[gimp] Bug 766683: Burn Mode on Paint tools are producing artefacts

[Massimo Valentini <mvalentini src gnome org>]

commit 7858eb3df2c30e1604ca6cb925551c5b2186df9a
Author: Massimo Valentini <mvalentini src gnome org>
Date:   Mon Jun 6 18:47:47 2016 +0200

    Bug 766683: Burn Mode on Paint tools are producing artefacts
    
    This operation produces NAN (0 / 0) and division
    by zero when a layer component is zero.
    
    Inline CLAMP to make sure it swallows also NAN.
    A NAN is mapped to 1 for backward compatibility.
    [the 2variate function comp (in[b], layer[b]) is
    discontinuous at layer[b] = 0]

 app/operations/gimpoperationburnmode.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/app/operations/gimpoperationburnmode.c b/app/operations/gimpoperationburnmode.c
index 8822e9d..c4a695e 100644
--- a/app/operations/gimpoperationburnmode.c
+++ b/app/operations/gimpoperationburnmode.c
@@ -109,7 +109,10 @@ gimp_operation_burn_mode_process_pixels (gfloat              *in,
           for (b = RED; b < ALPHA; b++)
             {
               gfloat comp = 1.0 - (1.0 - in[b]) / layer[b];
-              comp = CLAMP (comp, 0.0, 1.0);
+              /* The CLAMP macro is deliberately inlined and
+               * written to map comp == NAN (0 / 0) -> 1
+               */
+              comp = comp < 0 ? 0.0 : comp < 1.0 ? comp : 1.0;
 
               out[b] = comp * ratio + in[b] * (1.0 - ratio);
             }