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Old 3rd July 2011, 16:46   #1  |  Link
hello_hello
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AutoGK Color Correction

A few questions regarding AutoGKs color correction.....

What's the difference between the two options "fast" and "accurate"? Not in the obvious sense, obviously, but the way in which they actually work.

All other encoder GUIs I've used which color correct DVDs seem to do so by always loading the ColorMatrix.dll and using it to check the need for, and to correct the colors. I assumed that's the only way to do it but I've not been able to find any reference to ColorMatrix.dll having fast or accurate modes of operation.

Lately when converting DVDs I've been checking the scripts created by AutoGK to see if I could work out what it's doing for myself, but so far I've not seen anything which seems to indicate color correction is happening. The ColorMatrix.dll is never in the list of loaded plugins in AutoGK's scripts. Does AutoGK check for the need for color correction on it's own and then load the ColorMatrix.dll if required, or is AutoGK for some reason failing to load the colormatrix.dll as it should?

(I'm asking about the possibility of it failing to load the ColorMatrix.dll as a long time ago I had a problem with AutoGK failing to load the RemoveGrainSSE3.dll which I discovered required the presence of msvcr71.dll in the System32 folder. I'm wondering if for some reason there's a similar thing happening with ColorMatrix.dll or if AutoGK just does things differently to other encoder GUIs when it comes to color correction)

Thanks.

Last edited by hello_hello; 3rd July 2011 at 19:42.
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Old 3rd July 2011, 22:48   #2  |  Link
manono
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If you're doing DVD->AVI or Standard Def AVI->AVI encodes, assuming the AVIs you're making are also standard def, you don't need the colormatrix.dll anyway as the colorimetry is the same both before and after. So, I'm not sure the point of the questions. Are you using some Hi-Def sources?
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Old 4th July 2011, 03:10   #3  |  Link
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I'm basically referring to DVD to AVI conversion. I thought the point of color correction was to ensure R.709 DVDs are converted to R.601 while encoding, or when encoding any mpeg2 video. If that's not what the color correction option is for, what does it do?
I haven't checked the scripts AutoGK creates when encoding non-mpeg2 sources as I assumed the color correction option only applies to mpeg2 video.

From AutoGK's help file:
"- "Color correction" option allows you to slightly change color gamut closer to what the source actually contains (gamut often is changed automatically when doing MPEG2 -> MPEG4 conversion because of the tool involved, i.e. avisynth). You may notice that sometimes your XviD/DivX encodes are looking rather dark comparing to the original input source. In those cases usage of this option will help making it a little bit lighter. Not every MPEG2 input source require this correction and this option doesn't force it but rather switches ON the check if color correction is needed and applied. If color correction is needed then the encoding process can be 10-20% slower than one without it."

I'll admit I don't understand the "gamut often is changed automatically by avisynth" bit and how that works. I didn't think avisyth on it's own would effect colorimetry.
Pretty much every encode I've checked displays using the same colorimetry as the original DVD on my PC, so I assumed most DVDs are R.601 anyway, and maybe AutoGK checks the need for color correction itself and doesn't add anything to the script unless required. Unless AutoGK's color correction is something else entirely?

A program such as MeGUI for example, enables it's color correction option be default when encoding mpeg2 video which loads the colormatrix.dll and adds the necessary stuff to the script, then color conversion takes place if the colorimetry info in the stream is R.709. I assumed AutoGK would work the same way so I'm curious as to how it does it, and why AutoGK is the only program I've seen with fast and accurate color correction options.

Thanks.
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Old 4th July 2011, 05:05   #4  |  Link
manono
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A lot of that stuff for both MeGUI and for AutoGK is left over from when many thought (mistakenly) that DVDs used the Rec.709 matrix. As far as I know there's no such thing as a Rec.709 DVD. therefore the Colormatrix.dll isn't necessary unless, perhaps, you're upscaling a DVD to a Hi-Def AVI. The doc included with Colormatrix is also old and incorrect in places.
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Old 4th July 2011, 08:28   #5  |  Link
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Fair enough. I kind of figured 99.999% of DVDs use R.601, but I was kind of curious as to how AutoGK handles color correction when need be. And what the difference between fast and accurate color correction might be. Just a "curiosity" thing.

One other question, off on a slight tangent......
I've read various old threads where poster have offered screen shots to show how their encodes seem to look darker or washed out compared to the original DVD. An example might be this one: http://forum.doom9.org/showthread.ph...olormatrix.dll
What I've been wondering for a while is how people are able to notice the difference when comparing the original DVD to the encode on a PC. My logic is this:
If the Windows renderers always display DVDs using R.601 (even if they're R.709) and the encode displays using R.601, then shouldn't they display using identical colors, even if they're both wrong? In order for the original DVD to display using the correct colors (so you can see the encode is wrong) wouldn't you have to be playing the DVD using software which respects the colorimetry info in the original video stream?

Even if AVISynth's help files where it discusses the colormatrix.dll it says this:

"ColorMatrix corrects the colors of MPEG-2 streams of dvds. More correctly, many MPEG-2 streams use slightly different coefficients (called Rec.709) for storing the color information than AviSynth's color conversion routines or the XviD/DivX decoders (called Rec.601) do, with the result that DivX/XviD clips or MPEG-2 clips encoded by TMPGEnc/QuEnc are displayed with slighty off colors (which looks like a small difference in brightness). This can be checked by opening the MPEG-2 stream directly in VDubMod."

My question would be, is the above correct and if so how does that work? How is VirtualDub able to display a R.709 DVD using R.709, which I assume it must? I know it's all fairly irrelevant now, but I'm curious.

Thanks.
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Old 4th July 2011, 08:46   #6  |  Link
manono
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Again, much of the information about the Colormatrix.dll is old, obsolete, and wrong.

If you really want to try and wrap your head around this, read these two posts and then follow the link contained in the second one:

http://forum.doom9.org/showthread.php?p=1089979
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Old 5th July 2011, 12:40   #7  |  Link
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Cheers. I pretty much understand the need for color correction and what colormatirix does etc, it's just trying to work out how AutoGK does it which keeps me awake at night. It's just one of those "want to know" things.
When I do sleep I dream about AutoGKs fast and accurate color correction methods, but each time I wake up I seem to have forgotten what the difference is.
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Old 5th July 2011, 13:03   #8  |  Link
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Like you, I don't know the difference. And the tutorial isn't any help at all. Unlike you I don't dream about it.
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