Download Project File

 

This tutorial is a brief primer on the 15 different transparency settings, what they do, and when they should be used.  Before you get into this there are three things you should do.

 

1) Read my first tutorial “The Basics of Keying.”  It gives an overview of how keys work.  I’ll be assuming you’ve read it already and won’t be re-explaining the concepts I wrote about in it.

 

2) Once you’ve done that download the project file and its associated clips for this tutorial.  Once you’ve done that unzip the file to your C drive.  I know it’s a bit messy, but if we give the project a common point of reference it should load up fine on everybody’s machine.  Open up the project file.  The source clips should all slot into place.  Then feel free to play along at home.

 

3) If you don’t have it already, you’ll need Quicktime to view the JPG’s I use in the project.  You can get it from http://www.quicktime.com/.

 

As always, with all my tutorials, if you read through and know a better way to do something I’ve shown here, or want to correct something I’ve got wrong, please let me know.  I’d much prefer to know I’m wrong and correct it than continue to steer people the wrong way.

 

One last thing before we get into this.  Most of the information for this tutorial is gathered from your very own Adobe Premiere User Guide (also known as the f#*@ing manual) which just goes to show that reading it can actually be beneficial.

 

I just hope Premiere 7 doesn’t come along and change everything I’ve just written about.

But then again, it may not be a bad thing.  As you’re about to read, there’s a few things that could do with some changing.

 

Part 1

Finding Your Way Around The Transparency Window

 

When you open up a clip’s transparency settings you are faced with a standard box.  The problem is that the check boxes and options tend to change depending on what key type you select which can make things a little confusing.  Open up the project file for this tutorial and take a look at example clip 1.  Open the transparency settings either by right clicking on the clip, then selecting “video options/transparency”, or by clicking on the clip, then the “transparency settings” button in the effects controls palette in the top right corner.  So, starting from the top left corner and working our way clockwise we have…

 

Matte

 

Only used for the Image Matte and Difference Matte transparencies, this box allows you to browse for a previously created matte shape.  A pity it only allows you to choose still frames, but I guess moving vision is what Track Matte transparencies are for (which you’d already know, having read my previous tutorial, right?).

Pretty straight forward, you hit the “choose” button and browse for the frame.

 

Color

 

Used for the Chroma and RGB Difference transparencies, this box allows you to select a color from your frame to be made transparent.  Hold the cursor over the thumbnail and it becomes a color picker/eyedropper.  Click to select a color.

 

Sample

 

This is where you see your key put together.  Once you’ve selected your key type and have it working, this box is where you check your levels and settings to get a good clean key.  To do this you have a couple of tools to help.

 

Garbage Matte Handles

At the four corners of the sample thumbnail are small white boxes.  These are handles to decide which part of the keyed clip you want to actively key, and which parts you’re happy to just completely get rid of.  There’ll be more on this in the sections on color based keying and split screens, but for now the negatives.

First up, there’s only four handles so the garbage matte must be a quadrilateral.  It doesn’t have to be completely square, but four sides means quadrilateral.  O.K., you could theoretically put one of the handles directly in line with two others and make a triangle, but why would you?

Second problem, this is not a “mask” in the After Effects idea of the word.  Yes you use it to mask off a piece of the clip, but you can’t animate the mask over time, nor can you feather it or independently give it its own transparency.  But then, if you wanted all that you’d do the work in After Effects.

 

Preview Slider

This slider directly below the sample thumbnail allows you to scroll through in time to make sure that the transparency is OK for the length of the clip.

 

Transparency background icons

Clicking on one of these three buttons allows you to change the image displayed under your keyed clip in the Sample window.  They include –

 

  Black or White Background: This puts either a white or black color matte behind your key.  This is especially helpful with chroma keys to check fringes around the edges of the key.  Clicking this button more than once toggles between black and white.

 

 Checkerboard Background: To quote the manual,

“Displays a checkerboard pattern to help you view transparency in areas that may be difficult to see against a solid background or against the underlying image.”

Clicking this button more than once reverses the checkerboard.  Handy for close up work when you want to see if your key is starting to become transparent in areas you don’t want it to.

 

 Actual Background Button: This is the button to select to see the actual background behind the keyed clip.  Using this option may be slower to display when you drag the preview slider, according to the manual.

