commit 328fc724a64ead26f13ccb97d1ce6075b6fee173
author David Davidović <david@davidovic.io> Thu Jul 25 00:26:41 2024 +0200
committer GitHub <noreply@github.com> Wed Jul 24 15:26:41 2024 -0700
tree ca2525f675add7b5bd1367156a513a8c126c44ec
parent c4cb2254eca3c70199f1de5e39e3872c8c42e473

Apply blend modes on layer level and add Multiply blend mode (#2519)

This commit improves blend mode support in lottie-android in two ways:
* Applying blend modes on layer-level, instead of fill level
* Adding support for the Multiply blend mode

## Applying blend modes on layer level

The Lottie format defines blend modes as attributes on a layer. However, lottie-android is presently applying the layer blend modes on a solid color fill only. Notably, this causes any stroked or gradient-filled shapes or image layers to blend incorrectly, such as in this file:

[stroke-blending-test.json](https://github.com/user-attachments/files/16346206/stroke-blending-test.json)

(The file contains a filled + stroked shape that renders as a pink square on other platforms, but renders with a visible stroke on lottie-android since its blend mode is applied only on the fill.)

Instead, we move this decision to `BaseLayer` by analogy to transparent layer handling, which is closer to how the format specifies the property and fixes these cases.

## Multiply support

`BlendModeCompat` is designed to resolve to either a `BlendMode` (added in Android Q, supporting most modern blend modes) or `PorterDuff.Mode` (always available, but a smaller choice of modes as it is mostly focused on alpha compositing).

We use `BlendModeCompat` to support Lottie layer blend modes (`bm` key) to ensure compatibility on all platforms. For consistency, we don't  support values which don't have a `PorterDuff.Mode` equivalent.

Our support for Lottie blend modes did not include Multiply due to a slightly different behavior between the `PorterDuff.MULTIPLY` (exposed as `BlendModeCompat.MODULATE`) and `BlendModeCompat.MULTIPLY` variants. Namely, the formula used for `PorterDuff.MODULATE`, combined with alpha-premultiplication done by Skia, means that a layer with an alpha < 1.0 and multiply blend mode will also darken the destination:

![Incorrect-Blend](https://github.com/user-attachments/assets/6a2113ef-4bac-4bbc-830b-1353adf4ee2b)

(Multiply-blended layers with < 1.0 alpha on the left, Screen-blended layers with < 1.0 alpha on the right)

However, what we can do instead is clear the canvas with a solid white color instead of transparent before drawing the layer's contents as normal. When blending the resulting bitmap over an opaque background using `PorterDuff.MULTIPLY` (i.e. `BlendModeCompat.MODULATE`), the end result will be as if we had used `BlendModeCompat.MULTIPLY`, since all-1.0 (white) is a multiplication identity:

![Correct-Blend](https://github.com/user-attachments/assets/126022ef-6e47-48ee-b803-1d9800ca2c75)

This PR implements the latter solution and adds a consistent support for the Multiply blend mode for all Android versions.

*Test file used:*  [blendmode-tests-multiply+screen+bg.zip](https://github.com/user-attachments/files/16365843/blendmode-tests-multiply%2Bscreen%2Bbg.zip)