Swivel.

swivel.c

// This function, is one of the BIGGEST endeavors I have ever taken on.
// This function is something I have spent DAYS on.
// I think it might have started on Thursday, or even earlier.
// I was able to convert `swivel_vert` to matrix form, with one catch:
// z' = z is not possible. So z' = -zcoord here.
// I wonder if after successfully implementing this function,
// I'll even want to work on the game anymore.
// What if I feel burnt out, before I even started making the game?
// Hopefully that does not happen.
// I want to make a video on this WHOLE thing, so that happening would be an absolute SHAME.
// mid (or (mx, my)): The "vanishing point" for the perspective projection
// angle (radians): The amount to rotate the plane by, on the Y-axis
// pfactor (f): Larger pfactor means more perspective effect. pfactor=0.001 is fine.
// zcoord (K): The transformed z-coordinate will equal -K.
static void swivel_begin(Vector2 mid, float angle, float pfactor, float zcoord) {
    // My apologies for these abbreviations.
    // I derived my work using math notation (single-letter variables),
    // so it's easier for me if I write it this way.
    float mx = mid.x, my = mid.y;
    float a = angle;
    float f = pfactor;
    float k = zcoord;

    float c = cos(angle);
    float s = sin(angle);

    Matrix mat;

    // TODO should I do a*b*c or a * b * c here?
    mat.m0  = c - (f * mx *s);
    mat.m1  = -f * my * s;
    mat.m2  = k * f * s;
    mat.m3  = -f * s;

    mat.m4  = 0.0f;
    mat.m5  = 1.0f;
    mat.m6  = 0.0f;
    mat.m7  = 0.0f;

    mat.m8  = 0.0f;
    mat.m9  = 0.0f;
    mat.m10 = 0.0f;
    mat.m11 = 0.0f;

    mat.m12 = mx * (1 - c + (f * mx * s));
    mat.m13 = f * mx * my * s;
    mat.m14 = -k * (1 + (f * mx * s));
    mat.m15 = 1 + (f * mx * s);

    rlPushMatrix();
    rlLoadIdentity();
    rlMultMatrixm(mat);
}

static void swivel_end() {
    rlPopMatrix();
}

// A matrixless solution (I made this before making swivel_begin())
// (this is actually more efficient.
// I think the vertex shader approach to doing this, could be better than using a transformation matrix)
static void swivel_vert(float x, float y, float mx, float my, float angle) {
    // Assume middle is z = -1

    float xrot = mx + (x - mx) * cos(angle);
    float yrot = y; // rotating horizontally, so y is unchanged here
    float zrot = -1 + 0.001 * (x - mx) * sin(angle);

    float xnew = ((xrot - mx) / -zrot) + mx;
    float ynew = ((yrot - my) / -zrot) + my;

    rlVertex2f(xnew, ynew);
}