ha! this isn''t about 3D graphics. my other post wasn''t getting any attention , even though it was a problem all you hard-core programmers could solve in 3 nano seconds. here it is; its not hard (well, to me it is). i am working on my polygon engine, and for some reason there is a delema. i have a function that rotates a polygon, but however, whenever i pass the function a positive angle, the polygon rotates the polygon clock-wise, and vise versa, which is wrong. i know i can easily get around it by negating the angle, but it''s still bothering me. heres the code (BTW, i have look-up tables for the sin and cos functions for optimization, and i''m SURE they work): int Rotate_Polygon(POLYGON_PTR poly, int angle) { int index; if (poly->side == NULL) { return(-1); } if (angle < 0) { angle += 360; } for (index = 0; index < poly->num_sides; index++) { float xr = (float)poly->side[index].x1*cos_look[angle] - (float)poly->side[index].y1*sin_look[angle]; float x2r = (float)poly->side[index].x2*cos_look[angle] - (float)poly->side[index].y2*sin_look[angle]; float yr = (float)poly->side[index].x1*sin_look[angle] + (float)poly->side[index].y1*cos_look[angle]; float y2r = (float)poly->side[index].x2*sin_look[angle] + (float)poly->side[index].y2*cos_look[angle]; float cx = (float)poly->side[index].Cen_X*cos_look[angle] - (float)poly->side[index].Cen_Y*sin_look[angle]; float cy = (float)poly->side[index].Cen_X*sin_look[angle] + (float)poly->side[index].Cen_Y*cos_look[angle]; poly->side[index].x1 = xr; poly->side[index].y1 = yr; poly->side[index].x2 = x2r; poly->side[index].y2 = y2r; poly->side[index].Cen_X = cx; poly->side[index].Cen_Y = cy; } return(1); }

I could be going mad, but it looks like you have a left handed matrix transformation going on, and you need a right handed one.

This is the rotation formula:

xr = x cos a - y sin ayr = x sin a + y cos a 

From what I can see this is exactly what you are using. What you are missing is that the computer screen has the origo at the top-left corner, as opposed to your normal cartesian system that has it in the bottom left corner.





Edited by - Spellbound on 3/13/00 4:34:55 PM

unfortunatly, im no trig genius. all i know is that the formula works. when i was programming in DOS in Mode X 320x240, i used the exact same code and it worked.

however, i understand what you mean. so how do i fix it? switch sin and cos? switch signs? both?

If I remember it correctly it should be enough to transpose the transform matrix. Or as in the case with separate formulas:

xr = x cos a + y sin a
yr = -x sin a + y cos a

I''m not sure though. Try it, if it wasn''t correct then ask again.

thats what ive been doing to solve the problem. i just wasn''t sure if that was the most "professional" way to do it.

In my opinion, the professional way is to understand what goes on.