CODEWITHSUNDEEP
javaopenglopengl-es-2.0
Ogen
Ogen

Reputation: 6709

Rotate a triangle continuously

I have a float[] newCoords variable that has a size of 9. The first 3 entries represent one vertex, the next 3 represent the second vertex and the last 3 represent the last vertex.

I have some code that is supposed to rotate a triangle anywhere in space when I feed it the coordinates. It looks like this:

float s = (float) Math.sin(0.5);
float c = (float) Math.cos(0.5);

float[] centroid = getCentroid(newCoords);

newCoords[0] -= centroid[0];
newCoords[1] -= centroid[1];
newCoords[3] -= centroid[0];
newCoords[4] -= centroid[1];
newCoords[6] -= centroid[0];
newCoords[7] -= centroid[1];

newCoords[0] = (newCoords[0] * c) - (newCoords[1] * s);
newCoords[1] = (newCoords[0] * s) + (newCoords[1] * c);

newCoords[3] = (newCoords[3] * c) - (newCoords[4] * s);
newCoords[4] = (newCoords[3] * s) + (newCoords[4] * c);

newCoords[6] = (newCoords[6] * c) - (newCoords[7] * s);
newCoords[7] = (newCoords[6] * s) + (newCoords[7] * c);

newCoords[0] += centroid[0];
newCoords[1] += centroid[1];
newCoords[3] += centroid[0];
newCoords[4] += centroid[1];
newCoords[6] += centroid[0];
newCoords[7] += centroid[1];

The problem is, its not rotating it properly, the triangles are spinning and getting smaller and smaller until they disappear for some reason, can anyone see why this is happening?

EDIT: whoops, almost forgot, here is my getCentroid() method.

private float[] getCentroid(float[] p1) {
    float[] newCoords = new float[] {(p1[0] + p1[3] + p1[6]) / 3.0f,
                                     (p1[1] + p1[4] + p1[7]) / 3.0f, 0};
    return newCoords;
}

Upvotes: 2

Views: 262

Answers (1)

leemes
leemes

Reputation: 45725

I see two problems with your code. Both are fixed with a little change.

  1. You try to apply a rotation operation, taking X and Y coordinates as input and having the new X and Y as output. For every vertex you rotate, you have two lines of code: the first computes the X, the second the Y coordinate. But when computing the Y coordinate, you use the already rotated X coordinate! That's wrong.

  2. There is also a numerical problem. You reuse the old values again and again, resulting in a chain of rotation computations a value makes though, so the numerical errors sum up. Never rely on such computations to work as expected. Instead, you should work with the original values and increase the angle in each frame. This makes sure that each value only participated in a single rotation computation.

For fixing both problems, keep the original coordinates somewhere in your code, I call them coords, and rewrite the code such that you take that array as input (keep newCoords as the output). Increase the rotation angle in each frame to achieve a rotation animation.

This fixes both problems because you get rid of that chain and also you have different arrays for input and output in your rotation function.

Pseudo-code:

// initial:
angle = 0.0;
coords = (initial coordinates)

// per frame:
angle += 0.5;
newCoords = rotate(coords, angle);
draw(newCoords);

Also, please note that 0.5 is a large angle if you want to rotate by that angle frame by frame. The math functions expect angle in radians (not degrees), so you might want to use a lower value depending on what you want to visualize in particular.

You might wonder why I reuse the old angle in each frame, as according to the above mentioned problem 2., it should introduce numerical problems, since it's also a chain of computations. That's not a problem with the rotation angle, as a simple summation doesn't show such bad numerical errors you experience with applying rotations. Yet it has some problems, but they only show up at very long running times when the angle reaches some billions. The reason why such a summation in general is not that bad is because you're changing the variable in the same direction in each frame as well as a slightly off rotation angle isn't noticed very much by the user.

Upvotes: 4

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