Detect when we can skip clamps on gamut transforms.

This is the same logic from the matrix color filter, scaled down from 4x5 to 3x4.
I'm seeing cases in GMs where we can skip clamps in both directions, but never both together.

Change-Id: I515c5e207d35ed23f1e267d55460fe4d41e1a8f1
Reviewed-on: https://skia-review.googlesource.com/5456
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

1 files changed, 28 insertions, 3 deletions

diff --git a/src/core/SkPM4fPriv.h b/src/core/SkPM4fPriv.h
index db8d478f13..26d8b7d4e0 100644
--- a/src/core/SkPM4fPriv.h
+++ b/src/core/SkPM4fPriv.h
@@ -75,6 +75,29 @@ static inline float exact_srgb_to_linear(float srgb) {
     return linear;
 }
 
+static inline void analyze_3x4_matrix(const float matrix[12],
+                                      bool* can_underflow, bool* can_overflow) {
+    // | 0 3 6  9 |   |r|   |x|
+    // | 1 4 7 10 | x |g| = |y|
+    // | 2 5 8 11 |   |b|   |z|
+    //                |1|
+    // We'll find min/max bounds on each of x,y,z assuming r,g,b are all in [0,1].
+    // If any can be <0, we'll set can_underflow; if any can be >1, can_overflow.
+    bool underflow = false,
+          overflow = false;
+    for (int i = 0; i < 3; i++) {
+        SkScalar min = matrix[i+9],
+                 max = matrix[i+9];
+        (matrix[i+0] < 0 ? min : max) += matrix[i+0];
+        (matrix[i+3] < 0 ? min : max) += matrix[i+3];
+        (matrix[i+6] < 0 ? min : max) += matrix[i+6];
+        underflow = underflow || min < 0;
+        overflow  =  overflow || max > 1;
+    }
+    *can_underflow = underflow;
+    *can_overflow  =  overflow;
+}
+
 
 // N.B. scratch_matrix_3x4 must live at least as long as p.
 static inline bool append_gamut_transform(SkRasterPipeline* p, float scratch_matrix_3x4[12],
@@ -98,10 +121,12 @@ static inline bool append_gamut_transform(SkRasterPipeline* p, float scratch_mat
     *ptr++ = m44.get(0,2); *ptr++ = m44.get(1,2); *ptr++ = m44.get(2,2);
     *ptr++ = m44.get(0,3); *ptr++ = m44.get(1,3); *ptr++ = m44.get(2,3);
 
+    bool needs_clamp_0, needs_clamp_a;
+    analyze_3x4_matrix(scratch_matrix_3x4, &needs_clamp_0, &needs_clamp_a);
+
     p->append(SkRasterPipeline::matrix_3x4, scratch_matrix_3x4);
-    // TODO: detect whether we can skip the clamps?
-    p->append(SkRasterPipeline::clamp_0);
-    p->append(SkRasterPipeline::clamp_a);
+    if (needs_clamp_0) { p->append(SkRasterPipeline::clamp_0); }
+    if (needs_clamp_a) { p->append(SkRasterPipeline::clamp_a); }
     return true;
 }