ndi consumer straight alpha

[ducthiem90]

Expected behaviour

CasparCG output rgb error when alpha != 1
I think mixer needs divide by alpha when alpha != 0 and alpha != 1
color.rgb /= color.a

Current behaviour

When I try play with alpha and export to NDI, color receive in NDI Studio error

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Steps to reproduce

ADD 1 SCREEN
ADD 1 NDI
PLAY 1 #FF0000
MIXER 1 OPACITY 0.5

Environment

• Server version: 2.3.3
• Operating system: Windows 10

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Screenshots

[gizahNL]

To me this looks like it is working correctly.

You set opacity to 50%, so 50% translucent. NDI (supposedly, never seen it working with linux based receiver) transports Alpha channel. NDI Monitor has a black background, and alpha blends your red color on top of it. Making it darker.

[ducthiem90]

To me this looks like it is working correctly.

You set opacity to 50%, so 50% translucent. NDI (supposedly, never seen it working with linux based receiver) transports Alpha channel. NDI Monitor has a black background, and alpha blends your red color on top of it. Making it darker.

I don't think so. Here are the results when I change alphas:
CasparCG:

NDI Receive:

You can see color 50% in caspar = color 25% in NDI

[hreinnbeck]

Can you check turning on the checkerboard background in studio monitor and seeing how it multiplies with white? I don't remember how studio monitor handles alpha. Or perhaps test in OBS over another source? Perhaps also taking it back into CCG on another channel.

[hreinnbeck]

Looks to me like NDI wants straight alpha but CCG (>=2.3) only does pre-multiplied (and studio monitor likely only supports straight). Various software that works with NDI have options to support pre-multiplied.

[ducthiem90]

Here are results with white color:

• CCG:
  

• NDI Studio receive from CCG:
  

• NDI Studio receive from OBS:
  

[mint-dewit]

This is with the NDI receiver set to straight alpha:

This is with the NDI receiver set to premultiplied

I used VMix in this case but the studio monitor looks the same as the first picture. So this clears up the issue, Studio Monitor does not handle premultiplied alpha and that's not a bug in CasparCG.

(Just as a side note, it is quite annoying that the warnings on the test pattern are incorrect, however the top bar fading from black to white at 50% opacity is a good indication of what is going on)

[ducthiem90]

This is with the NDI receiver set to straight alpha:

This is with the NDI receiver set to premultiplied

I used VMix in this case but the studio monitor looks the same as the first picture. So this clears up the issue, Studio Monitor does not handle premultiplied alpha and that's not a bug in CasparCG.

(Just as a side note, it is quite annoying that the warnings on the test pattern are incorrect, however the top bar fading from black to white at 50% opacity is a good indication of what is going on)

(SIGHS)! I think we need to send to ndi as yuv format to not incorrect