Fix get_q, which was distorting reciprocal space previously

2 files changed, 9 insertions, 11 deletions

diff --git a/data/diffraction.cl b/data/diffraction.cl
index b0f7b6b6..bac0c5c7 100644
--- a/data/diffraction.cl
+++ b/data/diffraction.cl
@@ -48,7 +48,7 @@ float4 get_q(int x, int y, float cx, float cy, float res, float clen, float k,
              float *ttp, float4 z, int sampling)
 {
 	float rx, ry, r;
-	float ttx, tty, tt;
+	float az, tt;
 	float4 q;
 
 	rx = ((float)x - sampling*cx)/(res*sampling);
@@ -56,13 +56,12 @@ float4 get_q(int x, int y, float cx, float cy, float res, float clen, float k,
 
 	r = sqrt(pow(rx, 2.0) + pow(ry, 2.0));
 
-	ttx = atan2(rx, clen);
-	tty = atan2(ry, clen);
 	tt = atan2(r, clen);
-
 	*ttp = tt;
 
-	q = (float4)(k*sin(ttx), k*sin(tty), k-k*cos(tt), 0.0);
+	az = atan2(ry, rx);
+
+	q = (float4)(k*sin(tt)*cos(az), k*sin(tt)*sin(az), k-k*cos(tt), 0.0);
 
 	return quat_rot(q, z);
 }
diff --git a/src/diffraction.c b/src/diffraction.c
index 6df00164..bcfee476 100644
--- a/src/diffraction.c
+++ b/src/diffraction.c
@@ -141,7 +141,7 @@ struct rvec get_q(struct image *image, unsigned int xs, unsigned int ys,
                   unsigned int sampling, float *ttp, float k)
 {
 	struct rvec q;
-	float twothetax, twothetay, twotheta, r;
+	float twotheta, r, az;
 	float rx = 0.0;
 	float ry = 0.0;
 	int p;
@@ -164,14 +164,13 @@ struct rvec get_q(struct image *image, unsigned int xs, unsigned int ys,
 
 	/* Calculate q-vector for this sub-pixel */
 	r = sqrt(pow(rx, 2.0) + pow(ry, 2.0));
-	twothetax = atan2(rx, image->det.panels[p].clen);
-	twothetay = atan2(ry, image->det.panels[p].clen);
-	twotheta = atan2(r, image->det.panels[p].clen);
 
+	twotheta = atan2(r, image->det.panels[p].clen);
+	az = atan2(ry, rx);
 	if ( ttp != NULL ) *ttp = twotheta;
 
-	q.u = k * sin(twothetax);
-	q.v = k * sin(twothetay);
+	q.u = k * sin(twotheta)*cos(az);
+	q.v = k * sin(twotheta)*sin(az);
 	q.w = k - k * cos(twotheta);
 
 	return quat_rot(q, image->orientation);