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Moving image formats

From Wikipedia, the free encyclopedia

This article discusses moving image capture, transmission and presentation from today's technical and creative points of view; concentrating on aspects of frame rates.

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Transcription

How's it going everyone this is Chris from Spoon Graphics back with another video tutorial. Today we're going to have some fun creating what's known as a Plotagraph, which is a motion picture effect similar to Cinemagraphs, but it's created from a single static image, rather than a video clip. The name Plotagraph comes from the brand name of the software and the associated community based on this effect, which you can find at Plotagraph.com. The full software is pretty expensive, so I've been playing around in Adobe Photoshop to figure out how to create the effect manually. It works by stretching a certain portion of the image using keyframes in the Photoshop animation timeline. Repeating this simple transformation in a loop gives the illusion that the picture is moving. The effect works particularly well with landscape scenes of rivers or waterfalls that have continuous movement, which is what I'll be showing you as part of my example using this image by Kym Ellis from Unsplash.com. Begin by opening up your chosen image in Adobe Photoshop. This picture is pretty large by default, so I'm going to scale it down using the Image Size menu so it's less CPU intensive when we get around to animating it. We first need to choose a portion of the photograph to animate. Turn on Quick Mask mode at the bottom of the toolbar, then set up the Brush tool with a soft tip. Begin painting around the area you want to move, which in my case is the waterfall. Paint with black to add to the selection, or press the X key to switch to white if you need to remove any areas from the selection. Click the Quick Mask button when you're finished to have the painted area transformed into a selection, that retains the soft outline. Go to the Select menu and choose Inverse, then Copy and Paste this clipping onto a new layer. Open up the Timeline panel from the View menu, which is where we'll animate the image. Create a new Video timeline. Shorten the length of the clipping layer to around 1 second, then click the little arrow to expand this clips options. You'll see that the first keyframe option is to animate the Position of the clip, but we want to be able to stretch it, not just move it around. Right click on the layer within the Layers panel and choose Convert to Smart Object. If you check the clip options again you'll now see the option for Transform. Move the playhead to the start of the timeline, then click the stopwatch icon to apply a Transform keyframe. Move the playhead along and add another keyframe by clicking the little diamond icon. Zoom out so the image fits in the screen area and use the CMD+T shortcut for Transform to manipulate the waterfall clipping. Drag the bottom corner handle downwards to stretch the element in the direction the water is flowing. In the timeline, drag this keyframe all the way to the right of the clip, then scrub back and forth to see the basic animation effect. To help the animation play more smoothly, add a Fade transition to each end of the clip. Give it a test by clicking the Play icon. It doesn't look realistic yet, but the fading eliminates the harsh jump back to the start. Select the Background layer and use the Quick Mask to make a selection of a second portion of the image to animate. In my example it will be the mist and spray at either side of the waterfall. Apply the mask, inverse the selection, then Copy and Past the clipping on to a new layer. Right click and convert this layer into a Smart Object. Make sure the clip runs from the start of the timeline, and trim it to the same length as the other clip. Move the playhead to the start and add a Transform keyframe. Move the playhead along a little and add another keyframe. Press CMD+T to Transform the image. Scale it upwards so the mist will appear to expand outwards towards the edge of the canvas area. Drag the keyframe right to the end and add the Fade transitions to each end of the clip. If you give it a test, you'll notice that some portions of the image are being animated that we don't want, like this person being swept away. Select the background layer and press CMD+J to duplicate it. Drag this copy right to the top of the layer stack. Make sure this new layer aligns with the start of the timeline, then add a layer mask. Fill the mask with black using the CMD+Backspace shortcut, then move the playhead to a point in the animation where you can see things moving out of place. Set up the brush tool with white and begin painting over the unwanted areas. This will restore the static image to act as a mask so the animated elements won't be seen in these areas. Once everything looks good within this short 1 second clip, we're ready to extend and loop the animation. Select all the layers of the various animated pieces and Group them together. Hit the CMD+J shortcut 4 or so times to make numerous duplicates of the group. In the timeline, begin dragging the groups and position them so they offset by half the length of the previous clip. You can expand the group to use the transitions as a reference point for the centre. Give this extended animation a play to test how the transformation repeats. To ensure the animation continuously loops, trim both the top mask layer and the work area to the halfway point of the last clip group. Hit CMD+J to make a final copy, then drag it back to the start of the timeline so far that the first half is completely cropped off. Expanding the group again will make it easy to see where that centre point is between the two transitions. Click the Loop Playback option in the Timeline settings to see the animation repeatedly play through. It's amazing how the illusion of movement can be created from a static image. The final effect can be exported as either a video file by Rendering the timeline, or as an animated Gif if you were to scale down the document and find suitable compression to bring the file size down. So I hope you have fun experimenting with this effect. Don't forget to check out my tutorials on creating a Cinemagraph and 2D Parallax for similar kinds of motion effects. Subscribe to the channel if you want to stick around for more, otherwise thank you very much for watching, and I'll see you in the next one!

Contents

Essential parameters

The essential parameters of any moving image sequence as a visual presentation are: presence or absence of colour, aspect ratio, resolution and image change rate.

