Roll skia/third_party/skcms 9e351c0..48c6ca0 (1 commits)

https://skia.googlesource.com/skcms.git/+log/9e351c0..48c6ca0

2018-04-25 mtklein@chromium.org evaluate skcms_PolyTF carefully


The AutoRoll server is located here: https://skcms-skia-roll.skia.org

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.



TBR=stani@google.com

Change-Id: I8fb39fbacfe84a0ceec668b5221f8f32cc225f26
Reviewed-on: https://skia-review.googlesource.com/123657
Reviewed-by: skcms-skia-autoroll <skcms-skia-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Commit-Queue: skcms-skia-autoroll <skcms-skia-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>

3 files changed, 9 insertions, 8 deletions

diff --git a/third_party/skcms/src/PolyTF.c b/third_party/skcms/src/PolyTF.c
index 451a4272b7..f4aaa4877d 100644
--- a/third_party/skcms/src/PolyTF.c
+++ b/third_party/skcms/src/PolyTF.c
@@ -14,20 +14,18 @@
 #include <stdlib.h>
 
 // f(x) = skcms_PolyTF{A,B,C,D}(x) =
-//     Cx                     x < D
-//     Ax^3 + Bx^2 + (1-A-B)  x ≥ D
+//     Cx                       x < D
+//     A(x^3-1) + B(x^2-1) + 1  x ≥ D
 //
 // We'll fit C and D directly, and then hold them constant
 // and fit the other part using Gauss-Newton, subject to
 // the constraint that both parts meet at x=D:
 //
-//     CD = AD^3 + BD^2 + (1-A-B)
+//     CD = A(D^3-1) + B(D^2-1) + 1
 //
 // This lets us solve for B, reducing the optimization problem
 // for that part down to just a single parameter A:
 //
-//     CD = A(D^3-1) + B(D^2-1) + 1
-//
 //         CD - A(D^3-1) - 1
 //     B = -----------------
 //               D^2-1
@@ -39,6 +37,9 @@
 //                  (x^2-1) (D^3-1)
 //  ∂f/∂A = x^3-1 - ---------------
 //                       D^2-1
+//
+// It's important to evaluate as f(x) as A(x^3-1) + B(x^2-1) + 1
+// and not Ax^3 + Bx^2 + (1-A-B) to ensure that f(1.0f) == 1.0f.
 
 static float eval_poly_tf(float x, const void* ctx, const float P[4]) {
     const skcms_PolyTF* tf = (const skcms_PolyTF*)ctx;
@@ -49,7 +50,7 @@ static float eval_poly_tf(float x, const void* ctx, const float P[4]) {
     float B = (C*D - A*(D*D*D - 1) - 1) / (D*D - 1);
 
     return x < D ? C*x
-                 : A*x*x*x + B*x*x + (1-A-B);
+                 : A*(x*x*x-1) + B*(x*x-1) + 1;
 }
 
 static void grad_poly_tf(float x, const void* ctx, const float P[4], float dfdP[4]) {
diff --git a/third_party/skcms/src/Transform_inl.h b/third_party/skcms/src/Transform_inl.h
index 529780ab7b..debdf68a88 100644
--- a/third_party/skcms/src/Transform_inl.h
+++ b/third_party/skcms/src/Transform_inl.h
@@ -249,7 +249,7 @@ SI ATTR F NS(apply_transfer_function_)(const skcms_TransferFunction* tf, F x) {
 SI ATTR F NS(apply_poly_tf_)(const skcms_PolyTF* tf, F x) {
     // TODO: handle x<0
     return (F)if_then_else(x < tf->D, tf->C*x
-                                    , tf->A*x*x*x + tf->B*x*x + (1 - tf->A - tf->B));
+                                    , tf->A*(x*x*x-1) + tf->B*(x*x-1) + 1);
 }
 #define apply_poly_tf NS(apply_poly_tf_)
 
diff --git a/third_party/skcms/version.sha1 b/third_party/skcms/version.sha1
index 613794e829..d55a1c447d 100755
--- a/third_party/skcms/version.sha1
+++ b/third_party/skcms/version.sha1
@@ -1 +1 @@
-9e351c034c2b8f6f3166efeaa65c0d4d4f4fa641
\ No newline at end of file
+48c6ca003569d11bdb00847ed2619e40af591f44
\ No newline at end of file