 

Beside the Transparency Background Buttons are the-

 

 Zoom: You can zoom up to 10x into the Sample thumbnail to check the edges of your key which is very, very handy.  To zoom out hold the Alt key (or Option key for both the Mac users out there) as you click.

 

To move your way around the Sample picture you need to use the

 

 Hand:  With this button selected you can drag the frame around if you’ve zoomed into it.  Quick shortcut – to go back to full frame in the Sample window double click the hand.

 

Below these buttons are three checkboxes that are only available in certain keys.  We’ll go through them now and get them out of the way.

 

Reverse Key

 

Just as the title suggests, this button reverses what’s transparent and what’s opaque.  In short it inverts the matte shape.  Handy for those moments when you’ve created a killer title in Photoshop only to find out you’ve created the Alpha Channel back to front (as I’ve done on example clip 13).  Also handy for Difference keying, but more on that later.

Only available for None, Alpha Channel, Image Matte, Difference Matte, and Track Matte transparencies.

 

Drop Shadow

 

Again, quite simple in theory.  This, in the words of the manual,

 

“adds a 50% grey, 50% opaque shadow to opaque areas, 4 pixels below and to the right of any contiguous opaque region.” 

 

The down side?  This option is available for Difference Matte and RGB Difference keys, but not the ones you’d most like to use it with like Alpha Channel.

What good’s a drop shadow for a Difference key anyway?

 

But I digress…

 

Mask Only

 

This feature is SOOOOOO handy.  As anyone who’s done Curt Wrigley’s excellent video tutorial on creating a virtual news background will know, this is very handy for setting your key.  It displays the black and white matte signal being created by the keyer, allowing you to see quickly and easily if you’re cutting through areas you want to be opaque.  Perfect for chroma keys, and for taking a look at alpha channels in graphics and video where it may not be readily apparent.

An added bonus is that it’s not just a display feature.  If you leave it checked the clip actually becomes a full frame matte based on your transparency.  Very handy for a little trick I’ll mention at the end.

Available for Chroma, RGB Difference, Alpha Channel, Black Alpha Matte, White Alpha Matte, Difference Matte, Blue Screen, Green Screen and Non-Red transparencies

 

Under these buttons is the

 

Smoothing

 

Sets a level of anti-aliasing to your transparency by blurring the edge.  None is handy for titles and keys that need to show sharp edges, and High is handy for blending edges of chroma keys and composites into the background.

Again, only available for Chroma, RGB Difference, Difference Matte, Blue Screen, Green Screen and Non-Red transparencies.

 

OK and Cancel buttons

 

If I need to explain these, give up now.

 

As we continue our trip around the Transparency window, we reach the

 

Settings Sliders. 

 

These vary depending on the type of key selected, so we’ll leave individual descriptions to the relevant key type sections.

 

Just above these sliders is the

 

Key Type

 

A drop down menu, this is where you select your key type from among the fifteen different options.

 

Which brings us neatly to our next section…

 

Part 2

Key Types

 

The Color-based Keys

 

I know that the chroma and blue screen keys are what most of you are really interested in so you can do cool special effects, so let’s get them out of the way first.  Color-based keys, or Chroma Keys, allow the user to designate a specific hue to be transparent, leaving everything else opaque.  Mainly used for compositing actors and objects shot in front of a blue or green screen, the most obvious of these is, of course…

 

Chroma

 

The Chroma transparency allows the user to designate a specific color from the clip using the color picker in the “Color” box mentioned earlier.  This color becomes the transparent area of the clip.  This key type should be used for difficult chroma keys or when you wish to preserve shadows cast onto the background.

In the project file open the transparency box for clip 1.  You’ll notice four sliders; similarity, blend, threshold and cutoff.

Similarity allows you to “widen” your selection of color beyond the initial hue chosen from the frame.  So, for example, instead of selecting a color with a Blue setting of 120, it selects all Blue between ranges of 115-125.  This helps in situations where your background is not an even color.  Even when it is, such as this example, you still need to tweak it slightly.

Blend softens the mask based on areas that are most like the hue selected.  If you set it too high the subject disappears altogether.  However, a bit of Blend is good for a chroma key.  It helps the subject look less like a cut out and more like it’s part of the original background.

