ID:2031198
 
Not a bug
BYOND Version:510.1322
Operating System:Windows 7 Home Basic 64-bit
Web Browser:Chrome 48.0.2564.103
Applies to:Dream Seeker
Status: Not a bug

This is not a bug. It may be an incorrect use of syntax or a limitation in the software. For further discussion on the matter, please consult the BYOND forums.
Descriptive Problem Summary:
In 1320, this block of code worked fine. I was placing a BLEND_SUBTRACT square over a mob with both on the same BLEND_OVERLAY plane. The mob was correctly subtracted out and the BLEND_SUBTRACT square itself was invisible. The turfs underneath were also drawn correctly. In 1321 and 1322, it looks completely different. The square is fully black and as though it was BLEND_OVERLAY. It's also rendered below the mob despite being on the same plane.

Code Snippet (if applicable) to Reproduce Problem:
/mob
plane = 5
layer = 1

/obj/master/swipe_master
icon_state = "square"
screen_loc = "1,1"
appearance_flags = PLANE_MASTER
plane = 5
color = "#fff"
alpha = 255

/mob/Login()
..()
client.screen += new /obj/master/swipe_master()

/mob/verb/teleport_effect_test()
var/obj/swipe = new(locate(x+1,y,z))
swipe.icon_state = "square"
swipe.blend_mode = BLEND_SUBTRACT
swipe.plane = 5
swipe.color = "#000"
swipe.layer = 10
var/obj/copy = new(locate(x-4,y,z))
copy.appearance = appearance
var/obj/copyswipe = new(locate(copy.x,copy.y,copy.z))
copyswipe.appearance = swipe.appearance

var/matrix/M = transform
M.Translate(-32,0)
animate(swipe, transform = M, time = 40)
animate(copyswipe, transform = M, time = 40)


The square icon_state is just a 32x32 fully white opaque square.

Expected Results:
1320 looks like this:


Actual Results:
1321 and 1322 look like this:


Did the problem NOT occur in any earlier versions? If so, what was the last version that worked?

1320 worked fine.

Workarounds?
I haven't found anything that quite does what I was looking for that works on 1321.
Do you have a quick demo for this that I can check out? It'd be easier having the icons and such all ready to go.
Lummox JR resolved issue (Not a bug)
The behavior in question is not a regression; the BLEND_SUBTRACT behavior in 1320 (and earlier builds, but it wasn't an issue before) was simply incorrect. The top icon should never reduce the alpha of the bottom icon. This holds for BLEND_MULTIPLY as well.