DrawingSurface::DrawSurface doesn't process transparency properly

[fernewelten]

Describe the bug
In a true-colour game, when a DrawingSurface is drawn onto a DrawingSurface, the calculations don't process the transparency parameter and/or the alpha channel of the drawing surfaces properly. See screenshots below.

Also, apparently transparency cannot be 100. This should be an allowed value and result in an unchanged surface (because something is drawn onto it that is completely transparent).

AGS Version
Build 3.5.1.14
Edit: A quick test seems to show that AGS4 does handle DrawingSurface::DrawSurface() properly.

Game
Test351.zip

Here's the relevant code of Room 1:

DynamicSprite *ds;

function test(int transp)
{
    DrawingSurface *dsu = ds.GetDrawingSurface();
    dsu.DrawingColor = Game.GetColorFromRGB(255, 255, 255);
    dsu.DrawRectangle(10, 10, 50, 50);
    DrawingSurface *dsu_copy = dsu.CreateCopy();
    dsu.DrawingColor = Game.GetColorFromRGB(250, 100, 100);
    dsu.DrawCircle(20, 20, 20);
    dsu.DrawSurface(dsu_copy, transp);
    dsu.Release();
    dsu_copy.Release();
    player.Say("This is DrawSurface with %d %% transparency", transp);
}

function room_AfterFadeIn()
{
    ds = DynamicSprite.Create(50, 50, true);
    oTestTest.Graphic = ds.Graphic;
    
    test(99);
    test(70);
    test(20);
    test(1);
    test(0);
}

The setup in Room 1:

To Reproduce
Run game, observe screens:

Screenshots and expected behaviour

The square seems to be completely cut out of the circle although it is nearly completely transparent as per the transparency parameter (99 %).
I'd expect some of the circle to show through.

The circle has completely disappeared.
Outside of the white square, the alpha channel of the copy should be "transparent", so copying should not obliterate the circle in that area.

Transparency 20 % seems to do exactly the same as transparency 70 %. That can't be right.

Even transparency 1 % seems to do exactly the same as transparency 70 %

At 0 % transparency, the copy seems to replace the surface completely.
But, seeing that most of the copy has a completely transparent alpha channel, I'd expect the red circle to stay on surface wherever the white square isn't.

Desktop (please complete the following information):

• Windows 10, German edition

Smartphone (please complete the following information):

• n/a

[ericoporto]

Curiously the Test351.ags game in Compiled/Data did not ran for me. I rebuilt the attached game project and ran it with the latest 3.6.0.X editor release. And the bug doesn't happen in the master, so I tried 3.5.1.14, and it happened there.

Edit: A quick test seems to show that AGS4 does handle DrawingSurface::DrawSurface() properly.

Yeah, I think AGS4 is building from master merges, this is why it's fixed there already.

This could be added to the SDL2 milestone to be tracked as things fixed by it.

[ivan-mogilko]

I recall merging DrawingImage and DrawingSurface internal implementation to go through the same draw function in the end, so maybe that actually fixed that.

This maybe could be fixed in 3.5.1 too, and a new patch released later while 3.6.0 is still in alpha/beta. I wonder when it got broken though.

[ivan-mogilko]

Fixed in release-3.5.1 with 2710b99.

Don't know if/when there will be a next 3.5.1 patch release yet, I'd rather wait for a while in case someone finds another bug there.

[fernewelten]

I wonder when it got broken though.

I seem to have unearthed a function that hasn't been used for a long time, or else somebody would have complained already. 😀

[ivan-mogilko]

The fix was included to 3.5.1.15.