Threshold controls the amount of shadow in the selected hue.  The higher the setting, the more shadows retained.

Cutoff darkens or lightens the shadows you let through using the threshold slider.  Make sure you don’t drag it past the Threshold slider though.  Doing so inverts grey and transparent pixels.

Example Clip 3 is a good example of this.  Open the transparency settings for this clip and play with the threshold and cutoff sliders.  Notice how it affects the shadows cast by the subjects.

Actually, this clip is handy for another reason.  It shows an example of a very difficult clip to chroma key due to light spillage onto the subject.  It’s nearly impossible to get a good key on the left subject without cutting into some areas or leaving others with a blue fringe.  This is, unfortunately, a failing of the keyer’s ability, but it points out the necessity of good lighting for chroma key subjects.

A way to work around this shortcoming is shown in clip 4.  In this example I’ve used three copies of the clip, separated with garbage mattes.  Each is then keyed to work with what it needs most.  Eg, the bottom half needs to be keyed a bit looser to avoid cutting into the dress, and the top half needs to be keyed harder and with more use of the Blend slider to key out the spill in her hair.  The final section is a small area around the bottom of the dress to avoid cutting into it.

It’s not perfect, but it’s better than just a single pass key.  And besides, it’s possible to do as many copies as you like, all with individual settings.  As a follow-along-at-home exercise, try creating another video track, copying the clip and doing something with that hair.  Don’t forget that you can choose different shades of blue for each clip to take into account the variations of shade in the area you’re trying to key. And don’t forget that you can’t animate the garbage matte to follow a moving subject.

That’s a job for After Effects.

 

There’s plenty of resources on the internet relating to lighting subjects for chroma keying, and there’s a list of resources on the net at the end of this tutorial, but the basics you need to know is to keep the background color as evenly lit as possible and the foreground and background as separated as possible.

Of course you should also use the Smoothing option mentioned above to help blend the edges into the background.

 

One thing to remember with the Chroma transparency is that it isn’t limited to pulling blue or green screen keys.  While these colors are chosen because they’re less likely to occur in natural skin tones, etc, I’ve thrown in a shot on a red background (clip 5) to show you can pull a nice key from just about any even colored source where the background color isn’t in the foreground subject.

 

Directly beneath the Chroma transparency is the

 

RGB Difference

 

This is a simpler version of the Chroma transparency.  It only has the Threshold slider, which works as above.  It’s good for simple keying where you just want to do a simple chroma key for draft purposes or when you don’t need to hold onto shadow detail.  However it does give you the option of adding a drop shadow!  Whoopee!!

You can also use Smoothing to help blend those edges.

Clip 6 is there for you to have a play with.

 

Next up in the color-based keys we jump halfway down the list to the

 

Blue Screen and Green Screen

 

No prizes for guessing what these do.  It will automatically pick the hue based on what it considers to be proper blue or proper green.  What you have to do is play with the levels to get it right.

Take a look at the settings for clip 7.  You’ll notice I have the Mask Only option selected so you can see what’s happening.

 

Imagine that you take the blue chroma channel and convert it to a greyscale image, so that absolute blue is white, and the absence of blue is black.  Then invert it.  What you wind up with is kind of what we have here.

The Threshold and Cutoff sliders can be likened to the black and white on a levels effect (Effects/Video/Adjust/Levels).  Drag the Threshold to the left to raise the overall black level of the matte.  The trick is to lower the black of the background without cutting into the foreground subject.

Once you’ve done that you can play with the cutoff slider.  This time the idea is to get the foreground as solid white as possible without raising the black level of the background.  You’ll find that the two sliders will wind up very close.  The trick is to try and balance between getting a good solid key with having no difference between the white and black.  The closer together they are the less difference in the matte between completely opaque and complete transparent.  Smoothing can help a bit with the edges, but it’s best to give a bit of leeway in the levels.

Together you should be able to get a nice key from a well lit background. 

 

Clips 7 and 8 are there for you to play with.

 

Finally in the color-based keys comes the

 

Non-Red

Another transparency for pulling keys from green or blue screens, this is very similar to the Blue Screen and Green Screen transparencies in the way its sliders work.

