Hi all,

I was wondering if there's a way to have a video sample with an alpha
channel inside veejay.

Here's what I want to do:

- have a set of backgrounds (an arena, a field...)
- have a foreground (a bull),
- insert the foreground onto the various background, without using a
chroma key.

The trick is:
a background consists of a travelling loop (ie scroll from left to
right, the last frame matches the first frame). I then have a
Processing program that cues the video to follow the movements of the
bull.

If it proves too complicated, I of course can do that offline and
calculate all the samples.

If alpha is not supported, does it sound feasible with a chroma key ?

Thanks,

--
Charlot

Hi Charles!

What you want is not so easily accomplished with veejay, at least not as a
direct FX you could apply
Inside the performer, there's no support for an alpha channel

However, I could create an FX that adds a 2nd sample as a mask and another
FX that would use
this mask to downmix the two inputs (it's very simple to do)

The chroma key might not be able to do what you want, the background is not
static and probably also not evenly colored
so it will be hard to select the keying color.

A possibility might the chroma or magic overlay, where you can choose
SELECT MIN/MAX operations as blending modes,
if that raging bull is moving on a black background...

See you,
Niels

Hi Niels,

Ok.

I'm not sure I understand :).

Ok, so a bull moving on a black background can be blended into a
background video. That sound doable, but I have to make sure there's no
pure black on the bull :).

I'll give it a try and tell you.

Thanks,
Charles

Hi,

Luma keying works quite nice for this type of blending, and it's a low
resource effect, AFAIk it used to in the "old" days when resources
were low :0)

The term "alpha channel" refers to separate transparency information
within the image (stream), and veejay has no support for these types
of streams ( am I still right, niels? ). Most video formats do no
support an alpha channel, except for some proprietary codecs ( e.g.
"Animation codec" in ffmpeg )

Hi,

thanks for your help.

However even thought it might seem obvious how to do, can you tell me
how to do that, and more particularly what effects to use ?

I know near to nothing about video mixing.

What is the SELECT MIN/MAX Niels is talking about ?

Many thanks,

--
Charlot

Hi,

Luma key is in the 'video effects' list ( opposed to image effects? ).
( as is chroma key )

Top of my head:

slider 0: effect opacity ( use this to fade-in/fade-out the key )
slider 1: threshold lower boundary
slider 2: threshold upper boundary
slider 3: feather ( edge blur ) setting
slider 4: feather on/off

I may have switched some of the sliders - it's ancient muscle memory
as I don't have veejay installed here ( work machine )
You select a lower bound ( e.g. zero is black, 255 is pure white ) and
an upper bound ( e.g. 25 is dark grey ).

Now the section of the image within those bounds becomes transparent,
and you can select a source for that slot.
You can play with the feather settings to make the blending look a
little smoother. This works very robust for images that have a dark
background and a light foreground.
( look for things called "luma dancers" in popular vj stock sets :0) )

Luma key works on the "luma channnel", it's inherent to veejay's
internal colorspace, YUV, which keeps a separate channel just for
"lightness" information.
Therefore, it's extremely efficient.

Chroma key ( or green screen ) uses the same logic, but by selecting a
color (region). It's slightly less efficient since colors need to be
calculated, but I doubt that would be a concern on modern hardware,
depending on the video quality you are using.

Hi Matthijs,

thanks for the quick info, I think luma key will do. I'll try as soon as
I have videos at hand and tell you.