Syphon > Syphon with opacity causing problems

[Richard Williamson]

Hi all

Having a funny issue with Syphon - found this issue on 5.3 but also tested on 5.2 and issue was there then.

We have a show where we are using a syphon only surface to route some video around and back in to qlab where it is sucked in via a pair of camera cues and put out over HDMI

When the content is at 100% opacity everything is fine, but it looks like something funny is happening when the opacity is >0% and <100% , I think that the fade is effectively being doubled

In the attached photo you will see on the left the monitor of a syphon output with what look like two blocks of the same colour, in reality the left hand block is an image at 50% opacity, while the right hand block is a screen shot of this, so played back at 100% opacity

On the right is what the camera cue is capturing from syphon - you will see that the image with opacity is massively darker than the one at 100%

I suspect this is an issue internal to qlab but if anyone has any ideas of how this could be fixed it would be much appreciated!

Thanks

Richard

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Richard Williamson
www.richard-williamson.com
ric...@richard-williamson.com
07957 457 362

[Richard Williamson]

As an addendum to this, I just tested with qlab 4 and don't see this issue so I assume it's a bug that's crept in with the new video engine.

I don't see the issue with NDI in qlab 5, however due to the size of our surfaces (about 7k pixels wide) the performance impact of NDI makes it problematic

[micpool]

I can replicate this, and it's quite baffling.

The Syphon output itself is at the same intensity as a physical display output.

But, as you say, an image sent to a  Syphon output,  to  a camera cue, to  a display,  fades twice as fast as  the image sent directly to the display.

My first thought was that you had the image and the camera cue in a group and were fading both together, but this is not the case. Only the 2 images are fading, the camera remains at 100 percent throughout.

Screen recording attached

[micpool]

Ah ,sussed whats going on.

The syphon output has an alpha channel. To make content sent on a syphon out/camera in route fade the same as a direct to display route, you need to put a black layer behind the camera cue.

Mic

[Richard Williamson]

That’s amazing! Thanks Mic - will give that a go tomorrow. Might be good to add an option to disable alpha on camera cues or something 

Sent from my iPhone

On 19 Oct 2023, at 20:59, micpool <m...@micpool.com> wrote:

Ah ,sussed whats going on.

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[micpool]

The black image behind the camera cue is a good workaround,  but I think the camera cue may be misinterpreting the alpha channel as it fades. I can't replicate the same fade behaviour in a video cue when fading a video with an alpha. The camera cue somehow seems to be fading to white as the alpha fades, alongside the RGB fading as expected.

Mic