Fix Siddon projection kernel missing return on missed rays (cone and parallel)

Common/CUDA/Siddon_projection.cu

@@ -176,8 +176,10 @@ __global__ void kernelPixelDetector( Geometry geo,
     float aM=fminf(fminf(axM,ayM),azM);
 
     // line intersects voxel space ->   am<aM
-    if (am>=aM)
+    if (am>=aM) {
         detector[idx]=0;
+        return;
+    }
 
     // Compute max/min image INDEX for intersection eq(11-19)
     // Discussion about ternary operator in CUDA: https://stackoverflow.com/questions/7104384/in-cuda-why-is-a-b010-more-efficient-than-an-if-else-version

Common/CUDA/Siddon_projection_parallel.cu

@@ -180,8 +180,10 @@ __global__ void kernelPixelDetector_parallel( Geometry geo,
     float aM=(fminf(axM,ayM));
 
     // line intersects voxel space ->   am<aM
-    if (am>=aM)
+    if (am>=aM) {
         detector[idx]=0.0f;
+        return;
+    }
 
     // Compute max/min image INDEX for intersection eq(11-19)
     // Discussion about ternary operator in CUDA: https://stackoverflow.com/questions/7104384/in-cuda-why-is-a-b010-more-efficient-than-an-if-else-version

Python/tests/test_siddon_missed_rays.py

@@ -0,0 +1,41 @@
+import numpy as np
+import pytest
+import tigre
+
+
+@pytest.mark.parametrize("mode", ["cone", "parallel"])
+def test_siddon_missed_rays(mode):
+    """Test that Siddon projection handles missed rays properly for cone and parallel geometry."""
+    geo = tigre.geometry_default(high_resolution=False)
+    geo.mode = mode
+    # Make detector large enough so that outer rays do not intersect the voxel volume
+    geo.nDetector = np.array([256, 256])
+    geo.dDetector = np.array([3.0, 3.0])
+    geo.sDetector = geo.nDetector * geo.dDetector
+
+    img = np.ones(tuple(geo.nVoxel.astype(int)), dtype=np.float32)
+    angles = np.array([0.0, 0.5], dtype=np.float32)
+
+    proj = tigre.Ax(img, geo, angles, "Siddon")
+
+    # Missed rays must be exactly 0
+    assert not np.isnan(proj).any(), f"NaNs detected in {mode} Siddon projection"
+    assert (proj == 0.0).sum() > 0, f"Expected missed rays in {mode} Siddon projection"
+    # Intersecting rays must be positive
+    assert (proj > 0.0).sum() > 0, f"Expected intersecting rays in {mode} Siddon projection"
+
+
+@pytest.mark.parametrize("mode", ["cone", "parallel"])
+def test_siddon_completely_missed_volume(mode):
+    """Test that Siddon projection returns all zeros when the volume is completely outside the beam."""
+    geo = tigre.geometry_default(high_resolution=False)
+    geo.mode = mode
+    # Offset the volume far away along Y (perpendicular to optical axis)
+    geo.offOrigin = np.array([0.0, 5000.0, 0.0])
+
+    img = np.ones(tuple(geo.nVoxel.astype(int)), dtype=np.float32)
+    angles = np.array([0.0], dtype=np.float32)
+
+    proj = tigre.Ax(img, geo, angles, "Siddon")
+
+    assert np.all(proj == 0.0), f"Expected all zeros for completely missed volume in {mode} Siddon"