pay some attention to apply_tf()
Our sign-handling code here is very first-draft.
This new version does the same thing with much cheaper instructions.
Should be a small, free performance win.
Change-Id: I858ae734b57a0595fbf3f72c97bbe5a5a6d3d6b6
Reviewed-on: https://skia-review.googlesource.com/144624
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/src/Transform_inl.h b/src/Transform_inl.h
index 49d324a..86d9c9b 100644
--- a/src/Transform_inl.h
+++ b/src/Transform_inl.h
@@ -236,13 +236,17 @@
// Return tf(x).
SI ATTR F apply_tf(const skcms_TransferFunction* tf, F x) {
- F sign = if_then_else(x < 0, -F1, F1);
- x *= sign;
+ // Peel off the sign bit and set x = |x|.
+ U32 bits = bit_pun<U32>(x),
+ sign = bits & 0x80000000;
+ x = bit_pun<F>(bits ^ sign);
- F linear = tf->c*x + tf->f;
- F nonlinear = approx_pow(tf->a*x + tf->b, tf->g) + tf->e;
+ // The transfer function has a linear part up to d, exponential at d and after.
+ F v = if_then_else(x < tf->d, tf->c*x + tf->f
+ , approx_pow(tf->a*x + tf->b, tf->g) + tf->e);
- return sign * if_then_else(x < tf->d, linear, nonlinear);
+ // Tack the sign bit back on.
+ return bit_pun<F>(sign | bit_pun<U32>(v));
}