According to the manual, this key

 

Helps reduce fringing around the edges of non-transparent objects.  The Non-Red key is recommended for keying out green screens when you need to control blending or when you are not able to produce satisfactory results with the Blue Screen or Green Screen keys.

 

Its other great feature is in its use off the blend slider.  This apparently allows the user to blend the subject with the background, allowing it to look more realistic.

 

Now, while my tests were hardly conclusive, I found the Non-Red transparency to do very little of what it actually promised.  Rather than getting rid of green fringing, I found it more difficult to pull a good key using green screen clips.  Instead I found it worked quite well with blue screen clips.

Similarly, all the blend slider seemed to do was desaturate the foreground image rather than make it appear to be more a part of the background.  This, I suppose, helps act as a sort of spill suppression around the edges. 

Have a look at clips 9, 10 and 11 and see what you think.  In fact, clip 11 pulled a very good key around that troublesome hair in the clip we played with earlier.

There’s no reason why, instead of using the same type of color based key, that you couldn’t take our idea of multiple key passes and use a different type for each.  Go back to clip 4, where you tried adding another track to get rid of the blue fringing in the hair, and try again using Non-Red.

 

Alpha Channel Keys

 

I’ve already been through Alpha channels, how they work and what they do, in my previous tutorial, but there’s a few more things you should know with regards to Premiere.

 

Alpha Key

 

Uses the alpha channel on a 32 bit image to act as the matte.  The Alpha Key transparency is used for straight alpha channels, such as files created in (according to the manual) Photoshop, Illustrator and After Effects.

Great for titles, this is what the Premiere titler uses for its files.

 

Take a look at clip 12 and clip 13.  Clip 12 is the full frame graphic without transparency; white text with a rather ostentatious blue/green stroke on a black background.  Clip 13 is the same title with the Alpha Key applied.  Note that the alpha channel cuts out the stroke, leaving just the white text.  This is what I meant in my previous tutorial when I said that the “Picture information can exist beyond the edges of the alpha channel.”

 

White Alpha Matte and Black Alpha Matte

 

Almost the same, but should be used for clips with premultiplied alpha channels.  Some clips are created with premultiplied alpha channels to help keep file size down.  Take a look at clips 14, 15 and 16.  Clip 14 is the file with no transparency.  As you can see it’s a giant megaphone shot on a white background.  Clip 15 is the same file with the Alpha Key applied to it.  It looks alright, but there’s a white fringe where the picture information has started to fade to white before the edge of the alpha channel.

Clip 16 fixes that, by making Premiere look at the file’s premultiplied alpha channel.  It figures out how much transparency a particular pixel should have based on the amount of white it has to unmultiply out of the clip.

 

You obviously use the White Alpha matte for files premultiplied with white, and the Black Alpha Matte for files premultiplied with black.  For more info on Alpha Channels check out the Reference section at the end of this tutorial.

 

Please note that for all three Alpha Channel transparencies the opacity and transparency is based on the full 256 color greyscale.  You can’t adjust this in the Premiere.  The only way around it is to alter the alpha channel in an imaging program like Photoshop.

 

External Matte Keys

 

These transparencies use a separate image to act as the matte shape.

 

Image Matte

 

You select a separately created frame in the transparency dialog box, and Premiere uses its luminance values to create transparency. (See clip 17)

Simple right?

 

Or is it?

 

What if your matte isn’t greyscale?  Well, according to the manual, if you were to use, say a 100% red matte, the keyed clip would take on a blue-green, or cyan, color because the keyer is keying out everything in the red channel.  Everything red becomes transparent, leaving only the blue and green channels.

Well, that’s not what I found.  Take a look at clip 18.  I’ve keyed a 100% white matte over the water background using a tri-color matte, starting with red, then blue, then green.  Each of the colors is pure.

You’ll notice that the white, rather than having the keyed color channel removed, seems to have picked up the color, as if the opposite channels have been made transparent.

Clip 19 shows this with an RGB picture over the same background.  To get it the way it’s described in the manual, you need to click the “reverse” checkbox and make sure that your matte has white in the areas you want transparent instead of black. (See clip 20)

 

Track Matte

 

The king of fun-time mattes, this is also known as a travelling matte.  The reason being that it allows you to put your matte shape on the timeline and treat it just like any other clip; effects, motion, whatever.  I describe some ideas on what you can do with track mattes to make them lots of fun in my previous tutorial.  As such, I haven’t put in any example clips.

