This is the long due fifth article in this series. If you aren’t comfortable with vectors, you might want to take a look at the first four articles in this series before: Introduction, Vectors 101, Geometrical Representation of Vectors, Operations on Vectors.
This cheat sheet will list several common geometrical problems found in games, and how to solve them with vector math.
Complete list of basic vector operations
But first, a little review.
For this, I assume that you have a vector class readily available. This is mostly 2Dfocused, but everything works the same for 3D, except for differences concerning vector product, which I will assume to return just a scalar in the 2D case, representing the “z” axis. Any case that only applies to 2D or 3D will be pointed out.
Strictly speaking, a point is not a vector – but a vector can be used to represent the distance from the origin (0, 0) to the point, and so, it is perfectly reasonable to just use vectors to represent positions as if they were points.
I expect the class to give you access to each of the components, and to the following operations (using C++ style notation, including operator overloading – but it should be easy to translate to any other language of your choice). If a given operation is not available, you can still do it manually, either by extending the class or creating a “VectorUtils” class. The examples below are usually for 2D vectors – but 3D is usually simply a matter of adding the z coordinate following the pattern of x and y.

Vector2f operator+(Vector2f vec): Returns the sum of the two vectors. (In a language without operator overloading, this will probably be called add(). Similarly for the next few ones.)
a + b = Vector2f(a.x + b.x, a.y + b.y); 
Vector2f operator(Vector2f vec): Returns the difference between the two vectors.
a – b = Vector2f(a.x – b.x, a.y – b.y); 
Vector2f operator*(Vector2f vec): Returns the componentwise multiplication of the vectors.
a * b = Vector2f(a.x * b.x, a.y * b.y);  Vector2f operator/(Vector2f vec): Returns the componentwise division of the vectors.
a / b = Vector2f(a.x / b.x, a.y / b.y); 
Vector2f operator*(float scalar): Returns the vector with all components multiplied by the scalar parameter.
a * s = Vector2f(a.x * s, a.y * s);
s * a = Vector2f(a.x * s, a.y * s); 
Vector2f operator/(float scalar): Returns the vector with all components divided by the scalar parameter.
a / s = Vector2f(a.x / s, a.y / s); 
float dot(Vector2f vec): Returns the dot product between the two vectors.
a.dot(b) = a.x * b.x + a.y * b.y; 
float cross(Vector2f vec): (2D case) Returns the z component of the cross product of the two vectors augmented to 3D.
a.cross(b) = a.x * b.y – a.y * b.x; 
Vector3f cross(Vector3f vec): (3D case) Returns the cross product of the two vectors.
a.cross(b) = Vector3f(a.y * b.z – a.z * b.y, a.z*b.x – a.x*b.z, a.x*b.y – a.y*b.x); 
float length(): Returns the length of the vector.
a.length() = sqrt(a.x * a.x + a.y * a.y); 
float squaredLength(): Returns the square of the length of the vector. Useful when you just want to compare two vectors to see which is longest, as this avoids computing square roots
a.squaredLength() = a.x * a.x + a.y * a.y; 
float unit(): Returns a vector pointing on the same direction, but with a length of 1.
a.unit() = a / a.length(); 
Vector2f turnLeft(): Returns the vector rotated 90 degrees left. Useful for computing normals. (Assumes that y axis points up, otherwise this is turnRight)
a.turnLeft = Vector2f(a.y, a.x); 
Vector2f turnRight(): Returns the vector rotated 90 degrees right. Useful for computing normals. (Assumes that y axis points up, otherwise this is turnLeft)
a.turnRight = Vector2f(a.y, a.x); 
Vector2f rotate(float angle): Rotates the vector by the specified angle. This is an extremely useful operation, though it is rarely found in Vector classes. Equivalent to multiplying by the 2×2 rotation matrix.
a.rotate(angle) = Vector2f(a.x * cos(angle) – a.y * sin(angle), a.x * sin(angle) + a.y * cos(angle)); 
float angle(): Returns the angle that the vector points to.
a.angle() = atan2(a.y, a.x);
Simple cases – warming up
Case #01 – Distance between two points
You probably know that this is done with the Pythagorean theorem, but the vectorial way is simpler. Given two vectors a and b:
float distance = (ab).length();
Case #02 – Alignment
Sometimes, you want to align an image by its center. Sometimes, by its topleft corner. Or sometimes, by its topcenter point. More generally, you can do alignment using a vector whose two components go from 0 to 1 (or even beyond, if you’d like), giving you full control of alignment.
// imgPos, imgSize and align are all Vector2f Vector2f drawPosition = imgPos + imgSize * align
Case #03 – Parametric Line Equation
Two points define a line, but it can be tricky to do much with this definition. A better way to work with a line is its parametric equation: one point (“P0″) and a direction vector (“dir”).
