Only use the gainmap shader if we might need it

Skip using the gainmap shader when drawing to a bitmap
with a known SDR colorspace as it'll be a no-op in
rendering results since W=1.0, but it'll be very expensive
in runtime due to CPU evaluation of the SKSL shader.

Before:
createScaledBitmapWithGainmap_median (ns): 179,455,037

After:
createScaledBitmapWithGainmap_median (ns): 15,643,742

Bug: 311085927
Test: benchmark in CL
Change-Id: I3f2506a2bd86bbe29c6e0837d31e4a231a7c182a

1 files changed, 32 insertions, 2 deletions

diff --git a/libs/hwui/SkiaCanvas.cpp b/libs/hwui/SkiaCanvas.cpp
index 31fc929dfcdf..008ea3aaa2e7 100644
--- a/libs/hwui/SkiaCanvas.cpp
+++ b/libs/hwui/SkiaCanvas.cpp
@@ -589,10 +589,40 @@ void SkiaCanvas::drawMesh(const Mesh& mesh, sk_sp<SkBlender> blender, const Pain
 // Canvas draw operations: Bitmaps
 // ----------------------------------------------------------------------------
 
+bool SkiaCanvas::useGainmapShader(Bitmap& bitmap) {
+    // If the bitmap doesn't have a gainmap, don't use the gainmap shader
+    if (!bitmap.hasGainmap()) return false;
+
+    // If we don't have an owned canvas, then we're either hardware accelerated or drawing
+    // to a picture - use the gainmap shader out of caution. Ideally a picture canvas would
+    // use a drawable here instead to defer making that decision until the last possible
+    // moment
+    if (!mCanvasOwned) return true;
+
+    auto info = mCanvasOwned->imageInfo();
+
+    // If it's an unknown colortype then it's not a bitmap-backed canvas
+    if (info.colorType() == SkColorType::kUnknown_SkColorType) return true;
+
+    skcms_TransferFunction tfn;
+    info.colorSpace()->transferFn(&tfn);
+
+    auto transferType = skcms_TransferFunction_getType(&tfn);
+    switch (transferType) {
+        case skcms_TFType_HLGish:
+        case skcms_TFType_HLGinvish:
+        case skcms_TFType_PQish:
+            return true;
+        case skcms_TFType_Invalid:
+        case skcms_TFType_sRGBish:
+            return false;
+    }
+}
+
 void SkiaCanvas::drawBitmap(Bitmap& bitmap, float left, float top, const Paint* paint) {
     auto image = bitmap.makeImage();
 
-    if (bitmap.hasGainmap()) {
+    if (useGainmapShader(bitmap)) {
         Paint gainmapPaint = paint ? *paint : Paint();
         sk_sp<SkShader> gainmapShader = uirenderer::MakeGainmapShader(
                 image, bitmap.gainmap()->bitmap->makeImage(), bitmap.gainmap()->info,
@@ -619,7 +649,7 @@ void SkiaCanvas::drawBitmap(Bitmap& bitmap, float srcLeft, float srcTop, float s
     SkRect srcRect = SkRect::MakeLTRB(srcLeft, srcTop, srcRight, srcBottom);
     SkRect dstRect = SkRect::MakeLTRB(dstLeft, dstTop, dstRight, dstBottom);
 
-    if (bitmap.hasGainmap()) {
+    if (useGainmapShader(bitmap)) {
         Paint gainmapPaint = paint ? *paint : Paint();
         sk_sp<SkShader> gainmapShader = uirenderer::MakeGainmapShader(
                 image, bitmap.gainmap()->bitmap->makeImage(), bitmap.gainmap()->info,