This document describes a bit how the angles work in D2TM, especially when comparing two things and how an entity (ie a unit) should face another entity.
In summary d2tm angles are clock-wise, starting from the top. You calculate
the degrees using fDegrees, and with that you can calculate a facing angle.
From the facing angle you can calculate a drawing index to be used for a bitmap.
D2TM considers 2 'points', a 'from' (the center of an imaginary cirlce) and a 'to' which is at the edge of the circle.
Given these two points you get a degrees. You get this by calling fDegrees.
It takes x1,y1 and x2,y2. Where x1,y1 is always considered to be the 'from', ie
center of the 'circle'.
angles.mp4
Once we have this degrees value you can convert this to "angles", which
are basicallys segments within that circle to determine what the 'facing angle'
would be for a unit or projectile. Use faceAngle for this.
This function takes the amoutn of degrees we had from fDegrees and
the amount of angles (segments) you have in the circle. Assuming 0 degrees is up.
float degrees = fDegrees(fromX, fromY, toX, toY);
int facingAngle = faceAngle(degrees);
When no angles are given, faceAngle assumes 8.
Consider this QUAD image:
The numbers (in yellow) are the D2TM face angles. Notice that the third picture of the Quad, facing up, has facing angle of 0. The third picture of the Quad is called a drawing index. These go from left to right from 0 and upwards. The first drawing index (0) is the Quad facing to the right.
As we move from left to right on the bitmap, the Quad's facing angle is going into a counter-clockwise direction. All D2TM bitmaps of units have this. However, rockets have a clockwise rotation (and 16 angles, instead of 8).
There are 2 things required to determine the drawing index on the Quad picture, to make it correspond with the facing angle we described above. Facing angle 0 means up, meaning it should correspond with drawing index nr 2. Then as we go to the right on the Quad's bitmap, our facing angle is going backwards.
In code, in order to convert between a faceAngle to a drawIndex on the bitmap you use convertAngleToDrawIndex
float degrees = fDegrees(fromX, fromY, toX, toY);
int facingAngle = faceAngle(degrees);
int drawIndex = convertAngleToDrawIndex(facingAngle);
Without any extra parameters convertAngleToDrawIndex assumes that the bitmap is using pictures going into a counter-clockwise direction (like the QUAD above).
When the drawing index goes to the left it will correspond with the facing angle.
Putting it all together, this is the result:
| Facing angle | QUAD draw index |
|---|---|
| 0 | 2 |
| 1 | 1 |
| 2 | 0 |
| 3 | 7 |
| 4 | 6 |
| 5 | 5 |
| 6 | 4 |
| 7 | 3 |
As you can see, the left column, the d2tm facing angle goes from 0 to 7. The QUAD index goes down (from 7 to 0), but it also has a different starting point. The 2nd picture has a QUAD facing upwards. This is called an offset.
By default convertAngleToDrawIndex assumes an offset of 2, and that the bitmap uses counter-clockwise rotation. You can overrule that if you want.
Projectiles (rockets) have a starting point of 0 and a clock-wise rotation.