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MSP 13: Rendered chroma-keyed clips don't keep transparency

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MSP 13: Rendered chroma-keyed clips don't keep transparency

Postby lingyai » Sun Dec 27, 2015 10:39 pm

[UPDATE: sort of solved ... please see my next post]

Hi all,

Sorry, I've tried to solve this first myself, by researching online, without success, unfortunately.

I am trying to do my first chroma-keying in MSP 13. I downloaded a stock green-screen clip, applied the BCC chroma key effect, and it worked fine -- the green became transparent, and it passed a test to show the space was transparent and not merely black – i.e. I could see another clip, which I'd added to a test video track below it, peeking through in the way it should.

After removing the second (i.e. test) track, I rendered the chroma-keyed clip (only). I chose to render it as AVCHD / Video for Windows / PAL DV. It rendered.

I then dragged the rendered *.avi file into a new project's Project Media bin, and there, I right clicked on the clip and under Properties / Media / Alpha Channel, changed it from "None" to "Straight (umatted)" as I'd read online.

No go -- the keyed-out empty spaces appear as opaque blank, not as transparent. I know this because I cannot see any part of test clip below it.

I hope I'm overlooking something very basic. If I can't make this work, this pretty much stops me in my tracks. Can anyone advise?
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Re: MSP 13: Rendered chroma-keyed clips don't keep transpare

Postby lingyai » Sun Dec 27, 2015 11:54 pm

Update: good news and ... probably not bad news, but new related questions....

The good news is that I searched further, found a video which showed me one way to render and keep the alpha channel so that the chroma-key renders transparently as it should, yay! Here is the video:

https://www.youtube.com/watch?v=NaMqpipqeQQ

As for the related question, or maybe questions ... using all the render parameters shown in this video resulted in a suprisingly big file. The 7-8 second clip came out as 364 MB. It took my Win7, 64 bit Pro PC with an i7-3740QM , 2.7 Ghz, 16 GB of RAM, and Intel HD Graphics 4000, around 5 and half minutes to render. The FX on the clip included the BCC chroma key and I believe the NewBlue Dream Glow.

Now, I'm just returning to video after about 6 years away, and so am not sure what benchmarks to expect here. Should this have taken so long to render? If so, good thing I got an adult coloring book for Xmas so I'll have something to do on the side ;-)

Nor am I sure whether all the settings shown in the video had to be followed to a T, or rather whether some just reflected personal preferences. 364 MB for 7.5 seconds seems ... a bit much, no? I noticed for example that he chooses the 3 mbps option (the highest) and Best quality. This leads to the question of what quality level is needed for what I am doing.

So guess I better tell you what I'm doing...

I am creating a buffet of chroma-keyed clips -- I expect maybe 20 for this project -- from green screen clips, some shot by me, and high quality (the format anyway, Lumix HD set to max, *mts files) and others medium quality royalty-free *mp4s from the web. I render each individually, preserving transparency.

I then want to start pairing these with backdrop clips -- again, medium quality *mp4s. Each pairing will result in a new buffet, i.e. one of composited mini-scenes. Again, created and rendered one by one.

Finally, I'd like to assemble these composited mini-scenes, adding transitions, FX etc in a project file dedicated to this purpose.

Now, since I will ultimately be rendering some clips effectively for a few generations, I want to use a really high-quality format, yes? But need I use as high (and heavy) settings as those shown in the video?

I'm sorry this question has segued so far from the original topic ... mods, let me know if you think this should be broken up into a separate posts.
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Re: MSP 13: Rendered chroma-keyed clips don't keep transpare

Postby Steve Grisetti » Mon Dec 28, 2015 8:46 am

You sort of use the terms chromakey and alpha channel video interchangeably, lingyai, and that may be leading to a little confusion.

Chroma key is a process that you use on a clip or even to make it transparent within a project. You do this by designating a color in the video (usually blue or green) as a "key" color and the program will make everthing that's that color transparent. This effect is unique to this particular event and doesn't affect the original clip or any other usage of the clip.

