[sift] Remove unreachable code for deprecated downscaling alternative

* [sift] Remove unreachable code

This piece of now unreachable code was a replicating buggy OpenCV behaviour that existed in 2016.
Its case was no reachable callable with any valid command line or library parameter.

---------

Co-authored-by: Carsten Griwodz <[email protected]>

Author: griwodz

CHANGES.md

@@ -21,6 +21,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Removed option to create top level of every octave from input image [PR](https://github.com/alicevision/popsift/pull/178)
 - Removed option to compute very narrow Gaussian filters called opencv [PR](https://github.com/alicevision/popsift/pull/179)
+- Removed unreachable code for alternative downscaling between octaves [PR](https://github.com/alicevision/popsift/pull/180)
 
 ## [0.10.0] - 2025-10-14
 

src/popsift/s_pyramid_build.cu

@@ -29,24 +29,6 @@ namespace popsift {
 
 namespace gauss {
 
-__global__
-void get_by_2_interpolate( cudaTextureObject_t src_data,
-                           const int           src_level,
-                           cudaSurfaceObject_t dst_data,
-                           const int           dst_w,
-                           const int           dst_h )
-{
-    const int idx = blockIdx.x * blockDim.x + threadIdx.x;
-    const int idy = blockIdx.y * blockDim.y + threadIdx.y;
-
-    if( idx >= dst_w ) return;
-    if( idy >= dst_h ) return;
-
-    const float val = readTex( src_data, 2.0f * idx + 1.0f, 2.0f * idy + 1.0f, src_level );
-
-    surf2DLayeredwrite( val, dst_data, idx*4, idy, 0, cudaBoundaryModeZero ); // dst_data.ptr(idy)[idx] = val;
-}
-
 __global__
 void get_by_2_pick_every_second( cudaTextureObject_t src_data,
                                  const int           src_w,
@@ -214,35 +196,17 @@ inline void Pyramid::downscale_from_prev_octave( int octave, cudaStream_t stream
     h_grid.x = (unsigned int)grid_divide( width,  h_block.x );
     h_grid.y = (unsigned int)grid_divide( height, h_block.y );
 
-    switch( mode )
-    {
-    case Config::PopSift :
-    case Config::VLFeat :
-    case Config::OpenCV :
-        gauss::get_by_2_pick_every_second
-            <<<h_grid,h_block,0,stream>>>
-            ( prev_oct_obj.getDataTexPoint( ),
-              prev_oct_obj.getWidth(),
-              prev_oct_obj.getHeight(),
-              _levels-PREV_LEVEL,
-              oct_obj.getDataSurface( ),
-              oct_obj.getWidth(),
-              oct_obj.getHeight() );
-
-        POP_SYNC_CHK;
-        break;
-    default :
-        gauss::get_by_2_interpolate
-            <<<h_grid,h_block,0,stream>>>
-            ( prev_oct_obj.getDataTexLinear( ).tex,
-              _levels-PREV_LEVEL,
-              oct_obj.getDataSurface( ),
-              oct_obj.getWidth(),
-              oct_obj.getHeight() );
-
-        POP_SYNC_CHK;
-        break;
-    }
+    gauss::get_by_2_pick_every_second
+        <<<h_grid,h_block,0,stream>>>
+        ( prev_oct_obj.getDataTexPoint( ),
+          prev_oct_obj.getWidth(),
+          prev_oct_obj.getHeight(),
+          _levels-PREV_LEVEL,
+          oct_obj.getDataSurface( ),
+          oct_obj.getWidth(),
+          oct_obj.getHeight() );
+
+    POP_SYNC_CHK;
 }
 
 __host__

src/popsift/sift_conf.h

@@ -366,7 +366,7 @@ struct Config
     /// default: 1
     int  _filter_grid_size;
 
-    /// Modes are computation according to VLFeat or OpenCV,
+    /// Modes are computation according to VLFeat
     /// or fixed size. Default is VLFeat mode.
     GaussMode _gauss_mode;