[Mesa-dev] [PATCH] glsl: improve the accuracy of the atan(x, y) builtin function.

[Paul Berry]

The previous formula for atan(x,y) returned a value of +/- pi whenever
|x|<0.0001, and used a formula based on atan(y/x) otherwise.  This
broke in cases where both x and y were small (e.g. atan(1e-5, 1e-5)).

This patch modifies the formula so that it returns a value of +/- pi
whenever |x|<1e-8*|y|, and uses the formula based on atan(y/x)
otherwise.
---
 src/glsl/builtins/ir/atan |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/src/glsl/builtins/ir/atan b/src/glsl/builtins/ir/atan
index cfecc1f..7b5ea13 100644
--- a/src/glsl/builtins/ir/atan
+++ b/src/glsl/builtins/ir/atan
@@ -54,7 +54,9 @@
     )
     (
       (declare () float r)
-      (if (expression bool > (expression float abs (var_ref x)) (constant float (0.000100))) (
+      (if (expression bool >
+           (expression float abs (var_ref x))
+           (expression float * (constant float (1.0e-8)) (expression float abs (var_ref y)))) (
         (assign (x) (var_ref r) (call atan ((expression float / (var_ref y) (var_ref x)))))
         (if (expression bool < (var_ref x) (constant float (0.000000)) ) (
           (if (expression bool >= (var_ref y) (constant float (0.000000)) )
-- 
1.7.6