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Tut_HoldoutMattes
DISCLAIMER:
While this tutorial does describe how to do a traditional burn-in for a TV screen like in old TV-shows and movies. It should be known by the user that as of this writing, the VOP is quite limited in how it handles the gamma and bit-depth on the projection side. The gamma on the projection side is assumed to be linear. But without proper mapping of the brightness response of the projection screen, it's impossible to get a technically correct image that accurately transfers the image to the camera. Likewise, since the projection screen likely is limited to 24 bit or 8 bpc, we are limited to that when it comes to tonal accuracy as well.
As has been reiterated in several other sections, The VOP is a strictly additive compositing system. This means that you cannot do an exposure and expect the pixels to grow darker. All light added only makes it darker.
But what if you wanted to do a composite with a holdout matte? Why, I am glad you asked!
In order to make this work, you need a video file that you can upload to the VOP, that video needs to have a need for a second exposure where the second image needs to have darker pixels than the image it is to be combined with. In our example here we'll use the task that used to be common in the days before video compositing: A TV set that we'll add the contents for in post.
Either use a still image of a TV set and generate a video from it that's long enough in time to cover the footage we're adding to it. Or go out and do a locked down video shot of the TV set you want to use. For authenticity to how this used to look, we do not want the camera or TV to move. Back in the days, it used to be a still image.
What you also will need is the holdout matte itself. This is a simple binary black and white image where the white sections show where image goes and black where it doesn't go. Essentially, it's an alpha channel as a separate image. We want it to be the same resolution and aspect ratio as the image in the Cam Mag.
And lastly, you will need the footage that you want to add to the TV screen.
So to recap:
- Locked down video of TV.
- The Holdout Matte.
- The footage to be shown on the TV.
As this tutorial doesn't need any smears, it doesn't really matter if you are in SSS or MDS mode. You can do fancy things here but let's stick to the simplest form for this one, ok?
So if it's not set to SSS, you can go ahead and choose it now. I'll be assuming you are in this mode throughout the tutorial.
And while you are at it. You can in the PROJECTION BIPACK 2 section find the visibility eyeball and disable it, we won't be needing it for this tutorial. And make sure that both PROJECTION MAG and PROJECTION BIPACK 1 are visibility enabled with their own eyeballs.
Firstly, we'll load the camera mag with the TV screen. As mentioned before. We need this footage to be at least as long as the footage we intend to add to it.
Go to the CAM MAG section and hit the UPLOAD button
Since the incoming image is a video, the VOP will recognize this and start to process it to a TIFF sequence. This can take a while depending on the storage speed of the host device. When it's done. You should see the footage counter of the CAM MAG go from showing "---- / ----" to showing the actual frame count of the loaded footage.
And since this is more than one frame. We will need to expand the exposure sheet to fit the sequence.
In the EXPOSURE SHEET section. Click the + ADD SSS KEYFRAME (or + ADD MDS KEYFRAME if you are in MDS smear mode). You should now have two keyframes. Go ahead and put in the end frame number in the second keyframes FR column.
In order for the next step to work as intended we need to prepare the video that's in the CAM MAG to treat incoming bright pixels as darkness. We do that by sending the contents of the CAM MAG to a fictional LAB. And this is done with the button labeled LAB / INVERT. Please do press this button and accept the prompt that comes up.
This tells the VOP to open each of the TIFF files that are in the CAM MAG folder, invert its color and overwrite the file it came from before moving on to the next frame. When it's all done, you can use the CAM PROBE button to see the result. What you should see is the selected frame's inverted colors. Do not worry, this is just to make bright pixels go dark later.
This is a 2 step process.
First. You'll go into the section called PROJECTION BIPACK 1 and use the UPLOAD button to upload your holdout matte image there. As this can be a still image that will be reused throughout the shot, it can be a single still image.
If you want to be fancy and use a matte that follows a moving shot, you can load it here as a video file. The VOP will convert it to TIFF frames as well and unless mucked around with in the Exposure Sheet, it will just play 1:1 with the camera footage. But for this tutorial. We'll stick to a single still holdout matte.
And assuming you do have the holdout matte in the same resolution and aspect ratio as the footage loaded in the CAM MAG you should be able to hit the button labeled FIT FOV in order to reset its scaling in the 3D world so that when at position 0,0,-1 and rotation 0.0.0 in the exposure sheet, the holdout matte should be in the correct spot to cover the area we want.
