{"action":"created","comment":{"url":"https://api.github.com/repos/flutter/engine/pulls/comments/130001193","pull_request_review_id":52831743,"id":130001193,"diff_hunk":"@@ -1418,6 +1418,44 @@ class Canvas extends NativeFieldWrapperClass2 {\n   /// entire group can be made transparent using the [saveLayer]'s paint.\n   ///\n   /// Call [restore] to pop the save stack and apply the paint to the group.\n+  ///\n+  /// ## Using saveLayer with clips\n+  ///\n+  /// When a rectanglular clip operation (from [clipRect]) is not axis-aligned\n+  /// with the display, or when the clip operation is not rectalinear (e.g.\n+  /// because it is a rounded rectangle clip created by [clipRRect] or an\n+  /// arbitrarily complicated path clip created by [clipPath]), the edge of the\n+  /// clip needs to be anti-aliased.\n+  ///\n+  /// If two draw calls overlap at the edge of such a clipped region, without\n+  /// using [saveLayer], the first drawing will be anti-aliased with the\n+  /// background first, and then the second will be anti-aliased with the result\n+  /// of blending the first drawing and the background. On the other hand, if\n+  /// [saveLayer] is used immediately after establishing the clip, the second\n+  /// drawing will cover the first in the layer, and thus the second alone will\n+  /// be anti-aliased with the background when the layer is clipped and\n+  /// composited (when [restore] is called).\n+  ///\n+  /// This point is moot if the clip only clips one draw operation.\n+  ///\n+  /// ## Performance considerations\n+  ///\n+  /// Generally speaking [saveLayer] is very expensive.","path":"lib/ui/painting.dart","position":35,"original_position":35,"commit_id":"8d5e7b0f5fc66b6feecf2a842fc0bb5bc0daa2e6","original_commit_id":"8d5e7b0f5fc66b6feecf2a842fc0bb5bc0daa2e6","user":{"login":"abarth","id":112007,"avatar_url":"https://avatars3.githubusercontent.com/u/112007?v=4","gravatar_id":"","url":"https://api.github.com/users/abarth","html_url":"https://github.com/abarth","followers_url":"https://api.github.com/users/abarth/followers","following_url":"https://api.github.com/users/abarth/following{/other_user}","gists_url":"https://api.github.com/users/abarth/gists{/gist_id}","starred_url":"https://api.github.com/users/abarth/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/abarth/subscriptions","organizations_url":"https://api.github.com/users/abarth/orgs","repos_url":"https://api.github.com/users/abarth/repos","events_url":"https://api.github.com/users/abarth/events{/privacy}","received_events_url":"https://api.github.com/users/abarth/received_events","type":"User","site_admin":false},"body":"I'd say \"relatively expensive\" rather than \"very expensive\".  I think that's the terminology we've used elsewhere to describe this cost (e.g., in https://docs.flutter.io/flutter/widgets/Opacity-class.html).","created_at":"2017-07-28T02:28:59Z","updated_at":"2017-07-28T02:28:59Z","html_url":"https://github.com/flutter/engine/pull/3932#discussion_r130001193","pull_request_url":"https://api.github.com/repos/flutter/engine/pulls/3932","_links":{"self":{"href":"https://api.github.com/repos/flutter/engine/pulls/comments/130001193"},"html":{"href":"https://github.com/flutter/engine/pull/3932#discussion_r130001193"},"pull_request":{"href":"https://api.github.com/repos/flutter/engine/pulls/3932"}}},"pull_request":{"url":"https://api.github.com/repos/flutter/engine/pulls/3932","id":132861569,"html_url":"https://github.com/flutter/engine/pull/3932","diff_url":"https://github.com/flutter/engine/pull/3932.diff","patch_url":"https://github.com/flutter/engine/pull/3932.patch","issue_url":"https://api.github.com/repos/flutter/engine/issues/3932","number":3932,"state":"open","locked":false,"title":"More documentation, mainly around 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