 

One extra point to make though, Track Mattes and Image Mattes are similar to alpha channels in that they use the full 256 color grayscale for creating transparency and opacity.  If you want to adjust it, you need to find an alternate method.

Of course, the Track Matte allows you to use effects filters, so use of the Levels effect or Brightness/Contrast effect isn’t out of the question.

 

Difference Matte

 

A bit of a mystery, the Difference Matte allows you to create a key based on the difference in picture information between the pixels of the clip, and a still frame selectable from the “Choose” dialog box.

 

Rocky has created a pretty in-depth tutorial on how to use this to key out a subject that’s not been shot against a blue or green screen, so go and have a look at it.

 

Surefire tutorials Difference Keying

 

Other than that, I’ve found it pretty useless and as such I haven’t bothered to set up an example clip.  It does, however, get an award for being the only transparency in Premiere that gives access to all three checkboxes; reverse key, drop shadow and mask only.

 

Other

 

Luminance

 

The basic key, and the one that started it all, the Luminance key uses, well, the luminance of the clip to create its own key signal.  The threshold slider sets the luminance level that is opaque, and the cutoff slider sets what is transparent.  Everything in between these two levels is then stretched to fit the transparency scale from 100 to 0%.  As you slide the two closer together the difference between completely transparent and completely opaque gets smaller, with smaller increments filling out the range.

For example, clip 21 shows the water clip with the sliders at their original positions; threshold at 100%, cutoff at 0%.  In this case, there are 256 steps to create levels of transparency between completely opaque and completely transparent.

Clip 22 has the threshold set at 40% and the cutoff at 35%.  In this case, there’s only 5%, or about 13 gradient steps between completely opaque and completely transparent.  That’s why the clips are so highly delineated from each other.

 

Finally, on the topic of Luminance transparencies, I’ve set up clip 23.  In this case I’ve placed the title clip from the Alpha Channel section over the background and used the luminance key.  As expected, the lighter areas are nicely keyed through.  Unfortunately the darker areas of the bevel are transparent, and the background can be seen through the holes.

To fix this, try sliding the threshold slider to the left.  You’ll notice the dark areas starting to reappear, but you’ll also notice a black fringe around the stroke start to develop as the slightly-higher-than-black edges start to creep up the transparency scale.  It’s a bit of a tradeoff and it highlights the preference of using the alpha channel.  This is just a handy workaround to know, just in case.

 

Screen

 

Similar to the Photoshop and After Effects transfer mode of the same name, Screen doesn’t so much create transparency as it does interact with the background to create a new composite.

 

According to the manual,

 

“…the Screen key creates transparency in the areas that correspond to the dark areas of the underlying image.”

 

Not real helpful, and I think you need to work in one of the aforementioned programs to really understand what it does.  It’s also kind of misleading as it doesn’t actually “create transparency” as much as affect colour and luminance information.

In a nutshell, the luminance values of the Screened clip look at the corresponding pixel beneath it.  It then adds itself to the color and luminance values of the background clip to create something that is overall lighter.  Obviously, black in either clip is unaffected as anything added to zero just winds up as itself and white in either clip is completely opaque.

To avoid hitting peak white too soon (RGB 255), the transparency does a bit of mathematical hocus pocus to “roll off” so the lighter areas don’t blow out the detail.  It’s been likened to projecting two slides from two projectors onto the one screen.

I’ve given an example in clip 24, but it doesn’t really show itself off well in this instance.  It works best in clips with lots of high contrast and movement.

However, it does a lovely job in creating an adaptation of what After Effects gurus Chris and Trish Meyer refer to as “Instant Sex.”  Basically you copy your clip to a higher track, add blur, then give it the Screen transparency.  It creates a nice, soft, slightly blown out look, as can be seen in clip 25.

You can also use it to key objects shot on black, in much the same way the luminance transparency works.  Handy for explosion clips on a black background with no alpha channel.

