#896 fbGFX: Paint statement 'overflows', with SreenRes: GFX_ALPHA_PRIMITIVES

[MrSwiss]

Precondition: GFX_ALPHA_PRIMITIVES is set, in ScreenRes statement.

A color (ULong) 32 bit, used with Alpha other than 255/&hFF as border-color is used.
The result of which is, coloring of the entire user-screen, not respecting the set border.

' Paint-Leak_ex1.bas    -- (c) 2019-04-06, MrSwiss
'
' compile: -s gui
'
Const As UShort wid = 1024, hei = 768, cd = 32, pg = 2, sflg = 64

' ===== Start-Main =====
ScreenRes(wid, hei, cd, pg, sflg)       ' sflg = GFX_ALPHA_PRIMITIVES
Width wid \ 8, hei \ 16
Color(&hFF000000, &hFFFFFFFF) : Cls     ' black (fg) on white (bg)

Line (100, 100)-(wid-101, hei-101), &hFFFF0000, B   ' works correctly, fully opaque
Paint(101, 101), &h3fFFFF00, &hFFFF0000 ' fill color can be anything!
Flip
Sleep : Cls

Line (100, 100)-(wid-101, hei-101), &hBFFF0000, B   ' 25% transparent border = LEAK!!! 
Paint(101, 101), &h3fFFFF00, Point(100, 100)    ' adapted as per counting_pine's advice
Flip
Sleep

As discussed with counting_pine at: https://www.freebasic.net/forum/viewtopic.php?f=3&t=27537

[Matthew]

So as concluded on the forum, the problem is actually line overdraw with the box - the corners are drawn twice, changing their value - and the Point chosen for the border colour is on the corner. If e.g. Point(100,101) is chosen, the fill stops at the border colour as expected.

[MrSwiss]

If e.g. Point(100,101) is chosen, the fill stops at the border colour as expected.

It's by far NOT as simple, as further testing revealed: the simplistic method from fxm, works only with rectangular shapes (90 degree angles), whenever different shapes are drawn using LINE, there will (with high probability) a overlap of pixels happen, which will be causing a leak by a following PAINT statement. One single Pixel is enough to do it; e.g. closing a ellipse / star / polygon e.t.c..
This fact should at least be documented, on the LINE doc-page, of the FB-reference.
The only truely working workaroud, at this point in time is, to set the given border-color to 'fully opaque', by something like:

' assure that clr has Alpha set to opaque
    If clr Shr 24 <> &hFF Then clr Or= &hFF000000

[MrSwiss]

Since a fix for this behaviour is unlikely, but at least the documentation has been updated, to explain the noted behaviour, with LINE / CIRCLE in conjunction with PAINT, this ticket may be declared as: "closed".

See changed pages in documentation: https://www.freebasic.net/forum/viewtopic.php?f=3&p=259899#p259899

[Jeff Marshall]

Thank you MrSwiss. Issue resolved through documentation.

• KeyPgPaint → fxm [Added note about eventual bad filling behavior when using a transparent color for the border]
• KeyPgCircle → fxm [Added note about bad filling behavior by Paint when drawing a border with a transparent color]
• KeyPgLinegraphics → fxm [Added note about bad filling behavior by Paint when drawing a border with a transparent color]