From f4ee5c43ae44cf059fab91451044eef885e3d23b Mon Sep 17 00:00:00 2001
From: Hanno Schwalm <hanno@schwalm-bremen.de>
Date: Thu, 25 Dec 2025 14:21:58 +0100
Subject: [PATCH] Interpolator maintenance

const added where possible
---
 src/common/interpolation.c | 32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

diff --git a/src/common/interpolation.c b/src/common/interpolation.c
index 9a0c87caf68b..31afdd30bb70 100644
--- a/src/common/interpolation.c
+++ b/src/common/interpolation.c
@@ -340,7 +340,7 @@ static float _maketaps_lanczos(float *taps,
     dt_aligned_pixel_t sign;
     for_four_channels(c)
     {
-      int a = (int)vt[c];
+      const int a = (int)vt[c];
       r[c] = vt[c] - (float)a;
       sign[c] = (a & 1) ? -1.0f : 1.0f;
     }
@@ -429,7 +429,7 @@ static inline float _compute_upsampling_kernel(const dt_interpolation_t *itor,
   // yielding an incorrect result for the slightly-negative positions
   // that can occur at the top and left edges when doing perspective
   // correction
-  int f = (int)floorf(t) - itor->width + 1;
+  const int f = (int)floorf(t) - itor->width + 1;
   if(first)
   {
     *first = f;
@@ -474,10 +474,10 @@ static inline void _compute_downsampling_kernel(const dt_interpolation_t *itor,
   }
 
   // Compute first interpolator parameter
-  float t = xin * outoinratio - (float)xout;
+  const float t = xin * outoinratio - (float)xout;
 
   // Compute all filter taps
-  int num_taps = *taps = (int)((w - t) / outoinratio);
+  const int num_taps = *taps = (int)((w - t) / outoinratio);
   itor->maketaps(kernel, num_taps, itor->width, t, outoinratio);
   // compute the kernel norm if requested
   if(norm)
@@ -510,8 +510,8 @@ float dt_interpolation_compute_sample(const dt_interpolation_t *itor,
   float DT_ALIGNED_ARRAY kernelv[MAX_KERNEL_REQ];
 
   // Compute both horizontal and vertical kernels
-  float normh = _compute_upsampling_kernel(itor, kernelh, NULL, x);
-  float normv = _compute_upsampling_kernel(itor, kernelv, NULL, y);
+  const float normh = _compute_upsampling_kernel(itor, kernelh, NULL, x);
+  const float normv = _compute_upsampling_kernel(itor, kernelv, NULL, y);
 
   int ix = (int)x;
   int iy = (int)y;
@@ -544,7 +544,7 @@ float dt_interpolation_compute_sample(const dt_interpolation_t *itor,
       s += kernelv[i] * h;
       in += linestride;
     }
-    r = fmaxf(0.0f, s / (normh * normv));
+    r = s / (normh * normv);
   }
   else if(ix >= 0 && iy >= 0 && ix < width && iy < height)
   {
@@ -582,14 +582,14 @@ float dt_interpolation_compute_sample(const dt_interpolation_t *itor,
       s += kernelv[i] * h;
     }
 
-    r = fmaxf(0.0f, s / (normh * normv));
+    r = s / (normh * normv);
   }
   else
   {
     // invalid coordinate
     r = 0.0f;
   }
-  return r;
+  return fmaxf(0.0f, r); // make sure we don't push NaNs
 }
 
 /* --------------------------------------------------------------------------
@@ -612,8 +612,8 @@ void dt_interpolation_compute_pixel4c(const dt_interpolation_t *itor,
   float DT_ALIGNED_ARRAY kernelv[MAX_KERNEL_REQ];
 
   // Compute both horizontal and vertical kernels
-  float normh = _compute_upsampling_kernel(itor, kernelh, NULL, x);
-  float normv = _compute_upsampling_kernel(itor, kernelv, NULL, y);
+  const float normh = _compute_upsampling_kernel(itor, kernelh, NULL, x);
+  const float normv = _compute_upsampling_kernel(itor, kernelv, NULL, y);
 
   // Precompute the inverse of the filter norm for later use
   const float oonorm = (1.f / (normh * normv));
@@ -1102,7 +1102,7 @@ void dt_interpolation_resample(const dt_interpolation_t *itor,
     int hkidx = 0; // H(orizontal) K(ernel) I(n)d(e)x
 
     // Number of lines contributing to the output line
-    int vl = vlength[vlidx++]; // V(ertical) L(ength)
+    const int vl = vlength[vlidx++]; // V(ertical) L(ength)
 
     // Process each output column
     for(size_t ox = 0; ox < (size_t)roi_out->width; ox++)
@@ -1111,12 +1111,12 @@ void dt_interpolation_resample(const dt_interpolation_t *itor,
       dt_aligned_pixel_t vs = { 0.0f, 0.0f, 0.0f, 0.0f };
 
       // Number of horizontal samples contributing to the output
-      int hl = hlength[hlidx++]; // H(orizontal) L(ength)
+      const int hl = hlength[hlidx++]; // H(orizontal) L(ength)
 
       for(size_t iy = 0; iy < vl; iy++)
       {
         // This is our input line
-        size_t baseidx_vindex = (size_t)vindex[viidx++] * in_stride_floats;
+        const size_t baseidx_vindex = (size_t)vindex[viidx++] * in_stride_floats;
 
         dt_aligned_pixel_t vhs = { 0.0f, 0.0f, 0.0f, 0.0f };
 
@@ -1146,7 +1146,7 @@ void dt_interpolation_resample(const dt_interpolation_t *itor,
       // Negative RGB are invalid values no matter the RGB space (light is positive)
       dt_aligned_pixel_t pixel;
       for_each_channel(c, aligned(vs:16))
-        pixel[c] = MAX(vs[c], 0.f);
+        pixel[c] = fmaxf(0.0f, vs[c]);
       copy_pixel_nontemporal(out + baseidx, pixel);
 
       // Reset vertical resampling context
@@ -1565,7 +1565,7 @@ void dt_interpolation_resample_1c(const dt_interpolation_t *itor,
       // Output pixel is ready
       float *o = (float *)((char *)out + (size_t)oy * out_stride
                            + (size_t)ox * sizeof(float));
-      *o = vs;
+      *o = fmaxf(0.0f, vs);
 
       // Reset vertical resampling context
       viidx -= vl;