Creating a clip with alpha means that you're outputting a video that includes the transparency as part of its color information, and it's a bit more complicated a process -- which is why the process takes much longer. It's also a file that includes little or no compression, which is why the file is going to be about five times the size of the regular video file. The advantage of doing this is that this video will be transparent every time you use it in a project.

So, just to be clear: Are you trying to do a series of chroma key effects with green screen or blue screen video clips or are you trying to create a library of video clips that are semi-transparent?
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Re: MSP 13: Rendered chroma-keyed clips don't keep transpare

Postby lingyai » Mon Dec 28, 2015 2:08 pm

Steve,thanks for your post. Your exploding bulbs scared the crap out of me and my cat, btw :hyst:

Yes, I am, among other things, building up a gallery of semi-transparent clips, so that I can later drop them into various compositing projects. So these clips will get rendered in some form potentially a few more times. Hence the need to keep them pristine enough for the task of making videos whch not only look good on youtube at 1080p, but also good enough to submit things to competition / be shown on large TVs.

So I guess that's a rather high bar, which will mean I will have to accept that some files will be large and time-consuming to render. The question is, how large / long?

We are on the same page re the difference between chroma-keying and alpha-channel preservation, sorry if I didn't make that clear. At least I think we are ... you tell me please! My understanding is that for my purposes, chroma-keying is using a "live" FX (BCC's in this case) to make a clip look-semi-transparent in a particular session of MSP; alpha-key preservation lets me publish what I see in the preview window as a new clip which preserves the semi-transparency, so that I can then use it (for example, for compositing) in that session or any other session of MSP.

My question I guess now can be boiled down to this: for my goal of creating this gallery of semi-transparent clips as ingredients for future compositing projects, what is the optimal format for me to render clips?

Do I need to literally follow every step he takes in the video? He chooses a very specific combination of parameters: PNG, a specific frame rate, a custom frame dimension, the largest (3 mbps) Quicktime setting, NTSC, Best Quality, etc.

I followed the steps and sure enough, got something which meets my quality standards: i.e. it retains transparency and, after a few tests, seems to be quite pristine; I can use it in other projects, apply FX, render that, with no degradation I can see.

But did I have to follow each of those steps? If this is what it takes, ok, but I'd like to be sure before I resign myself to having to create such truly massive files (in terms of MB per second); maybe just somewhat massive, or sort of massive, would work as well?

This is where my ignorance of codec and other rendering details comes in, sorry. Did I need to choose 3 mbps, for example -- would 1 mbps do? Do I need to use his specified frame dimensions? Some years ago I recall facing this question and being steered to something which I think was called the lagarith lossless codec, but that was around 2009 on 32 bit XP, no idea if that's even relevant to the task today on MSP13, Win 7 64.

I could try to discover myself, by naivley and systematically test many combinations of codec settings, but, man, there are so many, that's reinventing the wheel. Instead of wandering in the dark, I'm asking any saintly gurus for a flashlight, a quick steer or at least some narrowed-down list of things to try.

Would be very grateful for any help. [-o<
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Re: MSP 13: Rendered chroma-keyed clips don't keep transpare

Postby Steve Grisetti » Mon Dec 28, 2015 3:23 pm

These are excellent questions, lingyai! And I wish I had the background to answer them.

I know that you can output an uncompressed AVI as well as an uncompressed MOV file with alpha -- but that's about all I know about.

Hopefully someone on this forum who's had more experience with creating transparent clips can chime in with an answer.
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Re: MSP 13: Rendered chroma-keyed clips don't keep transpare

Postby Bob » Mon Dec 28, 2015 3:33 pm

The clips with alpha channel will be larger. Even if there were no other changes, the alpha channel adds an extra byte to each pixel in each frame. A 1080p size clip will increase just under 2 MB per frame. The formats that support transparency aren't compressed as highly either. It's no surprise the files will be massive. If you want to do this, you'll need to live with the large files.

