Merge: gamnit: premultiply alpha for a nicer alpha blending
Premultiplying RBG pixels values by their alpha reduces visible artifacts when blending between different opacity levels. It also allows for mixing obstructing blending (like smoke hiding the background) and additive blending (like explosions amplifying each other, as in the second screenshot below).
Here are the old and new explosion effects in Jump 'n' Gun (using the same texture as Action Nitro):
 
By default, textures are expected to be straight (the classic not-premultiplied) and are premultiplied at loading. It can be deactivated for asset files that are already premultiplied (as with the explosion texture above).
The color services in both APIs are also affected. `Sprite::tint` remains the same, using straight colors, the easier alternative. However, `SmoothMaterial::ambient_color`, `diffuse_color` and `specular_color` should be premultiplied, giving more possibility to the clients. Alternatively, `Actor::alpha` is applied to material colors, so while RGB values of `ambient_color` are not premultiplied by `ambient_color[3]` they are premultiplied by `Actor::alpha`.
On Android, pixel values are automatically premultiplied when loaded from a file by the system, so we unpremultiply them as needed (with a strong possibility of data loss on additive blending effects). To fix this, this PR intro a new module `gamnit::android19` using services from the API 19 to disable the premultiply. This module is optional (for now) but it should be imported by all Android games with premultiplied texture asset files.
As a bonus, fix the support for Action Nitro on Android via F-Droid. The latest performance improvements to gamnit makes the game run nicely on mobiles devices!
Pull-Request: #2518
nitlanguage / nit.git / commit