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Alpha Channel doubles the size & data rate?

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joysprod
Posts: 201
Joined: 20 Jan 2012, 12:24
Location: United Kingdom

Alpha Channel doubles the size & data rate?

Post by joysprod » 23 Aug 2016, 14:23

I was playing around with outputting from Ventuz with a Huffy codec. Now that Ventuz correctly supports the alpha channel I decided to do some tests.

My thinking must be totally wrong as I thought that with no alpha channel it would be 24 bits of data. RGB or YUV with 8 bits each. Adding an alpha channel would be another 8 bits so data rate and file size would be about 1/3rd bigger. Why when I render out is the alpha version twice the file size?

When I use the animation codec it is only about a 1/3 larger.

Regards

Peter

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Tammo
Posts: 5
Joined: 24 Aug 2016, 09:31
Location: Hamburg

Re: Alpha Channel doubles the size & data rate?

Post by Tammo » 24 Aug 2016, 10:02

This is one of the peculiarities of the HuffYuv codec - when there's no alpha channel it encodes the image as YUV with 4:2:2 sampling, but as soon as you add alpha it can only encode RGBA 4:4:4 which compresses way worse.

So this behaviour is unfortunate but expected.

Best regards.

Tammo

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