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Digital Media. Lecture 10: Video & Compression Georgia Gwinnett College School of Science and Technology Dr. Jim Rowan. Coping with Video Size. Consider human vision limitations 1) Chrominance sub-sampling Compression - two versions 2) Spatial 3) Temporal differencing vectoring.

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Digital media

Digital Media

Lecture 10: Video & Compression

Georgia Gwinnett College

School of Science and Technology

Dr. Jim Rowan


Coping with video size

Coping with Video Size

Consider human vision limitations

1) Chrominance sub-sampling

Compression - two versions

2) Spatial

3) Temporal

differencing

vectoring


Coping with video size1

Coping with Video Size

Consider human vision limitations

1) Chrominance sub-sampling

Compression - two versions

3) spatial

4) Temporal

differencing

vectoring


Chrominance sub sampling

Chrominance sub-sampling

  • Humans can’t distinguish changes in color as well as they can distinguish luminance changes

    • http://en.wikipedia.org/wiki/Chroma_subsampling

  • In our cameras…

    • Of every 4 frames

    • store the luminance for each frame

    • only store a proportion of the color info

    • 4:2:0


Chrominance sub sampling1

Chrominance sub-sampling

http://dougkerr.net/pumpkin/articles/Subsampling.pdf


Chrominance sub sampling luminance cr cb

Chrominance sub-samplingLuminance, Cr, Cb

http://en.wikipedia.org/wiki/Chroma_subsampling#Sampling_systems_and_ratios


Coping with video size2

Coping with Video Size

  • Aside from screen size and frame rate...

  • Consider human vision limitations

    1) Chrominance sub-sampling

  • Compression - two versions

    2) spatial

    3) Temporal

    differencing

    vectoring


Coping with video size3

Coping with Video Size

  • Spatial compression

    • Individual images can be compressed using the techniques discussed in the bitmapped section

    • Doesn’t result in very much compression for video

    • Doesn’t take into consideration the other frames that come before or after it

    • Our video cameras do this, compressing each frame to jpeg


Coping with video size4

Coping with Video Size

  • Aside from screen size and frame rate...

  • Consider human vision limitations

    1) Chrominance sub-sampling

  • Compression - two versions

    2) spatial

    3) Temporal

    differencing

    vectoring


Temporal compression differencing

Temporal Compression differencing

  • Use the Difference in two frames

    • A naive approach can result in good compression

    • Works well for a small amount of movement

      • Security cameras spend most of their time “seeing” the same thing all night long

    • A Tarantino film? not so much…

      • Most pixels change with nearly every frame


Image differencing

Image Differencing

  • To subtract one image from the next

  • Do it one pixel at a time

    • red minus red

    • green minus green

    • blue minus blue

  • Store the difference

  • To play it back

    • Play frame one 1

    • Add frame 2 to frame 1

  • Next, an example in black & white


Temporal compression differencing1

Temporal Compression differencing


Digital media

Example 1, the difference of two identical images


Example 1 the difference of two identical images

Example 1, the difference of two identical images

The result ===>


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Example 2, the difference of two similar images


Example 2 the difference of two similar images

Example 2, the difference of two similar images

The result ===>


Temporal compression vectoring

Temporal Compressionvectoring

  • When an OBJECT moves

    • compute its trajectory

    • fill in the resulting exposed background

Captured

Frame 1

Captured

Frame 2

Stored

Background

Stored

Object

Movement

Vector

  • BUT there’s a problem...

  • why isn’t this an easy thing to do?


More on differencing

More on differencing…

  • The differencing can happen in a forward manner and a backward manner

  • It might be more economical (in data size) to create a frame from a frame that follows it...


Mpeg terminology

MPEG terminology

MPEG-2

iFrame (interframe prediction)

pFrame (forward predicted)

bFrame (backward predicted)

GOP (group of pictures)

http://en.wikipedia.org/wiki/Inter_frame

http://en.wikipedia.org/wiki/I-frames#Intra_coded_frames_.28or_slices_or_I-frames_or_Key_frames.29


Mpeg terminology1

MPEG terminology

iFrame:

-a keyframe

-spatially compressed

(a fully specified image)

pFrame

-predicted frame

-contains only the difference

between the current frame and

the previous iFrame

(smaller in size than iFrame)

bFrame

-bi-predicted frame

-contains difference between

current and both the preceding

and following iFrames

-even smaller in size than iFrame


Mpeg terminology2

MPEG terminology

Group(s) Of Pictures (GOPs)

All spatially (intra-frame)

compressed

The largest

Smaller…

but more

computation

Spatially compressed

and predictive

(difference)

Spatially compressed forward and backward predictive

(difference)

Smallest…

but more

computation

and it is

transported

out of order!

Play sequence: 1 2 3 4 5 6 7 8 9 10

Transmit sequence: 1 4 2 3 7 5 6 10 8 9


Digital media

So…

How does this ===>

happen?


Digital media

http://en.wikipedia.org/wiki/Inter_frame


Video compression what does it

Video CompressionWhat does it?

  • Coder/Decoder - Codec

    • encodes and decodes video

  • Can be symmetric

    • it takes as long to compress as decompress

  • Can be asymmetric

    • it takes longer to compress or decompress than it does to decompress to compress


  • Video compression which codec to choose it s a tradeoff

    Video Compression:Which codec to choose?It’s a tradeoff…

    • compression technique

      • sorenson

      • DV

      • Cinepak

      • Intel Indeo

      • MPEG4

  • the compression result

    • asymmetric or symmetric

      • satellite provider’s choice?

    • larger or smaller final size

    • computational complexity

    • the artifacts generated


  • Which is the original notice the artifacts

    Which is the original?Notice the artifacts?

    http://www.100fps.com/codec_quality_comparison.htm


    Center one is the original left is sharpened right is blurred

    Center one is the originalLeft is “sharpened” Right is “blurred”

    http://www.100fps.com/codec_quality_comparison.htm


    Retrograde motion

    Retrograde motion

    http://wiki.ggc.edu/images/8/85/JrowanSpring2012CroppedRetrogradeMotionClip.mov

    http://wiki.ggc.edu/images/3/35/RetrogradeGMCarcadiaBroadbandHigh.mov


    Two ways to make moving pictures video animation

    Two ways to make Moving Pictures:Video & Animation

    • In this class:

      • Video

        • shot with a camera

        • captures images from the world

        • then play them back

      • Animation

        • create frames individually

        • using inkscape and blender

        • play them back


    Reading i n the supplemental text

    Reading in the supplemental text:

    • Moving images: Video


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