Image change rate

There are several standard image-change rates (or frame rates) used today: 24 Hz, 25 Hz, 30 Hz, 50 Hz, and 60 Hz. Technical details related to the backward-compatible addition of color to the NTSC signal caused other variants to appear: 24000/1001 Hz, 30000/1001 Hz, and 60000/1001 Hz.

The image change rate fundamentally affects how "fluid" the motion it captures will look on the screen. Moving image material, based on this, is sometimes divided into two groups: film-based material, where the image of the scene is captured by camera 24 times a second (24 Hz), and video-based material, where the image is captured roughly 50 or 60 times a second.

The roughly 50 and 60 Hz material captures motion very well, and it looks very fluid on the screen. In principle, the 24 Hz material conveys motion satisfactorily; but, because it is usually displayed at least twice the capture rate in cinema and on CRT TV (to avoid flicker), it is not considered capable of transmitting "fluid" motion. Nevertheless, it still is used to film movies, because of the unique artistic impression arising exactly from the slow image-change rate.

25 Hz material, for all practical purposes, looks and feels the same as 24 Hz material. 30 Hz material is in the middle, between 24 and 50 Hz material, in terms of "fluidity" of the motion it captures; but, in TV systems, it is handled similarly to 24 Hz material (i.e. displayed at least twice the capture rate).

Capture

The capture process fixes the "natural" frame rate of the image sequence. Moving image sequence can be captured at the rate which is different from presentation rate, however this is usually only done for the sake of artistic effect, or for studying fast-pace or slow processes. In order to faithfully reproduce familiar movements of persons, animals, or natural processes, and to faithfully reproduce accompanying sound, the capture rate must be equal to, or at least very close to the presentation rate.

All modern moving image capture systems either use a mechanical or an electronic shutter. Shutter allows the image for a single frame to be integrated over a shorter period of time than the image change period. Another important function of the shutter in raster-based systems is to make sure that the part of frame scanned first (e.g. the topmost part) contains image of the scene integrated over exactly the same period of time as the part of frame scanned last.

Early TV cameras, such as the video camera tube, did not have a shutter. Not using shutter in raster systems may alter the shape of the moving objects on the screen. On the other hand, the video from such a camera looks shockingly "live" when displayed on a CRT display in its native format.

Transmission

Analog broadcasting systems—PAL/SECAM and NTSC—were historically limited in the set of moving image formats they could transmit and present. PAL/SECAM can transmit 25 Hz and 50 Hz material, and NTSC can only transmit 30 Hz and 60 Hz material (later replaced by 30/1.001 and 60/1.001 Hz). Both systems were also limited to an aspect ratio of 4:3 and fixed resolution (limited by the available bandwidth). While the wider aspect ratios were relatively straightforward to adapt to 4:3 frame (for instance by letterboxing), the frame rate conversion is not straightforward, and in many cases degrades the "fluidity" of motion, or quality of individual frames (especially when either the source or the target of the frame rate conversion is interlaced or inter-frame mixing is involved in the rate conversion).

50 Hz television systems

Material for local TV markets is usually captured at 25 Hz or 50 Hz. Many broadcasters have film archives of 24 frame/s (film speed) content related to news gathering or television production.

Live broadcasts (news, sports, important events) are usually captured at 50 Hz. Using 25 Hz (de-interlacing essentially) for live broadcasts makes them look like they are taken from an archive, so the practice is usually avoided unless there is a motion processor in the transmission chain.

Usually 24 Hz material from film is usually sped up by 4%, when it is of feature film origin. The sound is also raised in pitch slightly as a result of the 4% speedup but pitch correction circuits are typically used.

  • Older technology allows an alternative option where every 12th film frame is held for three video fields instead of two mostly fixing the problem.
  • More modern film playback technology allows for every 25th frame to be interpolated, with less objectionable results and no need for pitch modification.
  • Each of these film oriented content transmission techniques has its own drawbacks. However modern motion compensation processors are considered to produce the least objectionable output.

With roughly 30 or 60 Hz material, imported from 60 Hz systems, is usually adapted for presentation at 50 Hz by adding duplicate frames or dropping excessive frames, sometimes also involving intermixing consecutive frames. Nowadays, digital motion analysis, although complex and expensive, can produce a superior-looking conversion (though not absolutely perfect).

60 Hz television systems

Because of higher television production budgets in the US, and a preference for the look of film, many prerecoded TV shows were, in fact, captured onto film at 24 Hz.

Source material filmed at 24 Hz is converted to roughly 60 Hz using the technique called 3:2 pulldown, which includes inserting variable number of duplicate frames, with additional slowdown by the factor of 1.001, if needed. Occasionally, inter-frame mixing is used to smooth the judder.

Live programs are captured at roughly 60 Hz. In the last 15 years, 30 Hz has also become a feasible capture rate when a more "film like" look is desired, but ordinary video cameras are used. Capture on video at the film rate of 24 Hz is an even more recent development, and mostly accompanies HDTV production. Unlike 30 Hz capture, 24 Hz cannot be simulated in post production. The camera must be natively capable of capturing at 24 Hz during recording. Because the ~30 Hz material is more "fluid" than 24 Hz material, the choice between ~30 and ~60 rate is not as obvious as that between 25 Hz and 50 Hz. When printing 60 Hz video to film, it has always been necessary to convert it to 24 Hz using the reverse 3:2 pulldown. The look of the finished product can resemble that of film, however it is not as smooth, (particularly if the result is returned to video) and a badly done deinterlacing causes image to noticeably shake in vertical direction and lose detail.

