Thank you Monty for your clear and informative explanations !
De: "Monty Montgomery" <[email protected]> :
>> Furthermore, am I right to think that colorspace conversion can be
>> avoided in cinelerra by choosing "YUV-8 Bit" or "YUVA-8 Bit" as
>> Color model in the project parameters, when working with 8-bits
>> YCbCr footage ?
>
> No, unfortunately. You could minimize the loss by working in floating
> point intermediaries, but Cinelerra doesn't support them AFAIR. The
> chroma resampling aspects complicate it further (floating point won't
> save you there).
You're right - as usual.
I did a little test to know if I could avoid colorspace conversions, and if
not, how much loss it would induce.
My typical workflow for SD video editing is :
1. Load footage (mpeg2 from my camcorder, sometimes DV from another one) into
cinelerra using "YUVA 8-Bit" as color model ; edit video ; render it with
yuv4mpegpipe to uncompressed "YUV" 4:2:0 stream.
2. Load this uncompressed stream into avisynth (running under wine) with
RawSource() ; apply some nice postprocessing filters.
3. Pipe frames served by avisynth to an encoder (usually x264) with avs2yuv.exe.
I reproduced steps 1 and 2 of this workflow in my test :
1. Cinelerra load & render
1.1. Load mpeg2 footage into cinelerra ("YUVA 8-Bit" color model)
1.2. Render -> File format : YUV4MPEG Stream, Use Pipe : 'ffmpeg -i - -an
-vcodec rawvideo %'
2. Comparison with avisynth
2.1. Load original mpeg2 footage (with DGDecode plugin) as "src"
2.2. Load uncompressed "YUV" file rendered by cinelerra (with RawSource
plugin) as "rdr"
2.3. Create clip "a" containing the difference between src and rdr, using
Subtract()
2.4. Create clip "b" by increasing contrast of clip a difference to make it
blatantly obvious, using Levels()
2.5. Display a and b side-by-side + timecode information
(hours:minutes:seconds:frame)
Here's my avisynth script :
"""
src = DGDecode_MPEG2Source("D:\M2U00070.d2v")
rdr = RawSource("D:\render1.yuv",pixel_type="I420")
a = Subtract( src , rdr ).Subtitle("visible differences")
b = a.Levels(127, 1, 129, 0, 255).Subtitle("amplified differences")
StackHorizontal( a , b )
ShowSMPTE()
"""
Here's a still image (png) showing the output of this script :
http://img241.imageshack.us/img241/2590/000310.png
Only grey pixels can be considered identical between the two clips. The left
part of the screenshot is totally grey, with only very slight noise, meaning
differences are almost not noticeable. But when these differences are
(over)amplified, they become clearly visible on the right part of the picture.
Conclusion : there are very slight differences between original footage and
uncompressed "YUV" file rendered by cinelerra. Since these differences are
almost unnoticeable to the human eye, I think I won't change my workflow for
that :-)
-----
De: "Monty Montgomery" <[email protected]> :
> In the last vid I made, I had Cinelerra accidentally set to use RGB
> internally for editing. No compression involved. My source was HDV
> (a YUVish colorspace). I was rendering out to YUV4MPEG (which allows
> the same colorspace as HDV, so it should be zero conversion). The
> difference in output quality between the 'Use RGB for rendering',
> which caused Cinelerra to convert to 8-bit RGB and back, and 'Use YUV
> for rendering', which involved no conversion, was stark. I'd be happy
> to post an example if people care, though I'd have to load the raw
> video back off the USB drive and rerender.
I also noticed a visible color shift when I used RGB for editing and YUV4MPEG
for rendering.
-----
De "Burkhard Plaum" <[email protected]> :
> yuv420p has video range for luma and chroma. yuvj420p has full range
> (0..255). So the conversion simply maps the video range values to full
> range (e.g. with a loopup table).
It seems to me that yuv420p has full range (0-255) for luma and chroma. It can
be demonstrated with an histogram.
>> Furthermore, am I right to think that colorspace conversion can be
>> avoided in cinelerra by choosing "YUV-8 Bit" or "YUVA-8 Bit" as Color
>> model in the project parameters, when working with 8-bits YCbCr
>> footage ?
>
> You save the conversion to RGB if you use these.
> But these colormodels are 4:4:4, so you'll upsample the chroma planes,
> which doubles the memory footprint of the video frames compared to 4:2:0.
You're right, but choosing another "Color model" (eg. "RGB 8-Bit" or "RGBA
8-Bit") would also at least double the memory footprint, so there's no way to
avoid it.
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