Video Revision

8/2/2019 Video Revision

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Video revision

Contents

Technical ................................................................................................................................................. 2

Digital Compositing ............................................................................................................................. 2

Traditional compositing method..................................................................................................... 2

Processed foreground method ....................................................................................................... 3

Blending modes............................................................................................................................... 4

Gamma ................................................................................................................................................ 5

Slice Graph .......................................................................................................................................... 7

Matte creating & manipulation .......................................................................................................... 9

Rotoscoping .................................................................................................................................... 9

Luma keying .................................................................................................................................... 9

Chroma keying .............................................................................................................................. 10

Colour difference keying ............................................................................................................... 10

Matching the light space ................................................................................................................... 11

Essay ...................................................................................................................................................... 13

Digital revelation ............................................................................................................................... 13

History of 3D ..................................................................................................................................... 15

Motion graphics ................................................................................................................................ 16


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Technical

Digital Compositing

Traditional compositing method

The traditional compositing method is used to create combine two images together The formula for compositing is:

C = (m x fg) + ((1-m) x bg)

C = Composite

m = Matte

fg = Foreground

bg = Background

The composite operation consists of three steps : The foreground is scaled (multiplied) by the matte The background is scaled my the inverse of the matte Then the results are summed together to create the final composite (add)

Flow graph for the traditional compositing method:

BG

FG

invert

multi

multi

addmatte

Scaled

BG

Scaled

FG

Composite


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Processed foreground method

The process foreground method of compositing takes a different approach: A separate plate, known as the backing plate, is formed which is the same

colour as the backing colour of the foreground image.

This is multiplied by the inverted matte to form the scaled background platewhich is then subtracted from the foreground to create the processed

foreground image.

The processed foreground method is used for better edge qualityo But a downside is that it is vulnerable when there are variations in luminance levels

of the background colour.

o So it the background needs to be uniformly lit so it is all the same colour

Sub is the processed foreground because you are subtracting to get the final result Subtracting the scaled backing plate from the foreground image to get excellent edge quality Multi = is the scaled backing plate:

o Multiply is just:O = A x B

FG

Pull

matteinvert

multi

sub

Backing plate

Scaled

backing plate

Process

foreground


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Blending modes

Maximum

Maximum operation chooses the brightest pixel between the 2 images This is a very clean way to create beautiful composites. Most useful when you are combining the bright areas of one image to the dark areas of

another

Minimum

The minimum operation is the opposite of the maximum operation.

It picks the darkest pixel between the 2 images Again a great way to create digital composites Most useful when compositing the dark areas of one image onto the light areas of another.


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Gamma

This is the formula for gamma:O = (i)

g

O = output pixel

i = input pixel

g = gamma factor

This is the inverse formula for gammaO = (i)

1/g

O = output pixel

i = input pixel

g = gamma factor

What is gamma?o A mathematical operation that is performed on image pixels that alter the

brightness of the mid-tones without affecting the black and white points.

o In the early days display devices had a non-linear display characteristico Studies have shown that humans like a little bit of gamma in our image displayo The amount of gamma depends on local lighting, this means that the amount of

gamma changes from video to film

o Gamma is a power function that is applied to pixel data in order to lighten or darkenan image.

o In other words, the new pixel value is the original pixel value raised by the power ofgamma.

o For example if a pixel value of 128 represented as 0.5 with a gamma value above 1will darken an image and below 1 will lighten it.

Most software packages have a gamma operation and this implies using the inverse of thegamma, so that when the values is above 1 it will brighten an image,

o This is done to conform to what the user would expect Gamma changes are used rather than scaling operations to alter the brightness of an image

for 2 reasons:

o Firstly gamma changes effect an image in a way similar to the non-linear response ofthe eye, so it seems more natural.

o Secondly they do not clip the image.


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It is cheaper for TV studios to broadcast a non-linear signal that was gamma corrected thanto build gamma correction into TV sets.

Monitor gammao Has a non-linear characteristic and is an inherent feature of a cathode ray tubeo Increasing the voltage to the electron gun increases the stream of electrons to the

phosphors on the screen which in turn glow brighter.

o However, the stream of electrons does not increase linearly with gun voltageo Instead monitor gamma has a characteristic corresponding to a gamma value

between 2.35 and 2.5 depending on the monitors construction.

