Test for alpha channel?

[terrypin]

Is there a definitive test for detecting whether any video has an alpha channel please?

Can MediaInfo or GSpot show it?

--
Terry, East Grinstead, UK

[racer-x]

Not sure how definitive, but mediainfo detects the RGBA color space in this Lagarith clip.

General
Complete name : D:\ScreenCap\RX_RGBA.avi
Format : AVI
Format/Info : Audio Video Interleave
File size : 84.2 MiB
Duration : 19s 958ms
Overall bit rate : 35.4 Mbps
Writing library : This file was made by Adv.Editing AVI/BMP Export (RGBA).

Video
ID : 0
Format : Lagarith
Codec ID : LAGS
Duration : 19s 958ms
Bit rate : 35.4 Mbps
Width : 1 280 pixels
Height : 720 pixels
Display aspect ratio : 16:9
Frame rate : 24.000 fps
Color space : RGBA
Bits/(Pixel*Frame) : 1.600
Stream size : 84.2 MiB (100%)

[terrypin]

Thanks. But this MOV doesn't have a ColorSpace line in MediaInfo like yours:

General
Complete name: C:\Docs\My Videos\Misc Video\AlphaChannel\PSY Alpha Channel Template 2 - Dancing (No Feathering).mov
Format: MPEG-4
Format profile: QuickTime
Codec ID: qt
File size: 4.49 MiB
Duration: 1s 333ms
Overall bit rate mode: Variable
Overall bit rate: 28.2 Mbps
Encoded date: UTC 2012-11-05 19:33:32
Tagged date: UTC 2012-11-05 19:33:32
Writing library: Apple QuickTime

Video
ID: 1
Format: PNG
Format/Info: Portable Network Graphic
Codec ID: png
Duration: 1s 333ms
Bit rate mode: Variable
Bit rate: 28.2 Mbps
Width: 375 pixels
Height: 760 pixels
Display aspect ratio: 0.493
Frame rate mode: Constant
Frame rate: 23.976 fps
Compression mode: Lossless
Bits/(Pixel*Frame): 4.125
Stream size: 4.48 MiB (100%)
Language: English
Encoded date: UTC 2012-11-05 19:33:27
Tagged date: UTC 2012-11-05 19:33:32


Maybe the PNG here is the indicator?

[smrpix]

Open the file in Quicktime. The movie inspector will show the format as PNG, (dimensions),millions+ if it has alpha.

[terrypin]

Thanks smrpix. But presumably that's only for MOV files, and as I mentioned, MediaInfo seems to show that too. I was wondering if there was a universal test for all file types.

[poisondeathray]

The presence of an alpha channel indicated by various programs might just be a "dummy" alpha (it's there for memory alignment purposes, not valid data). Presumably you don't care about that

So the only definitive way to know for certain if it's a valid alpha channel is open it up and look at the alpha channel. You can do that in various compositing applications, some NLE's .

For some types of formats, there are different alpha interpretations e.g. prematted vs. straight alpha - so what you see might depend on how you have it set up

[smrpix]

Sorry Terry, I was responding to your second post specifically mentioning mov, didn't read the first one as closely, I guess.

FWIW, I just exported a Lagarith avi with an alpha channel from After Effects. Quicktime player couldn't open it -- missing component -- but it still correctly identified the color space as Millions+. Not as simple as Mediainfo or Gspot obviously, but it might help.

[terrypin]

Thanks both. Guess the best way then is to simply import it into my NLE, MAGIX Movie Edit Pro 2014 Premium and take a close look.