On this page i will discuss aspect ratios in DCPs, masking and Title Safe Area. Pictures will get larger when the mouse is on it.
Aspect ratio is a name for the ratio of the width and the height of a picture. There are a few ways of describing this. One is like 16:9, meaning the width of a picture is 16 units when the height is 9 units. Another way of expressing this is dividing the first number by the last and thus expressing how many times the height will give you the width. For 16:9 this will be 1.17777777777etc:1. mostly abbreviated to 1.77 or 1.78 or even 177 or 178. In cinema people are used to express aspect ratios in this form. Possibly because the height of a cinema screen was fixed, and it was easy to mask the sides as the aspect ratio changed. Most common aspect ratios in 35mm cinema are 1.33:1, 1.37:1 (also called Academy, since this aspect ratio was put to standard by the Academy of Motion Picture Arts and Siences in 1932), 1.66:1, 1.75:1, 1.85:1 (all three called Wide Screen or WS) and 2.39:1 (Cinema Scope or CS). In the video world people are used to 4:3, 16:9, 21:9 and the like. It is good to know that 4:3 is 1.33:1, 16:9 is 1.78:1 and 21:9 is almost 2.39:1 (it’s 2.33:1 in fact)
In DCP there are only three DCI compliant aspect ratios. Being Flat (1998 x 1080 pixels = 1.85:1), Scope (2048 x 858 px = 2.39:1) and Full Container (2048 x 1080 pixels is 1.9:1). You can double all pixel counts for the 4k variants, but on this page i will not for the sake of clarity. Though Full Container is DCI compliant most cinema projectors are not set up to show this correctly. In fact, Full Container would be using all the pixels of the projector image chip. See drawing. Black is the chip (2048 x 1080 px), red is the part of the chip used for Scope (2048 x 858 px), blue is the part used for Flat (1998 x 1080 px). But there is more to know.
Here we see what happens when a projector is tilted downwards because it is positioned higher than the screen. The expanding beam of light from the projector has to travel farther to reach the bottom of the screen. There it has expanded more than the top, resulting in the trapezium shape. If a projector is turned sideways to get the picture on the screen a similar thing will happen, but now one side will be too big for the screen.
To be sure the picture on the screen has straight sides and it is not outside of the screen the picture is masked inside the projector (grey parts in image). The effect is that a part of the picture will not be shown.
This is the place where i would like to say something about Title Safe Area and Action Safe area. Originally these terms were used in television and video, because the old CRTs inherently left edges of the picture invisible. As we have seen, in DCP projection the edges of the picture are possibly not visible on the screen because of the masking, necessary for a nice rectangular projected picture. So be sure titles stay away from the edges, or letters might go missing during screening as shown in this picture (left). It is wise to keep titles within a 1728 x 972 px box (the Title Safe Area), centered in the picture. So that’s 135 px from the sides and 54 px from top and bottom that should not contain text. There is also a thing called Action Safe. This means that you should not put important visual information (think of a waving hand, a blinking eye) close to the edge of the picture. This safety is less critical. The borders of the Action Safe Area could be set about half way between the pictures edges and the Title Safe Area.
A lot of video is shot on 16:9 which equals 1.78:1 or 1920 x 1080 px. This will not fill Flat (nor Scope). The only good procedure is to add small black borders to the sides, each 34 px wide, making the picture 34 + 1920 + 34 = 1998 px wide. This way it sticks to the Flat specification. In the same way video material in 4:3 ratio can be made fitting the Flat container by adding 279 px of black on either side making the picture 279 + 1440 + 279 = 1998 px wide. And here’s why you better put that information in the CPL name.
Green is the 1.78:1 picture from the tilted projector. The grey area is the masking for Flat. The black areas are the 34px borders that make the difference between 1998 (Flat) and 1920 (HD).
So on the screen the projected picture will look like this.
I guess that’s not what you want to see after months or years of hard work on your film. So put the 178 in your DCP title (..._F-178_...) and check if the theatre is able to show it the right way (they should). The projectionist then has to select the 178 masking on the projector (if necessary) so the result will be like this. You lose a small part of the picture on the lower sides, but it looks much better on the screen! Again, take care not to put text close to the egdes, use the safety as i explained above on Title Safe Area.
For best projection keep to the Flat 185 and Scope 239 aspect ratios. But you are a creative person so now you may want to have your film in some very nice aspect ratio you made up yourself. (I once screened a film changing from square to scope to a circular image and back). There is no problem with that as long as you are aware of the things that might happen when your film is screened in a less than ideal situation. If you made a film that does not have a standard aspect ratio it might be better to make the DCP in such a way that the whole picture is inside the Title Safe Area (whether Flat or Scope). Always make sure the aspect ratio that should be used in projection is in the CPL name. And provide for some information about your film, its aspect ratio and the way it should be shown. This can be written, printed or online, as long as it’s easily accessible for the projectionist. A decent projectionist will be able to adjust her/his equipment so that your picture will fill the screen as you intended.
When your picture has an aspect ratio of 1.85:1 or smaller put you film in a Flat container. If its aspect ratio is 2.33:1 or bigger put it in Scope. But what about something in between, like 2:1? You must make a choice with this in mind: Most regular theatres will not take the effort of making settings in the projector for content that is not either 858 px high (Scope) or 1080 px high (Flat). If your film will be distributed to regular theatres i think it is better to put any content with aspect ratio under 2.33:1 in a Flat DCP. Be sure the CPL name contains the aspect ratio (i.e. ..._F-200_...) so the projectionist knows what format to expect and adjust projector settings. It will have a higher resolution in projection (for 2:1 on the screen each frame will consist of 1.9 Mpx in Flat where in Scope it will be 1.4 Mpx). In theatres where this special setting will not be made most likely the film will be shown as Flat (185) which will result in all of the picture being visible but with black borders on top and bottom of the picture on the screen. (There is a small chance the projectionist still wants to fill the screen top to bottom and decides to show the film on Scope, which will cut of top and bottom parts of the picture. The picture will be trapezium shaped in the theatres with a tilted projector. Clear instructions on how to project the film should prevent this). If possible you might make two versions, one in Scope and one in Flat, with, as always, clear instructions and appropriate naming of the DCP.
Read more here:
ISDCF on aspect ratio naming convention for DCP
Wikipedia on image aspect ratios
An entertaining 18 minute film (YouTube) about how we got to the different aspect ratios we now are used to.
A list of motion picture film formats and aspect ratios on Wikipedia.
Scott Norwood made a simple and very clear chart with most cinema aspect ratios in 2K DCP. You can find it on this page. For those working in projection he made a DCP that can help you get the masking right. Thanks Scott!