Gamma Correctiono Gamma correction is used to compensate for the monitor gammas non-linear

characteristic

o Most systems refer to this as Display Gammao The correction is applied by using a look up table (LUT) for the monitor, with values

being determined by using the formula 1/gamma.

o The original image is first loaded into the frame buffer for a work station.o The Look up table is used to convert the input values to output values.o In practice we tend to use a gamma correction value of2.2 instead of 2.5

End-to-End gammao Display devices are improved if they have a slight amount of gamma to them,

because of this gamma that is applied that does not cover the gamma range of the

device.

o For example, if a device has a gamma value of 2.5 and a gamma correction value of2.2 may be applied to give an overall value of 1.1

o This overall value is known as end-to-end gamma, and this is the most important asit determines how an image will look on a display device.

o Formula for end-to-end gamma is:End-to-end gamma = monitor gamma / gamma correction


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Slice Graph

The slice graph is a very important tool for analysing digital images The graph is created by using a slice tool to create a line across a region of interest in an

image.

Then the pixel values from under the line are plotted onto a graph This graph shows the colour levels relative to each other and provides insight for pulling

mattes and despill operations

This is the slice graph for a mid-grey circle on a green background

Formula for the raw colour difference mate is:rm = G max(R,B)

rm = Raw matteG = Green

R = Red

B = Blue


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The flow graph for colour difference mate is:

Raw matte does not have enough density for the final matte, As the final matte will need a backing region of 100% white and a foreground region of 0%

black.

This is done by scaling the density of the raw matte It is scaled using a colour corrections curve. Scaling the white for the matte must not be overdone as it can harden the matte edges if

over scaled.

If non-zero black pixels start to become lights (go grey), then this again is dealt with in thecurve.

To get the foreground region to 0% back for the final matte

GS Sub

Max (R,B)

R

G

B


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Matte creating & manipulation

Rotoscoping

To draw mattes frame by frame, it can manually but it is very time consuming. In the past mattes were drawn by hand,

o An excellent example of this is the scape station, orbiting Earth in 1968 2001: ASpace Odyssey

Now there are software packages that aid this process and increase productivity. The tools in these packages often include interpolation routines to work out the evolution of

travelling mattes between key frames.

Essentially it is a matte that changes its shape frame by frameLuma keying

When a matte is created based on the original RGB images luminance information.

Using a single threshold value can create very hard edged mattes, this is more than oftennot wanted

So in a lot of luma-keyers a second threshold if offered to introduce a softer edge to thematte

Luminance + 0.30R + 0.59G + 0.11B For a single threshold, a threshold value is set and pixel values greater than the threshold

are set to 100% white and pixel values below are set to black

A second threshold is added to soften matte edges,o So this is an inner density edge of 100%o And an outer density edge of 0%

With a gradient in between them, this gradient allows for softer edges in the matte


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Chroma keying

This is a matte which is based on colour, it takes an RGB image and converts it into HSV(Hue, Saturation and value(or colour, saturation and value))

HSV colour model

A range of colours are selected and this is used as a basis on which to define the matte Also it is possible to select a brightness range to give more control.

Colour difference keying

Most popular keying method as it gives superior edge characteristics to the matte It relies of the background being one of the 3 primary colours (Red, Blue, Green) The idea is that the difference between the green record and the other 2 records is relatively

larger in the green backing (green screen) region but is zero or very small in foreground

regions

The dark grey partial density is then scaled to 100% white

If there are some non-zero pixels in the foreground region of the matte then they are pulleddown to zero to give a solid black matte

Simple to invert the matte if needed.


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Matching the light space

Matching the light space is a process of making two images that were shot separately looklike they were shot together in the same light space.

The method is to approach matching the light space in a logical way.