Use the COMP VIEW to make temporary exposures that mixes the new exposure with the image in the CAM MAG. What you want to see is the inverted image with the TV screen's glass completely blown out to white.
You can if you need, adjust position and rotation and do repeated COMP VIEW previews to make sure it's lined up correctly. This button is temporary and doesn't write any new image data to the CAM MAG. When happy. Make sure both keyframes have the correct POS and ROT input values for BP1
And adjust the EXP (this takes float numbers for seconds of exposure time) columns so that theholdout matte is completely white in the composite when using COMP VIEW
Once you are certain things are lined up properly, you can go ahead and hit the dreaded EXECUTE SEQUENCE. This will burn in the white of the holdout matte onto the inverted.
Since this tutorial assumes it's a still image used for the video footage loaded to the CAM MAG and the holdout matte is also a lone still image, checking frames in the exposure should yield just that. The same image over and over again in throughout the sequence.
And when this is done. you can go ahead and hit the LAB / INVERT button a second time. This will make the VOP go into the CAM MAG folder again and invert the colors of all the images in it. The result should now be that the footage of the TV set has normal colors everywhere except the area inside the holdout matte where the image is blacked out completely.
It's a lot of steps to essentially just overlay black on an image, but welcome to the wonderful world of optical compositing!
Now that we have the background image prepared, we can finally load in the image that we want to show on this screen.
In the section called PROJECTION MAG, upload the footage you want to show on the screen.
Note:
Technically, all three projection side mags (Projection Mag, Bipack 1 and Bipack 2) do the same thing. It doesn't really matter if you use Bipack 2 for the new footage. I just thought it would be more understandable to treat bipacks more as as holder of mattes in this tutorial.
Since the footage we add there is likely a video, we'll just wait until it's finished processed as a TIFF sequence.
Once uploaded, we can hit FIT FOV for the PROJECTION MAG and use the positioning and rotation inputs in the Exposure Sheet to position the footage correctly and like before. We can use the COMP VIEW button to temporarily preview what the composite will look like.
Since we didn't unload the Holdout Matte from step 4. It should still be there helping us out here in this step. As the VOP treats the three layers on the Projection side as film strips. Meaning that the images are multiplied together stopping more and more of the projection light before being shown to the camera, the holdout matte now automatically becomes our mask that blacks out all that's outside the matted area. If you do a CAM PROBE you should therefore see black outside the holdout matte. And if you do a COMP VIEW you will see the image burned into the black area of the TV screen.
If you do need to move the image around. Make sure to adjust the POS and ROT columns for both keyframes for the PM columns as well. And adjust the EXP column again to make sure the exposure time in seconds is correct for the image you want to achieve.
Digression:
Here's a thing about the old ways of doing a shot like this. Traditionally, it was kind of impossible to actually warp the image for correct perspective if the camera isn't pointed head on to the TV screen. This meant that if the camera is not pointed straight on to the TV set, there'd be a strange mismatch in perspectives as the TV could be slightly angled but the TV presenter still was facing the film camera. This is technically an error. But the only way to fix it would have been to reproject the image with the presenter onto a screen that's in the correct perspective and then refilm that with a film camera to produce a rendering of the perspective that can be used in the multiple exposure in an optical printer like we are doing here. Most films and TV-shows did not have the facilities or budgets to do this. Which is why, in old films, when we are shown a TV screen we often enough simply see a still image with the TV image pasted onto it with no over the shoulders in the foreground.
End of digression.
Once you are happy with the alignment, wether it's realistically applied or not. I won't judge. You can go ahead and hit the EXECUTE SEQUENCE again for a second time and see how the new footage is correctly matted into the latent images in the CAM MAG while preserving the darkness of the projected image.
Once it's all rephotographed, you should be presented with a link to view the latest workprint. If you watch it and indeed feel it ran correctly, you can use the RENDER PRORES button to get a high quality ProRes 4444 render of the current latent image sequence in the CAM MAG to use in your NLE of choice.
If you are not happy with the result. Go back and do it all over again.... The VOP doesn't have undo's. Once something is committed to the CAM MAG, it's destructive. This is indeed a feature. Not a bug. :)
Repo run by: https://jmalmsten.com