Changing The "premultiplied Matted Color" In A Cg Generated Rgba Image

[gcpnmax]

Hey everybody,

this is kind of an embarassing question, but I wasn't able to figure it out in a nice way.

I got a CG image rendered out of maya with rgba (premultiplied on GREY) where in the rgb channel I got instead of a BLACK background something greyish (r: 0.6 G: 0.6 B: 0.6). The Alpha is clean as expected. How Do I replace the wrong Background color in my rgb channels so that I get clean edges (with NO grey left in the semi transparent edges) to go on with my comp? (in AFX for example I would choose to interpret the footage differently and switch the "Matted with color" to grey)

I tried a alpha masked (by the original alpha to leave out the foreground objects) color lookup where I pull down everything by R:0.4 G: 0.4 B: 0.4 but I seem to get dirty edges this way when I premultiply the resulting rgb with the unchanged alpha afterwards. I think I have to alter the Edges of my alpha too to get clean edges afterwards but how? When I start some messy edge detection stuff and then color correct the edges until its "ok" I have the feeling do do it an unnecessarely messy way and of course the Motion Blur gets hurt.

Any ideas? I'm sure it's not that hard (and solvable by a clean math operation) but I seem to be a little lost here.

Thx for your times.

Louis

[user_deleted]

you just have to reverse what an Over does internally, so try this:
invert the alpha and copy it into a Constant node that is the colour your cg was rendered on, then premultiply that.
Subtract the result from your cg element using a Merge node set to minus and the cg connected to input A. Make sure you uncheck the rgba.alpha channel for the B input in that Merge node so the alpha isn't effected by the subtraction.
This should give you a clean, properly premultiplied element that is ready for comping.

the same workflow can be used to cancel out a background image that your cg was rendered on given you have the clean bg image available for subtraction.

[user_deleted]

try this:

BackdropNode {
inputs 0
name BackdropNode2
tile_color 0x3f00
label fix
note_font_size 50
selected true
xpos -62
ypos -238
bdwidth 428
bdheight 416
}
BackdropNode {
inputs 0
name BackdropNode1
tile_color 0x28284600
label "CG render"
note_font_size 50
selected true
xpos -62
ypos -794
bdwidth 290
bdheight 512
}
CheckerBoard2 {
inputs 0
full_format "512 512 0 0 512 512 1 square_512"
color0 {0.1 0.1 0.1 1}
color1 {0.5 0.5 0.5 1}
color2 {0.1 0.1 0.1 1}
color3 {0.5 0.5 0.5 1}
centerlinecolor {1 1 0 1}
name CheckerBoard1
selected true
xpos -12
ypos -694
}
add_layer {alpha rgba.alpha}
Radial {
output alpha
cliptype format
replace true
area {59 68 490 435}
p1 {100 400}
p0 {100 100}
name Radial1
selected true
xpos -12
ypos -613
}
Premult {
name Premult1
selected true
xpos -12
ypos -494
}
clone nodeefa3b640 Constant {
inputs 0
channels rgb
color 0.6
full_format "512 512 0 0 512 512 1 square_512"
name Constant1
selected true
xpos 108
ypos -461
}
set Cefa3b640 [stack 0]
Merge2 {
inputs 2
bbox B
name Merge1
selected true
xpos -12
ypos -440
}
set N9150e58 [stack 0]
push $N9150e58
Dot {
selected true
xpos 19
ypos -60
}
Invert {
channels rgba
name Invert1
selected true
xpos 88
ypos -61
}
clone $Cefa3b640 {
inputs 0
xpos 216
ypos -138
selected true
}
Copy {
inputs 2
from0 rgba.alpha
to0 rgba.alpha
name Copy1
selected true
xpos 216
ypos -68
}
Premult {
name Premult2
selected true
xpos 216
ypos 78
}
Merge2 {
inputs 2
operation minus
Bchannels {rgba.red rgba.green rgba.blue -rgba.alpha}
bbox B
name Merge4
selected true
xpos -12
ypos 78
}

[gcpnmax]

Thanks so much, I really was on the wrong path constantly trying to replace grey with white via a compositing operation. Glad you had the time.

Louis

[boraddd]

You right rueter but there is one problem. As soon as alpha and edges are fine, overal color of my CG element is affected by minus operation.
How to avoid that

Thanks

Chris

[user_deleted]

that should only be the case for pixels with an alpha less than 1 in which case the apparent colour is not the solid colour anyway, as it is mixed with the transparency information (i.e. multiplied by a values less than 1)