First by making a luminance version of the composite to correct the brightness and contrast, then the colour aspects of the image are addressed, following that light direction, quality and interactive lighting effects are explored to refine

the basic colour correction

Shadows are examined for edge characteristics also a task to handle atmospheric haze

Stages of matching the light space

Brightness and contrast are matched using the luminance (greyscale) version of the image.o This is done by looking at the black, white and mid-tones of both imageso Then adjusting the brightness and contrast until the foreground and background are

matching

o This is done using the curve tool.o This stage dramatically improves results when matching the light space

Example with numbers:Foreground Background

Blacks 0.15 0.08

Whites 0.83 0.94

The background levels are measured and the foreground levels are adjusted to matchthe background

So if the background black levels were 0.08 black and the foreground was only 0.15black the curve tool would be used to push the black levels in the foreground to 0.08

black so they would match the levels in the background.

When increasing contrast an issue of clipping the blacks and whites can occur, whenusing a simple colour curve,

o Instead an S curve is more elegant as it gently pushes the blacks and whites Mid-tones Even though the blacks and white points have been matched, mismatches in the mid-

tones can still arise due to the dissimilar gamma in the two images

However this can be fixed easily without disturbing the blacks and white points, becausetheir control points have already been set.

So it is just a matter of bending the middle of the colour curve to adjust the mid-tones.


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Colour issueso Matching the colour can be difficult depending on the content of the foreground

and the background layers

o Using Grey cards can help us match colours as they have equal levels of R, G and Bo Matching flesh tones, if possible, is also a good technique as these will be similar in

humans.

Adjusting foreground colour bias to match background colour bias. Light direction, quality and interaction

o Light direction is largely important if there are cast shadows, as the direction of lightshould obviously be the same direction in both the foreground and background to

create believable shadows.

o Light quality refers to the nature if the light source For example, hard or soft. So if the light source is casting soft shadows, it would make for a

unbelievable composite if there were hard shadows in the background and

soft shadows in the foreground.

o Interaction between layers can take a verity of forms, For example, a foreground object casting shadows on a background layer.

o It can be used as a means of embedding a foreground object into a composite For example, if a person in a white shirt walks next to a brightly lit red wall,

the shirt will pick up a red tint.

Shadowso Their edge characteristics depend on light source, the distance between the light

source and the object.

o Shadows cast from a coloured light source have colour themselves

Atmospheric haze

o Has the effect of blurring objects in the distance and turning them bluero Associated with light scattering


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Essay

History of video and film & The Digital Revolution

History of video and film

Until persistence of vision was understood it was difficult to consider the idea of movingimages being possible

Photographers such as Eadweard Muybridge helped with the understanding the nature ofmotion

Muybridge had cameras activated by trip wires to capture the motion of a horses gallop His findings are still used by animators today The principle behind being able to understand and visualize moving objects is known as

persistence of vision

The development ofcelluloid film provided a convenient way ofstacking series of images,which could be played to the eye at a suitable frame rate

The kinetoscope was one of the first devices to enable images to be stacked Lumire brothers were the first to project moving images in 1896 Special effects have been used in film making from the early days, for example stop motion

and split screen techniques

All early compositing techniques were optical, and this remained the case until the start ofthe digital revolution in the 1990s

Some optical composting techniques like blue screen colour separation and blue screencolour difference process

These early techniques were improved with more direct techniques, one of these being rearprojects

This involved foreground action being filmed against a screen where the background actionwas being projected

o It was important that the cameras shutter speed matched up to the camerao And that the action in the foreground was lit the same as the rear projection


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These were known as process shots Hitchcock used process shots in Vertigo 1958 Rotoscoping was another technique that was used instead of process shots, it was tedious to

do.

o But results could be spectacular, as shown in Kubricks2001: A Space Odyssey1968

With the rise of television in the 1950s experiments were made with film formats andprojector methods

In 1952 CinemaScope was patented and had an aspect ratio of 2.35:1o Was used in The Robe 1953

Larger film print sizes such as 70mm were used to improve the quality of filmDigital Revolution

One of the most important breakthroughs in film technology was the digital revolution By scanning celluloid film using lasers, it was possible to convert picture information into

data, which could then be manipulated using a computer

After the film had been manipulated, the final print was created by imprinting the data ontoraw film using a laser

The next step was to remove the film out of the process, by shooting using a digital videocamera and projecting the final result in a digital cinema

This opened up a whole host of possibilities Computer based-editing packages allowed editing to take place without any cutting of film Optical techniques were replaced with digital compositing techniques which allowed for

composites to be so realistic it is hard to see the joins in most cases

o This can be seen in the Lord of the Rings films Colour grading is now much easier and more versatile

o As well as grading whole frames, selective colour correction can be used on smallelements

Special effects are easier to use and more versatile, match moving can be used it compositecomputer generated images onto real life footage (or vice versa)

Collaborative post production become easier, with the advancement in the internet andcommunication

This caused a slight negative impact to storytelling however, as an emphasis on the visualspectacles became more apparent.


