+
+// CLASS IDENTICATION
+// BackprojectIntDiff3
+//
+// PURPOSE
+// Highly optimized version of BackprojectIntDiff2
+
+void
+BackprojectIntDiff3::BackprojectView (const double* const filteredProj, const double view_angle)
+{
+ double theta = - view_angle; // add half PI to view angle to get perpendicular theta angle
+ static const int scaleShift = 16;
+ static const kint32 scale = (1 << scaleShift);
+ static const kint32 scaleBitmask = scale - 1;
+ static const kint32 halfScale = scale / 2;
+ static const double dInvScale = 1. / scale;
+
+ const kint32 det_dx = nearest<kint32> (xInc * sin (theta) / detInc * scale);
+ const kint32 det_dy = nearest<kint32> (yInc * cos (theta) / detInc * scale);
+
+ // calculate L for first point in image (0, 0)
+ kint32 detPosColStart = nearest<kint32> ((start_r * cos (theta - start_phi) / detInc + iDetCenter) * scale);
+
+ // precalculate scaled difference for linear interpolation
+ double deltaFilteredProj [nDet - 1];
+ if (interpType == Backprojector::INTERP_LINEAR) {
+ for (int i = 0; i < nDet - 1; i++)
+ deltaFilteredProj[i] = (filteredProj[i+1] - filteredProj[i]) * dInvScale;
+ }
+
+ for (int ix = 0; ix < nx; ix++, detPosColStart += det_dx) {
+ kint32 curDetPos = detPosColStart;
+ ImageFileColumn pImCol = v[ix];
+
+ if (interpType == Backprojector::INTERP_NEAREST) {
+ for (int iy = 0; iy < ny; iy++, curDetPos += det_dy) {
+ const int iDetPos = (curDetPos + halfScale) >> 16;
+ assert(iDetPos >= 0 && iDetPos < nDet);
+ *pImCol++ += filteredProj[iDetPos];
+ } // end for iy
+ } else if (interpType == Backprojector::INTERP_FREQ_PREINTERPOLATION) {
+ for (int iy = 0; iy < ny; iy++, curDetPos += det_dy) {
+ const int iDetPos = ((curDetPos + halfScale) >> 16) * m_interpFactor;
+ assert(iDetPos >= 0 && iDetPos < nDet);
+ *pImCol++ += filteredProj[iDetPos];
+ } // end for iy
+ } else if (interpType == Backprojector::INTERP_LINEAR) {
+ for (int iy = 0; iy < ny; iy++, curDetPos += det_dy) {
+ const kint32 iDetPos = curDetPos >> scaleShift;
+ const kint32 detRemainder = curDetPos & scaleBitmask;
+ assert(iDetPos >= 0 && iDetPos < nDet - 1);
+ *pImCol++ += filteredProj[iDetPos] + (detRemainder * deltaFilteredProj[iDetPos]);
+ } // end for iy
+ } //end linear
+ } // end for ix
+}