[gegl] buffer: more improvements to sampler box-filter criterion

[Ell <ell src gnome org>]

commit 662b9c35f5e052f94268ea0aea59923ed23b10bb
Author: Ell <ell_se yahoo com>
Date:   Thu Nov 15 17:01:33 2018 -0500

    buffer: more improvements to sampler box-filter criterion
    
    Don't perform box filtering in the linear/cubic samplers if any of
    the transformed basis vectors are scaled down, that is, if the
    image is upscaled in either direction, since the box filter would
    effectively be performing nearest-neighbor sampling in this
    direction -- it's better to let the non box-filtered sampler do its
    thing.

 gegl/buffer/gegl-sampler.h | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)
---
diff --git a/gegl/buffer/gegl-sampler.h b/gegl/buffer/gegl-sampler.h
index 54d2724fc..40091eae8 100644
--- a/gegl/buffer/gegl-sampler.h
+++ b/gegl/buffer/gegl-sampler.h
@@ -237,7 +237,8 @@ _gegl_sampler_box_get (GeglSampler*    restrict  self,
       const gdouble v_norm2 = scale->coeff[0][1] * scale->coeff[0][1] +
                               scale->coeff[1][1] * scale->coeff[1][1];
 
-      if (u_norm2 >= 4.0 || v_norm2 >= 4.0)
+      if ((u_norm2 >= 4.0 && v_norm2 >= 1.0) ||
+          (v_norm2 >= 4.0 && u_norm2 >= 1.0))
         {
           gfloat  result[channels];
           gdouble uv_samples_inv;
@@ -260,8 +261,8 @@ _gegl_sampler_box_get (GeglSampler*    restrict  self,
               const gdouble v_norm         = fabs (scale->coeff[1][1]);
               const gint    u_norm_i       = ceil (u_norm);
               const gint    v_norm_i       = ceil (v_norm);
-              const gint    u_samples      = CLAMP (u_norm_i, 1, n_samples);
-              const gint    v_samples      = CLAMP (v_norm_i, 1, n_samples);
+              const gint    u_samples      = MIN (u_norm_i, n_samples);
+              const gint    v_samples      = MIN (v_norm_i, n_samples);
               const gdouble u_samples_inv  = 1.0 / u_samples;
               const gdouble v_samples_inv  = 1.0 / v_samples;
               const gdouble u_dx           = scale->coeff[0][0] * u_samples_inv;
@@ -301,8 +302,8 @@ _gegl_sampler_box_get (GeglSampler*    restrict  self,
               const gdouble v_norm         = sqrt (v_norm2);
               const gint    u_norm_i       = ceil (u_norm);
               const gint    v_norm_i       = ceil (v_norm);
-              const gint    u_samples      = CLAMP (u_norm_i, 1, n_samples);
-              const gint    v_samples      = CLAMP (v_norm_i, 1, n_samples);
+              const gint    u_samples      = MIN (u_norm_i, n_samples);
+              const gint    v_samples      = MIN (v_norm_i, n_samples);
               const gdouble u_samples_inv  = 1.0 / u_samples;
               const gdouble v_samples_inv  = 1.0 / v_samples;
               const gdouble u_dx           = scale->coeff[0][0] * u_samples_inv;