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History of 3D

In 1838 Charles Wheatstone published a paper on stereoscopic vision He made an important point that by presenting each eye with a slightly different vision of a

scene, the illusion of 3D can be achieved In 1844 Brewster created a practical stereoscopic viewing device

Oliver Wendell Holmes improved the design by adding convex lenses; the new lensesenhanced the depth perception.

In 1862 Holmes and Joseph Bates worked together to create a cheap stereoscopic viewer,that forms the basis for many modern designs.

Some magic lantern shows projected two images on a screen that were colour coded tocreate a stereoscopic effect.

o They needed to be viewed using glasseso This method is known as an Anaglyph image

The earliest film that used this technique was The power of love 1922 Hammond & Cassidy invented the teleview system, it had to be synced with the projector,

but did not last as it was uncomfortable for people watching the film

Died out in 1930 In the 1950s there was a boom in 3D and this was due to polaroid technology

o The screen had to be silver to protect the polarized lighto Two projectors needed to be used with polarizing filters

One of the most famous and successful 3D films was House of wax 2953 By 1954 3D films had ceased again, this time because of the rise in popularity of

CinemaScope

3D films were no longer making enough money.Present day

The success of Avatar 2009 has revived 3D in present day


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3D movies require 2x the amount of frames then a 2D film so there is a rise in productioncosts

Also need specialist 3D cameras to film footage. Investments needed to be made in cinemas in order to play the films Many films make a large amount of their income from home distribution, so development

and rollout of low cost 3D TV.s is vital for the success of 3D in the future

It will be interesting to see if 3D will die out again, like it has done before in the past

Motion graphics

In the early days film titles were developed using techniques from early animation as theyshowed the graphical potential for moving images

Early animations tried to mix animation with live action Max Fleisher pioneered this andalso invented Rotoscoping

These techniques were not used straight away in film titles and they were dominated bytext

o Romantic films had swirly texto Horror films would have straight edge font

Titles would only be used to introduce a list of characters to start with The potential of using the title sequence to set the mood of the film had not yet been

explored.

The Charlie Chaplin film City Lights 1931 made a huge step forward in motion graphicso As it made clever use of super imposition by means of optical printers

In 1940 Spook Sport began to use the early animation techniques in a title sequenceo Remarkable for its time

Saul Bass Revolutionized title sequence design in the 1950s As he saw the potential in establishing a mood and atmosphere for the film in the

sequence

He designed the titles for many popular films such as:o The man with the golden arm 1955

Where white lines move around the font in different patterns until the finalimage of a twisted arm is viable

o Anatomy of a Murder 1959 Synced a silhouette of a body with music

o Vertigo 1958 Considered his best work

These title sequences were impressive at the time but primitive it todays standards In the 50s creating title sequences took a lot of time as it has done by hand and cost a lot

of money


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Animation was further used in title sequences in the 60s by Friz Frelengo Who created the title sequence for The pink panther 1963

Moris Binder further developed the field in James Bond title sequences, as did RobertBrown John, who created the From Russia with Love title sequence.

o Using a projector to project text onto dancerso However the dancers had to dance accurately for this to work.

Binder further improved this technique using an optical printer to project colours onto atank in ThunderBall

ALL of the Bond title sequences perfectly reflected the theme of the films In 1995 Kyle Cooper produced the ground breaking title sequence for the movie se7en

o In contrast to the time when Saul Bass was designing titles, Cooper had access totechnology which allowed him to work faster with much less constraints

o Cooper is considered one of the best motion graphics artists in the world at themoment due to his ability to depict the nature and mood of a film

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