In twenty years I have never seen a film properly screened from a computer on an extended desktop, with VLC or QuickTime, even with a very good setup.
The projector receives a computer screen, not a video signal, and the film arrives distorted, for three reasons.
Reason 1 of 3
Gamma
Gamma is the speed at which the image comes out of the blacks.
Between the file and the projector, the player and the operating system can change that curve without warning. The image comes out of the blacks too fast, or too slowly.
- What the audience sees
- Too bright, with grey, milky blacks. Or too dark, with crushed shadows.
- Where to spot it
- In a night scene.
- too bright ≈ 1.8
- as graded 2.4
- too dark ≈ 3.0
Reason 2 of 3
Gamut
Gamut is the colour of the colours.
A video master is made in a precise colour space, for example Rec. 709 or P3. When the computer drives the projector as a screen, the operating system's colour management steps in. It converts the image for the profile it assigns to the projector, and nothing guarantees that's the right one.
- What the audience sees
- Hues shift. Greens above all, but reds, skies and skin too.
- Where to spot it
- On foliage, a meadow, then on a face in close-up.
- Rec. 709: the film's colour space
- Display P3: what the system thinks it's talking to
Reason 3 of 3
Frame rate
A film is 24 or 25 frames per second. A computer is often 60 Hz.
With an extended desktop, the computer sends the projector its own display refresh rate. 60 isn't a multiple of 24: one frame stays up for 3 refreshes, the next for 2, and so on. Frames no longer all last the same time. At 25 frames per second, same problem.
Anything that moves steadily starts to catch:
- a pan across a landscape
- a tracking shot down a street
- a car, a bike, a face crossing the frame
- the end credits rolling
- What the audience sees
- It stutters.
- Where to spot it
- On the end credits roll of a well-finished film. It should glide up in one piece. Here, it moves in jerks.
The same film, the same movement, two cadences.
Video output
every frame lasts the same time
Extended desktop, 60 Hz
3 refreshes, then 2, then 3: 50 ms, 33 ms, 50 ms
On the left, every frame lasts the same time. On the right, the same frames, shown at 60 Hz. In real time, watch the poles during the pan, or one line of the credits: on the right, it catches. In slow motion you can see why: the frames on the right last 50 ms, then 33 ms, in turn.
Your display: measuring. This page cannot show true 24p; it shows the difference between frames of equal duration and frames of unequal duration. On a 120 Hz display or faster, the 3-2 cadence is reproduced as a 60 Hz display would give it.
What the film deserves, for years to come
under€200
A small video output device sends the projector a real video signal. At the film's frame rate, in the film's format, without going through the system's colour management.
Thunderbolt
24p · 25p
The kit
| DaVinci Resolve | free version | €0 |
|---|---|---|
| UltraStudio Express Monitor 3G | SDI and HDMI outputs | ≈ €170 |
| An SDI or HDMI cable | ≈ €15 | |
| Total | < €200 |
Prices incl. VAT, seen in Europe. A Mac or PC with a USB4 or Thunderbolt 4 port is required.
In practice
- Install DaVinci Resolve and Desktop Video, the device's driver. Both are free.
- Plug the device into the computer, then into the projector over SDI or HDMI.
- In Resolve, create a project at the film's frame rate, 24 or 25 frames per second, before importing the file.
- In the preferences, pick the device as the video output. Press play.
The projector needs to be set up properly too. But at least it receives video.
The next question
What about DCP?
A DCP (Digital Cinema Package) is still more reliable. It's the cinema format, made and tested for this.
But from a ProRes 422 HQ master, this chain gives excellent quality, far closer to a DCP than to a film played in a window.