QUESTION: I really just wanted the red sweater and the picture frames to stand out, which is why I kept it so underexposed. I should have just done the underexposing in post. I tried using the Color Key plugin, but it's not working.

When you are capturing images that will require later work such as compositing, you want to capture with an eye toward maintaining detail, and minimizing noise.

With that in mind, expose your images so that you are using the available "image space" to best effect, meaning using the available latitude of the media. Increase exposure just below where you would clip highlights (clipping will lose detail), and in some cases, reduce contrast so that you improve detail in the darker areas.

Ideally, you want your histogram "filled" from left to right with image data, but with no image data being clipped of the left or right side. This will give you the greatest flexibility in post.

Caveat: When using 8 bit media, pushing this idea too far can result in other artifacts such as banding and related delta-E errors.

As for keying, don't use the "color key" plug in. Use Keylight - it's included in AE, or use Primatte — both are superior to the Color Key plugin. I have a quick sample below from your image, and a further discussion of how to improve the key. But first:

WHY USE GREEN?

I want to say for the record that green screen is not a cure-all for key and matte purposes. As far as I can tell in this present shot, there is no hair or semi transparent objects that would benefit from a green screen key, and the objects that you are keying (the frames) are so small, that you can easily do a simple animated mask as the foreground object passes each one.

Unless the camera is locked, you need to do a planar track of each picture anyway, and except for the foreground object passing in front there is no benefit to using green, since you might as well just do a corner pin for each. No keying needed.

In other words: it may have been more trouble to put green in the frames than you would save through keying instead of masking. (Judging by your simple foreground element of a person in a red hoodie).

Simple corner-pin-type tracked layers, with traveling masks to key the foreground red coat will give you the cleanest result for this shot, as it is presently shot (due largely to noise and lack of color channel separate, which I discuss below). If the camera is moving, use Mocha for a 2.5D planar track.

LET'S TALK ABOUT ALTERNATIVES TO GREEN:

The noise in this image looks like you shot on a camera with a high level of compression. The greater the noise/compression artifacts, the more challenging it will be to make a green key. So sometimes it is worth considering alternatives to green. (If I could get a sample of your camera original, I could provide better analysis).

Green is not the only valid choice here. Flocked black could work in some cases, white can work in some cases. In this case, an 18% grey could work, or a red matching the picture frame (all red square) might be just fine - and to wit, we are NOT going to color key with any of these choices.

When we use a BG like 18% grey or all red, it is just to provide some contrast to create the animated masks. then we key with masks not color, and when keying with masks we want the BG color to be close to what we are replacing it with, or in some cases close to foreground, to prevent any matte lines.

Flocked black (dense velvet or duv that reflects no light) are good if you have glass on top and you want to retain the reflections. You can corner pin/mask and ADD mode the images that go into the frames, and the reflections remain naturally. You may need to use traveling masks to matte the foreground element, though in some cases you can use a Luma key with a combination of garbage masks - again, depending on the shot.

For more on black screen, read this article: Black Screen and Gamma Blur

If instead of reflections you want to retain shadows over the images from other object in the scene, then you could use WHITE screen instead of flock black, and then you would use MULTIPLY to place the images in the frames, and the shadows and relative scene lighting on the white screen would be retained.

Now, there are certainly plenty of good reasons to use green — especially if you have a foreground object with complex edges (i.e. hair) that is hard to manually mask.

But if you choose green, there are parameters that you need to stay within for successful keying.

NOW LET'S DISCUSS YOUR SHOT, AS SHOT:

It is not that your green screens are too dark per se - it is a common misconception that a green screen must be "bright". Actually, a green screen can be two stops UNDER meter for a scene that is being exposed at meter (i.e. green screen can be in zone 3 - so if you spot meter an 18% grey card and expose it in zone 5, then spot meter on the green screen can be up to two stops lower).

This also assumes, however, that your camera and the compression used on the footage is noise free enough (it isn't in the example shot you posted).

As a general rule, if you want your scene dark, you should still shoot it brighter (so long as you don't blow highlights), and then bring it down in post. This will help to eliminate noise issues and maintain detail in the darker areas. This is particularly important when shooting on dSLRs and cheaper camcorders that record to a highly compressed 8 bit file. 

The biggest problem you are facing here with your shot is the extreme noise in the green screens due to the low exposure level.

Incidentally, it is a mistake to attempt to color correct a plate before keying - i.e. do NOT put levels or curves in before Keylight. In most cases, Keylight (pr primatte) should be the first plug-in. Pull the key using the camera original with minimal alterations, and then blend and color correct downstream.

COLOR CHANNEL SEPARATION:

The other important aspect is the separation of green from the red and blue channels. This is why proper narrowband green needs to be used as the screen material. You can't just stick any old green paper or whatnot in for a green screen and expect good results.

