Is this code correct or logically sound?

import std;
enum {////side/depth/height and side/height
x,//0
y,//1
z //2
}
/**
Indicates wether a triangle faces an imaginary view point.
*/
bool triangleFacesCamera(float[3] tA, float[3] tB, float[3] tC) {
float r1, r2;

r1 = atan2(tB[x] - tA[x], tB[z] - tA[z]);//tried swapping parameters without success
r2 = atan2(tC[x] - tB[x], tC[z] - tB[z]);//also tried with tA as substraction 2nd operand, with same/similar results.
//trying with both is still broken, but appears to change the breakage pattern.

r2 -= r1;

return r2 > 0;
}

For context, it is trying to reproduce an old algorithm that does the same thing :

//in D pseudo code
extern short sus(byte A, byte B){
r0 = sus(tB[y] - tA[y], tB[x] - tA[x]);
r1 = sus(tC[y] - tA[y], tC[x] - tA[x]);

r1 -= r0;
return r1 > 0;
}

sus is named so because it makes use of atan and a premade table that looks like tan(a*(a*5.09...)) but is impossible to exactly reproduce for me.(on a graph it looks like tan but crammed to loop only four/sixteen/sixtyfour times at τ/4,π,τ)

Trying to parse both atan2 and sus with (sin(angle), cos(angle)) as arguments[1] shows that atan2 outputs it as an angle in -π~π angle representation but sus's output differs in that it looks like sin(x*2).

I'd add images to better illustrate it but do not know how...

Big thanks

[1] https://run.dlang.io/is/xR8TcE

ps: is it normal that sometimes posting has a very long delay? when it happens it is as if the post has gone nowhere but then it suddenly appears much later, which can cause confusion and duplicate posting, oof.

On Thursday, 30 March 2023 at 10:29:25 UTC, z wrote:

You need to be exact on what 'correct' is. The comment above triangleFacesCamera says:

There's no view point / camera position defined anywhere in your code.
Instead, all it's doing is comparing the angle of one triangle side (A->B) with another (B->C) in the XZ plane. This suggests you want to know the triangle winding order: clockwise or counter clockwise.

If you search for "triangle winding order" you'll find simple and correct ways to do that. Your code needlessly computes angles, only considers the XZ plane, and doesn't compare the angles correctly.

You need to wrap (r2 - r1) into [-τ/2, τ/2] range, then you can compare with 0.

On Thursday, 30 March 2023 at 12:59:31 UTC, Dennis wrote:

Yes, the viewpoint/cam should be [0,0,0] for now. The old algorithm completely discards depth information after scaling triangle point data depending on depth distance from 0 hence why i ignored it.

I've tried to search before but was only able to find articles for 3D triangles, and documentation for OpenGL, which i don't use.

My description of the old algorithm was also badly written(it is confusing!) so here it is rewritten to be more understandable :

enum {x, y}//leftright/side, updown/height
alias tri = byte[2];
/**
Determines if a triangle is visible.
*/
extern bool oldfunction(tri tA, tri tB, tri tC) {
short r0, r1;//r, "result"
r0 = cast(short) ((tB[y]-tA[y]) * (tC[x]-tB[x]));
r1 = cast(short) ((tC[y]-tA[y]) * (tB[x]-tA[x]));
r1 -= r0;
return r1 > 0;//"visible" means result is neither negative or zero
}

(replacing my code with this but in float point "works" but there is an apparent "angle bias" and other problems that are difficult to pinpoint or describe.)
Does this match anything known? It doesn't look like cross product to me because the subtractions occur earlier than the multiplications.

By correct i mean that it can be used exactly like the old algorithm. (to ignore subshapes from a 3D model)

Thanks again

On Friday, 31 March 2023 at 13:11:58 UTC, z wrote:

The first function you posted takes a 3D triangle as input, so I assumed you're working in 3D. What are you working on?

You haven't defined what 'visible' means for a geometric triangle.

On Saturday, 1 April 2023 at 15:32:27 UTC, Dennis wrote:

explained to the best of my ability : https://d.godbolt.org/z/4a8zPGsGo
the "angle bias" i was trying to explain about is present, when rot = 0 the mask is supposed to only have three true values.(that of the "front" triangle.)

Problem is, all i have is an assembly dump and the data it interacts with, both of which are very(very) old.