 

Multiply

 

The opposite of Screen, Multiply darkens everything based on the picture values of the two images.  It’s been likened to projecting two slides from the same projector at the same time.  Clip 26 shows an example of what it does.  Don’t be afraid to try switching the clips around, so the background is the clip you’re keying.  You’d be surprised at some of the results.

A quick way to get video in a title over a black background is to create your title on black, then key it over the video using the Multiply transparency as seen in clip 27.

 

None

 

Listed as a transparency type, this basically allows you to get access to the garbage matte handles without having to select a specific transparency type.  I guess you could even look at it as the “default” transparency type.

 

The manual describes a simple split screen effect by using the garbage matte handles to create a wipe between the background and the keyed subject (clip 28).  It’s just one of a couple of ways to do this effect.

 

Conclusion

 

I hope this tutorial has been of some use in explaining some of the things that you just couldn’t quite figure out from the manual.  Of course, getting your head around keying and compositing is a mixed blessing.  Once you figure out the basics you find yourself figuring out all sorts of ways to get a specific effect you’ve seen on TV or the movies.  The down side is that Premiere can only follow you so far.  Then you find yourself needing to fork out the money to buy After Effects.

 

I had mentioned earlier that I was going to describe a bonus chroma key technique to assist your blue screen compositing, but I think I’ve gone on quite long enough.

 

Besides, I need something to write another tutorial on.

 

Reference Links

 

A lot of the clips used here are Star Wars based and have been downloaded off the net from various sources, but the majority have been gathered together at http://www.moonzdesigns.com/.  If you want to get other clips, and even videos, to practice chromakeying techniques have a poke around there.

 

Other links to stuff on keying and Premiere include;

 

Wrigley Video Tutorials:  A great learning place for beginners and intermediate users.

 

Keying with 4:1:1 DV footage:  Douglas Spotted Eagle runs through the issues regarding keying with the limited chroma sampling of DV.  

 

Another Keying with 4:1:1 footage article: This one, by the Rev. John Jackman, is interesting in that it describes some of the new hardware to assist with keying DV.  You’ll need to register on the DV.com website, but with all the information on that site it’s well worth it.

 

Keying Titles with DV codec:  Philip Hodgett’s article on getting the best out of your titles using DV.  Primarily written for After Effects, the basics hold true for Premiere.

 

Also worth reading is Understanding 4:1:1 and 4:2:2 Sampling Rates by Studio 1.

 

The Blue Screen Page: Steven Bradford’s very informative page on how chroma keying works and how to light for good results.

 

Building your own Blue Screen:  There’s tons of ways to do this on the cheap.  Here’s just one of them.

 

The DV Garage Composite Toolkit Samples Page: For a more detailed look on chroma keying and compositing using After Effects.  Also describes the theory behind how the After Effects Production Bundle “Color Difference Keyer” works.  If nothing else it’s fun to watch them put together a Star Wars combat scene, complete with light sabers.

 

Brain Dump Series Alpha Channels Sample Page:  From the same site, DV Garage.com, this series of Quicktime files runs through the basics of Alpha Channels.  It’s primarily for Photoshop, and it is just a few samples from a longer video series, but it has some good information and tips.

 

Straight and Premultiplied Alpha Channels: Mentioned in my previous tutorial, this primer from Creating Motion Graphics (see below) runs through the difference between Straight and Premultiplied Alpha Channels.

 

Index of Cool Stuff:  A bit dodgy really, but this little site has links to primers and PDF files on a range of software, including Premiere, After Effects and Photoshop.

 

Creating Motion Graphics with After Effects: The definitive After Effects guidebook by Chris and Trish Meyer.  It’s full of concepts that stretch way beyond the realms of just the one program.  The White Alpha Matte example clip used in this tutorial was obtained from its accompanying CD.

 

Steven L Gotz’s website:  If it’s cool and/or it has something to do with Premiere, chances are there’s a link to it on Steven’s site.

 

Ulead Freebies!:  Some of the clips in this tutorial were obtained from Ulead’s free downloads page.  While I don’t like plugging the competition, it’s worth dropping by and picking up.  Just scroll down near the bottom of the page for the link.  You’ll be prompted to give an email address, and the file is 20Mb.

 

And last of all, don’t forget our old friend Google.  You’d be surprised how much information on this subject is out there on the net.

 

Peace.