Sprite: Additive and Multiply color should take into account transparency

[fdefelici]

Understand if it is possible to avoid coloring transparent pixel when using Additive and Multiply coloring in Sprite and InstancedSprite

[DoctorWho406]

Taking transparency into account when using Additive Tint and Multiply Tint would be ideal for creating time-related fade effects. In particular with the Multiply. For now when the Multiply is invoked to modify only the transparency the whole sprite disappears. On the contrary with the AdditiveTint the Sprite becomes transparent but also the other colors change.

[fdefelici]

@DoctorWho406 Could you integrate the issue by providing some code samples that show off what your are describing?

[DoctorWho406]

@fdefelici Sure, as soon as I have a minute at work I'll do it.

[DoctorWho406]

For the Add I used the following code and as a result I got a slightly transparent Sprite but with deformed colors.

public override void Update() {
    sprite.SetAdditiveTint(0f ,0f, 0f, -0.1 * Game.DeltaTime);
}

For the Multiply instead I used the following code. The sprite disappeared immediately.

public override void Update() {
    if(Disappear) {
        sprite.SetMultiplyTint(0f ,0f, 0f, 0.9 * Game.DeltaTime);
    }
}

[fdefelici]

Additive and Multiply are 'storically' implemented this way:

frag_color *= mul_tint;
frag_color += vec4(add_tint.xyz * frag_color.a, add_tint.a);

This explain why Addictive distor color (by Alpha component)

Instead, Multiply should work as expected. It's normal that sprite disappear immediatly because you are multiply RGB by 0. This should work in this way:

public override void Update() {
    if(Disappear) {
        sprite.SetMultiplyTint(1f ,1f, 1f, 0.9 * Game.DeltaTime);
    }
}

[DoctorWho406]

Thank you @fdefelici. The Multiply method works!

I did not understand why the Additive should distort the colors. In my case (I used Either Additive OR Multiplay, never both) the mul_tin was not there so I suppose it is replaced with a vector (1, 1, 1, 1)

frag_color *= mul_tint;

while add_tin was a vector with (0,0,0, -1 * Game.Deltatime)

frag_color + = vec4 (add_tint.xyz * frag_color.a, add_tint.a);

then the frag_color would be incremented by a vector ((0,0,0) frag_color.a , -1 Game.DeltaTime) = (0, 0, 0, -1 * Game.DeltaTime) which should do exactly what I wanted.