The green paper you used isn't all that green. If you examine it in linear colorspace, you will see that the GREEN channel is only about one stop brighter than the RED and BLUE channel. This is very poor channel separation.

The example shot: has very poor channel separation.

See that your green channel is less than twice as bright as the other two, i.e. the green is less than a stop brighter than red and green. This will not generate a clean key.

Below are some real world examples of what you want to see in a green screen. In the first two cases, the green screen is exposed at meter (i.e. a spot meter reading of the screen is equal to a spot reading of an 18% grey card). In the third example, the screen is exposed 1 stop under meter, and in the fourth, it is 2 stops under. (Note that as a general rule for practical reasons (such as noise), you typically don't want the darkest useable portion of the screen much darker than -2 stops. This can be important when there are cast shadows you want to preserve and avoid noise.)

Example Channel Separations 
Example Channel Separations

In these examples, the green channel is three to four stops brighter than the next brightest channel, and more than 6 stops brighter than the darkest channel. This is the kind of channel separation needed for a good clean key.

But in your case, the green paper element is not narrow band, with less than a stop of separation between R/B and G. If you are looking for a paper product, you could try something that is a dark "florescent green" as a way to get a very narrow band of light reflection.

Ultimately, what you want to see on your monitor looks like this:

sRGB Ideal Green Screen Appearance:

 

While the accuracy of this green patch is limited by your monitor, this is about the color we want to see if viewed on an sRGB monitor.

On a properly calibrated sRGB monitor the border is approximately 18% grey, and the green is about a stop under the grey border (in terms of luminance), with no blue or red crosstalk. 

On an IRE scope, 18% grey is about IRE 43 (Rec709) to 48 (sRGB). The green patch on an RGB parade (i.e. green by itself) is also about 43 to 48 IRE. But the green patch is more like 30 IRE when viewed as luminance (all channels combined).

Ideally we like to see the IRE Luminance level for a green screen at 25 to 35 IRE, and never higher than 45 IRE Luminance. On the IRE RGB Parade, ideally the red and blue are at or near 0 (and not more than 10), and the green is 30 to 50 IRE. Different color spaces will cause these values to vary. 

sRGB Ideal Blue Screen Appearance:

 

While the accuracy of this blue patch is limited by your monitor, this is about the color we want to see if viewing a blue screen shot on an sRGB monitor. Again the border is 18% grey. This is the brightest pure blue possible on an sRGB monitor, which is about 11% luminance (lower luminance than the border, this is because blue only makes of 7% to 11% of luminance, depending on the color profile). 

In some colorspaces blue can actually be brighter than this (and that's not possible to display on an sRGB monitor). This is because when shooting BLUE screen it is best to overexpose relative to meter, by as much as a stop. And here it is set to maximum, #0000FF. But because blue only makes up 11% of luminance, it is actually lower in luminance than the 18% grey border.

On an IRE scope, The blue patch on an RGB parade (i.e. blue by itself) can easily be 100 IRE, as it is here. But this blue patch is less than 12 IRE when viewed as luminance (all channels combined, red and green at 0).

As the blue channel has much more noise than green, it does help to maximize the blue channel exposure. Because blue makes up so little of total luminance, we can increase exposure well above meter without much crosstalk from red and green (assuming a properly narrow band material).

Also, blue spill is often not noticeable in the scene, while green spill is an ever present problem.

While blue should be exposed brighter than key, it is important to avoid avoid clipping. It is important to never clip any channel when shooting a color key element, but you especially need to avoid clipping near the edges of the subjects being keyed. So adjust lighting and exposure for blue screens to be bright, but be careful never to clip.

Also ensure that an exposure increase does not introduce excessive red or green channel crosstalk. If increasing the exposure of the blue (or green) screen causes the red and/or blue channels to start to increase, then that means the screen brightness is as high as it can go without crosstalk into the other channels. This happens particularly as you approach clipping for the screen color's primary channel, due in part to limitations of the Bayer filter and debayering algorithm.

TIP: You can test for the bandwidth of a particular color by taking a full screen image of it with a dSLR and then looking at the RGB histogram in the camera. You want to see both the red and blue spikes far to the left of the green spike, with as much distance between the green, and the red/blue channels as possible. Take your camera to the art or fabric store when choosing the color (place the paper or cloth sample under a light source type similar to your shooting setup, or use daylight).

As such, using your shot as an example, exposing it brighter (to reduce noise), and using a more narrowband green element, should solve your issues.

Regardless, don't use the "color key" plug in. Use Keylight or Primatte: Here's a rough key after spending about a minute using Keylight:

Now, if this were on your QT file, and not a still, you would probably see crawling noise at the edges of the picture frames due to the noise in the element.

If I received this particular shot "as a shot to do", I would probably not use the green for a key at all, and just comp over the green with the images, then use animated masks for the foreground person passing by. Based on this situation that appears to be the best solution.

This article was originally written circa 2012, but minor updates in July of 2018 

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