Vector2f p0 = point1; Vector2f dir = (point2  point1).unit();
With this, you can, for example, get a point 10 units away by simply doing:
Vector2f p1 = p0 + dir * 10;
Case #04 – Midpoint and interpolation between points
Say you have vectors p0 and p1. The midpoint between them is simply (p0+p1)/2. More generally, the line segment defined by p0 and p1 can be generated by varying t between 0 and 1 in the following linear interpolation:
Vector2f p = (1t) * p0 + t * p1;
At t = 0, you get p0; at t = 1, you get p1; at t = 0.5, you get the midpoint, etc.
Case #05 – Finding the normal of a line segment
You already know how to find the direction vector of a line segment (case #03). The normal vector is a 90 degree rotation of that, so just call turnLeft() or turnRight() on it!
Projections using the Dot Product
The dot product has the incredibly useful property of being able to compute the length of a vector’s projection along the axis of another. To do this, you need the vector that you’ll project (“a“) and a unit vector (so make sure that you call unit() on it first!) representing the direction (“dir“). The length is then simply a.dot(dir). For example, if you have a = (3, 4) and dir = (1, 0), then a.dot(dir) = 3, and you can tell that this is correct, because (1, 0) is the direction vector of the x axis. In fact, a.x is always equivalent to a.dot(Vector2f(1, 0)), and a.y is equivalent to a.dot(Vector2f(0, 1)).
Because the dot product between a and b is also defined as abcos(alpha) (where alpha is the angle between the two), the result will be 0 if the two vectors are perpendicular, positive if the angle between them is less than 90, and negative if greater. This can be used to tell if two vectors point in the same general direction.
If you multiply the result of that dot product by the direction vector itself, you get the vector projected along that axis – let’s call that “at” (t for tangent). If you now do a – at, you get the part of the vector that is perpendicular to the dir vector – let’s call that “an” (n for normal). at + an = a.
Case #06 – Determining direction closest to dir
Say that you have a list of directions represented as unit vectors, and you want to find which of them is the closest to dir. Simply find the largest dot product between dir and a vector in the list. Likewise, the smallest dot product will be the direction farthest away.
Case #07 – Determining if the angle between two vectors is less than alpha
Using the equation above, we know that the angle between two vectors a and b will be less than alpha if the dot product between their unit vectors is less than cosine of alpha.
bool isLessThanAlpha(Vector2f a, Vector2f b, float alpha) { return a.unit().dot(b.unit()) < cos(alpha); }
Case #08 – Determining which side of a halfplane a point is on
Say that you have an arbitrary point in space, p0, and a direction (unit) vector, dir. Imagine that an infinite line goes by p0, perpendicular to dir, dividing the plane in two, the halfplane that dir points to, and the halfplane that it does not point to. How do I tell whether a point p is in the side pointed to by dir? Remember that dot product is positive when the angle between vectors is less than 90 degrees, so just project and check against that:
bool isInsideHalfPlane(Vector2f p, Vector2f p0, Vector dir) { return (p  p0).dot(dir) >= 0; }
Case #09 – Forcing a point to be inside a halfplane
Similar to the case above, but instead of just checking, we’ll grab the projection and, if less than 0, use it to move the object projection along dir, so it’s on the edge of the halfplane.
Vector2f makeInsideHalfPlane(Vector2f p, Vector2f p0, Vector dir) { float proj = (p  p0).dot(dir); if (proj >= 0) return p; else return p  proj * dir; }
Case #10 – Checking/forcing a point inside a convex polygon
A convex polygon can be defined to be the intersection of several halfplanes, one for each edge of the polygon. Their p0 is either vertex of the edge, and their dir is the edge’s innerfacing normal vector (e.g., if you wind clockwise, that’d be the turnRight() normal). A point is inside the polygon if and only if it’s inside all the halfplanes. Likewise, you can force it to be inside the polygon (by moving to the closest edge) by applying the makeInsideHalfPlane algorithm with every halfplane. [ops, this actually only works if all angles are >= 90 degrees]
Case #11 – Reflecting a vector with a given normal
Ponglike game. Ball hits a sloped wall. You know the ball’s velocity vector and the wall’s normal vector (see case #05). How do you reflect it realistically? Simple! Just reflect the ball’s normal velocity, and preserve its tangential velocity.
Vector2f vel = getVel(); Vector2f dir = getWallNormal(); // Make sure this is a unit vector Vector2f velN = dir * vel.dot(dir); // Normal component Vector2f velT = vel  velN; // Tangential component Vector2f reflectedVel = velT  velN;
For more realism, you can multiply velT and velN by constants representing friction and restitution, respectively.
Case #12 – Cancelling movement along an axis
Sometimes, you want to restrict movement in a given axis. The idea is the same as above: decompose in a normal and tangential speed, and just keep tangential speed. This can be useful, for example, if the character is following a rail.
