    else if ((theta > (PI - rectAtan)) || (theta <= -(PI - rectAtan)))
    {
        region = 3;
    }
    else
    {
        region = 4;
    }

    float xFactor = 1.f;
    float yFactor = 1.f;

    switch (region)
    {
        case 1: yFactor = -1; break;
        case 2: yFactor = -1; break;
        case 3: xFactor = -1; break;
        case 4: xFactor = -1; break;
    }

    EdgeLocation = FVector2D(0.f, 0.f); // This rese is nessesary, UE might re-use otherwise. 

    if (region == 1 || region == 3)
    {
        EdgeLocation.X += xFactor * (a / 2.f);              // "Z0"
        EdgeLocation.Y += yFactor * (a / 2.f) * tanTheta;
    }
    else // region 2 or 4
    {
        EdgeLocation.X += xFactor * (b / (2.f * tanTheta)); // "Z1"
        EdgeLocation.Y += yFactor * (b / 2.f);
    }
}

Answer 8

PYTHON

import math
import matplotlib.pyplot as plt

twoPI = math.pi * 2.0
PI = math.pi

def get_points(width, height, theta):
    theta %= twoPI

    aa = width
    bb = height

    rectAtan = math.atan2(bb,aa)
    tanTheta = math.tan(theta)

    xFactor = 1
    yFactor = 1
    
    # determine regions
    if theta > twoPI-rectAtan or theta <= rectAtan:
        region = 1
    elif theta > rectAtan and theta <= PI-rectAtan:
        region = 2

    elif theta > PI - rectAtan and theta <= PI + rectAtan:
        region = 3
        xFactor = -1
        yFactor = -1
    elif theta > PI + rectAtan and theta < twoPI - rectAtan:
        region = 4
        xFactor = -1
        yFactor = -1
    else:
        print(f"region assign failed : {theta}")
        raise
    
    # print(region, xFactor, yFactor)
    edgePoint = [0,0]
    ## calculate points
    if (region == 1) or (region == 3):
        edgePoint[0] += xFactor * (aa / 2.)
        edgePoint[1] += yFactor * (aa / 2.) * tanTheta
    else:
        edgePoint[0] += xFactor * (bb / (2. * tanTheta))
        edgePoint[1] += yFactor * (bb /  2.)

    return region, edgePoint

l_x = []
l_y = []
theta = 0
for _ in range(10000):
    r, (x, y) = get_points(600,300, theta)
    l_x.append(x)
    l_y.append(y)
    theta += (0.01 / PI)

    if _ % 100 == 0:
        print(r, x,y)

plt.plot(l_x, l_y)
plt.show()

Answer 9

Unity C# (Converted from Winter's Java code)

    public Vector2 DetermineRectangleEdge(float aDegrees, float aWidth, float aHeight) {

        if (aDegrees < -90)
            aDegrees += 360f;

        float ANGLE = Mathf.Deg2Rad * aDegrees;
        float diag = Mathf.Atan2(aHeight, aWidth);
        float tangent = Mathf.Tan(ANGLE);

        Vector2 OUT = Vector2.zero;

        if (ANGLE > -diag && ANGLE <= diag)
        {
            OUT.x = aWidth / 2f;
            OUT.y = aWidth / 2f * tangent;
            _ObjectRectTransform.sizeDelta = _VerticalSize;
        }
        else if(ANGLE > diag && ANGLE <= Mathf.PI - diag)
        {
            OUT.x = aHeight / 2f / tangent;
            OUT.y = aHeight / 2f;
            _ObjectRectTransform.sizeDelta = _HorizontalSize;
        }
        else if(ANGLE > Mathf.PI - diag && ANGLE <= Mathf.PI + diag)
        {
            OUT.x = -aWidth / 2f;
            OUT.y = -aWidth / 2f * tangent;
            _ObjectRectTransform.sizeDelta = _VerticalSize;
        }
        else
        {
            OUT.x = -aHeight / 2f / tangent;
            OUT.y = -aHeight / 2f;
            _ObjectRectTransform.sizeDelta = _HorizontalSize;
        }

        return OUT;
        
    }