References to "60 Hz" and "30 Hz" in this context are shorthand, and always refer to the 59.94 Hz or 60 x 1000/1001 rate. Only black and white video and certain HDTV prototypes ever ran at true 60.000 Hz. The US HDTV standard supports both true 60 Hz and 59.94 Hz; the latter is almost always used for better compatibility with NTSC.

25 or 50 Hz material, imported from 50 Hz systems, can be adapted to 60 Hz similarly, by dropping or adding frames and intermixing consecutive frames. The best quality for 50 Hz material is provided by digital motion analysis.

Modern digital systems

Digital video is free of many of the limitations of analog transmission formats and presentation mechanisms (e.g. CRT display) because it decouples the behavior of the capture process from the presentation process. As a result, digital video provides the means to capture, convey and present moving images in their original format, as intended by directors (see article about purists), regardless of variations in video standards.

Frame grabbers that employ MPEG or other compression formats are able to encode moving image sequences in their original aspect ratios, resolution and frame capture rates (24/1.001, 24, 25, 30/1.001, 30, 50, 60/1.001, 60 Hz). MPEG—and other compressed video formats that employ motion analysis—help to mitigate the incompatibilities among the various video formats used around the world.

At the receiving end, a digital display is free to independently present the image sequence at a multiple of its capture rate, thus reducing visible flicker. Most modern displays are "multisync," meaning that they can refresh the image display at a rate most suitable for the image sequence being presented. For example, a multisync display may support a range of vertical refresh rates from 50 to 72 Hz, or from 96 to 120 Hz, so that it can display all standard capture rates by means of an integer rate conversion.

Presentation

There are two kinds of displays on the market today: those which "flash" a picture for a short part of the refresh period (CRT, cinema projector), and those which display an essentially static image between the moments of refreshing it (LCD, DLP).

The "flashing" displays must be driven at least 48 Hz, although today, a rate significantly below 85 Hz is not considered ergonomic.

For these displays, the 24–30 Hz material is usually displayed at 2x, 3x, or 4x the capture rate. 50 and ~60 Hz material is usually displayed at its native rate, where it delivers a very accurate motion without any smearing. It can also be displayed at twice the capture rate, although moving objects will look smeared or trailed, unless intermediate frames are calculated using the motion analysis and are not just simply duplicated.

The "continuous" display can be driven at any integer multiple of the capture rate - it won't matter for the viewer, nor can it be visually discriminated. However, in general, "continuous" displays show noticeable smear over quickly-moving objects in 50 and ~60 Hz video material (even if their response time is instant). However, there are two emerging techniques to combat smearing of the video-based material in LCD display: it can be effectively converted into the "flashing" display by appropriately modulating its backlight; and/or it can be driven at double the capture rate while calculating intermediate frames using the motion analysis (see LCD television).

Obviously, when presentation rate is not an integer multiple of the capture rate, the "fluidity" of the motion on the screen will suffer to a varying degree (terribly for video-, unpleasantly for film-based material). This is usually the case with computer-based DVD players and PAL PC TVs, where the user does not switch the refresh rate either out of ignorance, or due to technical constraints; which sometimes are, in fact, artificial, made by manufacturers counting on that user's ignorance. For instance some laptop LCD panels cannot be (easily) switched to anything but a 60 Hz refresh rate, and some LCD displays with DVI input refuse to accept digital input signal if its vertical refresh rate does not fit between 58 and 62 Hz.

Most software DVD players do not assist with switching display modes, and even if it is switched manually, they hardly synchronize frame updating with the display's vertical retrace periods. (There is only soft synchronization using hardware double buffering, which is not enough to match hardware players in the stability of playback.)

50 vs. 60 Hz

60 Hz material captures motion a bit more "smoother" than 50 Hz material. The drawback is that it takes approximately 1/5 more bandwidth to transmit, if all other parameters of the image (resolution, aspect ratio) are equal. "Approximately", because interframe compression techniques, such as MPEG, are a bit more efficient with higher frame rates, because the consecutive frames also become a bit more similar.

There are, however, technical and political obstacles for adopting a single worldwide video format. The most important technical problem is that quite often the lighting of the scene is achieved with lamps which flicker at a rate related to the local mains frequency. For instance the mercury lighting used in stadiums (twice the mains frequency). Capturing video under such conditions must be done at a matching rate, or the colours will flicker badly on the screen. Even an AC incandescent light may be a problem for a camera if it is underpowered or near the end of its useful life.

The necessity to select a single universal video format (for the sake of the global material interchange) should anyway become irrelevant in the digital age. The director of video production would then be free to select the most appropriate format for the job, and a video camera would become a global instrument (currently the market is very fragmented).

See also

References

External links

This page was last edited on 15 October 2018, at 19:59
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