Only a few file formats will support transparency. Generally the Quicktime format is used along with the Animation or PNG codec. Try the animation codec and see if it produces smaller files than png. As far as settings. Make the frame rate and frame sizes match your footage. The compressed depth setting is critical. You must set it to 32 bpp. 24 bpp does not provide for the extra byte containing the alpha channel.
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Re: MSP 13: Rendered chroma-keyed clips don't keep transpare

Postby George Dean » Mon Dec 28, 2015 4:13 pm

Hi lingyai,

Using your workflow I did some short test. I used a h.264 green screen video I had shot last year as the sample. Cut it to 7 seconds. Applied the BCC chroma key (no Dream Glow effect). It took about 4 minutes to render and was 271 meg. I rendered another test making only one change to the Video Format (PNG codec). Selecting the Configure... button, under PNG Options there are seven options to choose from. In my system the option defaulted to "Best", which I used on the previously mentioned render test. I changed this option from "Best" to "None". The render time decreased to about 3 minutes, and the file size increased to 427 megs. With a quick look, there appeared to be no difference in the results using it against the same background, but I didn't make a final render and examine it close to determine what difference the change of filter made. I assume there would be at least a minor difference.

So, if you are looking to increase speed at the cost of storage space, changing the PNG Option to "None" may be a suitable alternative. If not, then you may want to leave that change to last during your experimentations.

In my limited experience with chroma keying, I think there is a lot of science, but also a bunch of art. From what I have tested and used, the better the original sub-sampling, the better the keying outcome.

Perhaps someone will provide you with the golden nugget you seek and save you from the hours of trying things out. I would be interested in a cookie cutter as well. Over the past 2 years using Movie Studio, I have only invested about 50 hours on chroma keying, and I have achieved the results I thought adequate, but I'm always open to learning an easier way. None of this will probably help you much, but I do feel your pain.

Best Regards.....George
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Re: MSP 13: Rendered chroma-keyed clips don't keep transpare

Postby Bob » Mon Dec 28, 2015 6:50 pm

PNG options affect compression. The options don't specify compression directly, however. Instead, they are a precompression filter that is used to optimize the pixel values in a scan line prior to processing the line in the compression engine. Setting it to "None" disables preprocessing and you are compressing unoptimized data. While there are situations where that is beneficial, they are more commonly associated with still photographs using low bit depth or indexed grayscale. For normal bit depth full color as would be used in most video, "none" is a poor setting. You're almost always better off with the default setting "Best".
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Re: MSP 13: Rendered chroma-keyed clips don't keep transpare

Postby lingyai » Mon Dec 28, 2015 10:29 pm

Gosh, want a helpful discussion, thanks very much all, especially you, Bob.

Ok, I'm happy to do what I've tried when rendering semi-transparent clips. Good thing 5 TB external drives are becoming affordable ;-)

If I may, then, ask one last question -- in light of what I want to do, i.e. keep things pristine, so that in the end I can render something which will look good anywhere -- on a big TV, on a phone ... can anyone suggest optimal render settings for keeping intermediate-stage files pristine when I do *not* care about alpha channels? Which I suspect will be much of the time.

Again, my ignorance of modern codecs and sub-codecs / settings shows itself. There are so many when you consider all the "layers" of settings, for example, ...

-- for the first "layer" ( following what you see under the Make Movie dialogue) I am choosing AVCHD ;

-- the next "layer" is the choice of Formats, followed
-- the last "layer" which is the choice of Templates.

I really have no clue which Formats / Templates to use (except in the case of rendering alpha channels to preserve semi-transprancy, as discussed) at this stage. Again, to be clear, by "at this stage" I am asking about what settings to use to when I am still preparing the film -- say, when rendering small scenes as clips, and then using the rendered clips later in another project i.e. a final arrangement. I am not asking about settings to use for rendering the final product, as that of course will depend on the device targeted.

Again, sorry to be so newb-ish, but seems these questions need to get sorted first before I start investing hours which I might have to repeat because of some bone-headed settings I'd been using :(
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Re: MSP 13: Rendered chroma-keyed clips don't keep transpare

Postby John 'twosheds' McDonald » Tue Dec 29, 2015 12:39 am

lingyai wrote:...sorry to be so newb-ish...

We were all newbies once. No need to apologise when using this site.
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