add helper to know when a colortype is always opaque

Now that surface creation is more picky about its imageinfo, we need to
allow clients to know when they should clean-up their alphatype (like
our own gm)

Bug: skia:
Change-Id: Ic110c75769e0154a8343d7e2160d3351f02cf48f
Reviewed-on: https://skia-review.googlesource.com/106320
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>

gm/imagealphathreshold.cpp

@@ -89,7 +89,7 @@ private:
 // Create a 'width' x 'height' SkSurface that matches the colorType of 'canvas' as
 // best we can
 static sk_sp<SkSurface> make_color_matching_surface(SkCanvas* canvas, int width, int height,
-                                                    SkAlphaType alphaType) {
+                                                    SkAlphaType at) {
 
     SkColorType ct = canvas->imageInfo().colorType();
     sk_sp<SkColorSpace> cs(canvas->imageInfo().refColorSpace());
@@ -98,9 +98,11 @@ static sk_sp<SkSurface> make_color_matching_surface(SkCanvas* canvas, int width,
         // For backends that aren't yet color-space aware we just fallback to N32.
         ct = kN32_SkColorType;
         cs = nullptr;
+    } else if (SkColorTypeIsAlwaysOpaque(ct)) {
+        at = kOpaque_SkAlphaType;
     }
 
-    SkImageInfo info = SkImageInfo::Make(width, height, ct, alphaType, std::move(cs));
+    SkImageInfo info = SkImageInfo::Make(width, height, ct, at, std::move(cs));
 
     return sk_tool_utils::makeSurface(canvas, info);
 }

include/core/SkImageInfo.h

@@ -93,6 +93,11 @@ enum SkColorType {
  */
 SK_API int SkColorTypeBytesPerPixel(SkColorType ct);
 
+/**
+ *  Returns true iff the colortype is always considered opaque (i.e. does not store alpha).
+ */
+SK_API bool SkColorTypeIsAlwaysOpaque(SkColorType ct);
+
 /**
  *  Tries to validate the colortype, alphatype pair. In all cases if it returns true, it
  *  will set canonical to the "canonical" answer if it is non-null, and ignore the parameter if
@@ -110,8 +115,8 @@ SK_API int SkColorTypeBytesPerPixel(SkColorType ct);
  *  one of them (e.g. kUnknown_SkAlphaType is not valid for any colortype except
  *  kUnknown_SkColorType), then the function returns false, and canonical's value is undefined.
  */
-bool SkColorTypeValidateAlphaType(SkColorType colorType, SkAlphaType alphaType,
-                                  SkAlphaType* canonical = nullptr);
+SK_API bool SkColorTypeValidateAlphaType(SkColorType colorType, SkAlphaType alphaType,
+                                         SkAlphaType* canonical = nullptr);
 
 
 ///////////////////////////////////////////////////////////////////////////////

src/core/SkImageInfo.cpp

@@ -79,6 +79,19 @@ static SkColorType stored_to_live(unsigned stored) {
     return kUnknown_SkColorType;
 }
 
+bool SkColorTypeIsAlwaysOpaque(SkColorType ct) {
+    switch (ct) {
+        case kRGB_565_SkColorType:
+        case kRGB_888x_SkColorType:
+        case kRGB_101010x_SkColorType:
+        case kGray_8_SkColorType:
+            return true;
+        default:
+            break;
+    }
+    return false;
+}
+
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 int SkImageInfo::bytesPerPixel() const { return SkColorTypeBytesPerPixel(fColorType); }