Rotations
Case #13 – Rotating a point around a pivot
If used to represent a point in space, the rotate() method will rotate that point around the origin. That might be interesting, but is limiting. Rotating around an arbitrary pivot vector is simple and much more useful – simply subtract the pivot from it, as if translating so the origin IS the pivot, then rotate, then add the pivot back:
Vector2f rotateAroundPivot(Vector2f p, Vector2f pivot) { return (pos  pivot).rotate(angle) + pivot; }
Case #14 – Determining which direction to turn towards
Say that you have a character that wants to rotate to face an enemy. He knows his direction, and the direction that he should be facing to be looking straight at the enemy. But should he turn left or right? The cross product provides a simple answer: curDir.cross(targetDir) will return positive if you should turn left, and negative if you should turn right (and 0 if you’re either facing it already, or 180 degrees from it).
Other Geometric Cases
Here are a few other useful cases that aren’t that heavily vectorbased, but useful:
Case #15 – Isometric world to screen coordinates
Isometric game. You know where the (0, 0) of world is on the screen (let’s call that point origin and represent it with a vector), but how do you know where a given world (x, y) is on the screen? First, you need two vectors determining the coordinate base, a new x and y axes. For a typical isometric game, they can be bx = Vector2f(2, 1) and by = Vector2f(2, 1)  They don’t necessarily have to be unit vectors. From now, it’s straightforward:
Vector2f p = getWorldPoint(); Vector2f screenPos = bx * p.x + by * p.y + origin;
Yes, it’s that simple.
Case #16 – Isometric screen to world coordinates
Same case, but now you want to know which tile the mouse is over. This is more complicated. Since we know that (x’, y’) = (x * bx.x + y * by.x, x * bx.y + y * by.y) + origin, we can first subtract origin, and then solve the linear equation. Using Cramer’s Rule, except that we’ll be a little clever and use our 2D crossproduct (see definition at the beginning of the article) to simplify things:
Vector2f pos = getMousePos()  origin; float demDet = bx.cross(by); float xDet = pos.cross(by); float yDet = bx.cross(pos); Vector2f worldPos = Vector2f(xDet / demDet, yDet / demDet);
And now you don’t need to do that ugly findrectanglethenlookuponbitmap trick that I’ve seen done several times before.
Jordan Trudgett
/ 20120704Thanks for making this! I’m doing some 2D vector collision detection at the moment and this helped.
Rodrigo Cardoso Rocha
/ 20120903Very useful! Thanks.
Will
/ 20120903very sweet! Love all your posts.
Keep it coming, lets have one with quaternions and camera control – how to do first and third person – next?
Captcha was 73LD, and I thought we were only on the 24th LD?
Rons
/ 20120903Omg, I wish I had read this article ten years ago. My code is full of the things that vectors would have solved. It’s like seeing the light. Almost made me cry:) Thanks!
Lord Voldemoo
/ 20120904Really well done! I had (am still having, actually) problems visualizing Case #10. An illustration here would be really helpful. I guess I also don’t yet know the “why” for this operation.
jefhebb
/ 20120904Well done. It takes me back many a year. When I use to do equations for aircraft simulators
Calvin
/ 20120904Cara seus posts são ótimos, achei que o site estivesse abandonado. Compartilhe conosco mais conhecimentos sobre a areá de desenvolvimento de jogos. Obrigado!
Vitor Almeida da Silva
/ 20120904Keep it coming Rodrigo.
The articles are very well written and useful. I’m really looking forward to more articles like this.Keep it coming Rodrigo.
The articles are very well written and useful. I’m really looking forward to more articles like this.
evetro
/ 20120907“Likewise, you can force it to be inside the polygon (by moving to the closest edge) by applying the makeInsideHalfPlane algorithm with every halfplane.”
This is not true.
amz
/ 20120907evetro, you’re right. This only works for polygons whose internal angles are all >= 90 degrees. Will update post.
Carsten
/ 20120911If you enjoy this series (I do) – you might be interested in raytracing as well – I wrote a small series explaining the math behind that and implementing it in F# some time ago.
You can find it here: http://gettingsharper.de/tag/raytracing/
Truck
/ 20130404This whole series has been very, very helpful. Thank you so much for your efforts. I hope that you shall continue to share your hard won knowledge.
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Meganopteryx
/ 20130809This is an EXCELLENT reference. Thanks for providing! Question on Case #10, isn’t a convex polygon by definition a polygon with all angles >= 90 degrees?
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/ 20140225I’ve been reading up on Vector math from a couple other sources, and wasn’t quite understanding just how I can use dot/cross products, but I finally think I get it thanks to these very well written explanations and examples. I think some diagrams like you put in your earlier posts would make some of these more complex cases easier to visualize. Thanks for taking the time to share this valuable knowledge!
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