Possibly demosaicers should not write negatives

Demosaicer input has negatives, also it's output can have negs due to internal
algorithms even if input didn't.

All OpenCL demosaicers read data safely (leaving -FLT_MAX) even if the image was not correctly initialized,
we had such issues on AMD systems with bad things following, nvidia/intel seemed to return zero in such cases.

If things in the pixelpipe work fine all should be good even with negs fed into the pipe, following code
in color maths must handle that. Remember there are colorsoaces with neg values so the interpolaters should
not try to keep data non-negative.

We can possibly add code prohibiting negative demosaicing data by uncommenting
DT_DEMOSAIC_POSITIVE

Author: jenshannoschwalm

src/iop/demosaic.c

@@ -52,6 +52,8 @@ DT_MODULE_INTROSPECTION(6, dt_iop_demosaic_params_t)
 #define DT_DEMOSAIC_XTRANS 1024 // masks for non-Bayer demosaic ops
 #define DT_DEMOSAIC_DUAL 2048   // masks for dual demosaicing methods
 
+// #define DT_DEMOSAIC_POSITIVE
+
 typedef enum dt_iop_demosaic_method_t
 {
   // methods for Bayer images
@@ -240,6 +242,7 @@ typedef struct dt_iop_demosaic_global_data_t
   int show_blend_mask;
   int capture_result;
   int final_blend;
+  int image_positive;
   float *gauss_coeffs;
 } dt_iop_demosaic_global_data_t;
 
@@ -904,6 +907,13 @@ void process(dt_iop_module_t *self,
     dt_iop_clip_and_zoom_roi((float *)o, out, roi_out, roi_in);
     dt_free_align(out);
   }
+
+#ifdef DT_DEMOSAIC_POSITIVE
+  float *oo = (float *)o;
+  DT_OMP_FOR_SIMD(aligned(oo : 64))
+  for(size_t k = 0; k < (size_t)roi_out->width * roi_out->height * 4; k++)
+    oo[k] = fmaxf(0.0f, oo[k]);
+#endif
 }
 
 #ifdef HAVE_OPENCL
@@ -1201,7 +1211,13 @@ int process_cl(dt_iop_module_t *self,
   }
 
   if(!direct)
-     err = dt_iop_clip_and_zoom_roi_cl(devid, dev_out, out_image, roi_out, roi_in);
+    err = dt_iop_clip_and_zoom_roi_cl(devid, dev_out, out_image, roi_out, roi_in);
+
+#ifdef DT_DEMOSAIC_POSITIVE
+  if(err == CL_SUCCESS)
+    err = dt_opencl_enqueue_kernel_2d_args(devid, gd->image_positive, roi_out->width, roi_out->height,
+        CLARG(dev_out), CLARG(dev_out), CLARG(roi_out->width), CLARG(roi_out->height));
+#endif
 
 finish:
   dt_opencl_release_mem_object(dev_xtrans);
@@ -1284,6 +1300,7 @@ void init_global(dt_iop_module_so_t *self)
   gd->kernel_rcd_step_5_2 = dt_opencl_create_kernel(rcd, "rcd_step_5_2");
   gd->kernel_demosaic_box3 = dt_opencl_create_kernel(rcd, "demosaic_box3");
   gd->kernel_write_blended_dual  = dt_opencl_create_kernel(rcd, "write_blended_dual");
+  gd->image_positive  = dt_opencl_create_kernel(rcd, "image_positive");
 
   const int capt = 38; // capture.cl, from programs.conf
   gd->gaussian_9x9_mul = dt_opencl_create_kernel(capt, "kernel_9x9_mul");
@@ -1357,6 +1374,7 @@ void cleanup_global(dt_iop_module_so_t *self)
   dt_opencl_free_kernel(gd->show_blend_mask);
   dt_opencl_free_kernel(gd->capture_result);
   dt_opencl_free_kernel(gd->final_blend);
+  dt_opencl_free_kernel(gd->image_positive);
   dt_free_align(gd->gauss_coeffs);
   free(self->data);
   self->data = NULL;