This document provides an overview of color theory and color models used in digital images and video. It discusses how the human visual system perceives color and light, and various color spaces such as RGB, YUV, YCbCr. The document also covers color decimation, packing, and conversions between different color formats like 4:4:4, 4:2:2, 4:2:0. Hands-on exercises demonstrate repacking video files between different color models and formats using FFmpeg.

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Hands-on Video CourseHands-on Video Course
Yossi Cohen
June 2013
Lecture 2
RAW Video

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Overview
Intro to color theory
Image Packing
Ex1- Hands on Image packing

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Losless Compression
Model Based Compression
Model
Based
Transform/s
Quantize /
Prioritize Reorder
Entropy
Coding
Sample +Pre
Processing
Bit rate control

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COLOR
Representation
Decimation
Conversion

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Human Visual System
The human eye has two basic light receptors:
Rods – Light Intensity receptors
Cons – Colored light receptors

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Rods and Cons Frequency Reception

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4.2 Color Models in Images
Colors spaces are used to store, display, print
images.
Images are represented using different:
Color spaces
Color Depth
Color Decimation Methods
Pixel/Plane Interleaving methods

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Color Spaces
 Color Spaces
 RGB – Each pixel is provided coordinates in 3D space.
Coordinates are in Red, Blue and Green. This is how we
perceive color
 YUV – Conversion of the color to a Light (Y) based system.
Y and V are Color Complements
 HSV – Hue, Saturation, Value
 YCbCr and more

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RGB->YUV Color Transform
R
G
B
R
B
G
Y
U
V

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Color matching
 How can we compare colors
so that the content creators
and consumers know what
they are seeing?
 Many different ways
including CIE chromacity
diagram

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Video Color Transforms
 Largely derived from older analog methods of coding
color for TV. Luminance is separated from color
information.
 YIQ is used to transmit TV signals in North America and
Japan.This coding also makes its way into VHS video
tape coding in these countries since video tape
technologies also use YIQ.
 In Europe, video tape uses the PAL or SECAM codings,
which are based on TV that uses a matrix transform
called YUV.
 Finally, digital video mostly uses a matrix transform called
YCbCr that is closely related to YUV

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YUV Separation

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YUV Color Model
•YUV codes a luminance signal (for gamma-corrected
signals) equal to Y , the “luma".
•Chrominance refers to the difference between a color
and a reference white at the same luminance. (U and V)
The transform is:

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YIQ Color Model
YIQ is used in NTSC color TV broadcasting.
Again, gray pixels generate zero (I;Q)
chrominance signal.
I and Q are a rotated version of U and V .
The transform is:

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YCbCr Color Model
1. The Rec. 601 standard for digital video uses
another color space YCbCr which closely
related to the YUV transform.
2. The YCbCr transform is used in JPEG image
compression and MPEG video compression.
For 8-bit coding:

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Color Decimation
Rods>>Cons
Spatial Representation(Light) >> SR(Color)
Current Representation of light by 3 coordinates
per pixel waste space.
Color Decimation is done only in Light based color
representation (not in RGB)

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Color Decimation
YUV 4:4:4 – No Color decimation
Used in studio editing
WiFi Display
YUV 4:2:2 – UV are decimated by two
High end video coding
YUV 4:2:0 – UV pixels are decimated by 4
Standard video coding
UY V
UY V
U
Y
V

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Color Packing
We could arrange the “Color” pixels in several
ways:
Packed:
 YUV|YUV|YUV
Planer:
 YYYYYY….UUUUU…..VVVVV
Semi Planer:
 YYYYYYY UVUVUV

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Common Color Packing formats
For video coding well use
Progressive input (and not interleaved)
YUV 4:2:0 Decimation
Planer Color Packing (yuv420p / I420)


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COLOR DECIMATION
PACKING AND CONVERSION
Lab 1

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Open the file with YUVPlayer
Just open it as default what do you get?

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YUV Player
Did you got this? Why?
1. It seems that YUV file-format does not hold
format information
2. There are many YUV formats
What to do?

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Play & Probe file
FFPLAY akiyo_cif.yuv
Cant play missing Image size
FFPlay –s 352x288 akyo_cif.yuv
ffprobe.exe -video_size 352x288 akiyo_cif.yuv
Input #0, rawvideo, from 'akiyo_cif.yuv':
Duration: 00:00:12.00, start: 0.000000, bitrate: 30412 kb/s
Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p,
352x288, 30412 kb/s, 25 tbr, 25 tbn, 25 tbc

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Play & Probe file
FFPLAY akiyo_cif.y4m
FFProbe akiyo_cif.y4m
Input #0, yuv4mpegpipe, from 'akiyo_cif.y4m':
Duration: N/A, bitrate: N/A
Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p,
352x288, SAR 128:117 DAR 1408:1053, 29.97 fps, 29.97
tbr, 29.97 tbn, 29.97 tbc

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Back to YUV Player
Select YUV420
And if we need it in UYVY (YUV422 Packed)
ffmpeg -s 352x288 -i akiyo_cif.yuv -pix_fmt
uyvy422 akiyo_cif_uyvy.yuv

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Ex 1
Create Akiyo YUV file which is opened using
YUV422 option of YUVPlayer
What is the difference between this 4:2:2 format
and the previous one?
Can we open this format using Y only setting?
If not create a Y only file
Is there an option for a YUV file with headers?
Save akiyo in raw YUV with headers

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EX1 answers
Check FFMPEG –pix_fmts
ffmpeg -s 352x288 -i akiyo_cif.yuv -pix_fmt
yuv422p akiyo_cif_422p.yuv
ffmpeg -s 352x288 -i akiyo_cif.yuv -pix_fmt gray
akiyo_cif_y.yuv

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Y4M filel Format
Y4M is a YUV file with a header.
Header states
YUV decimation and packing
Frame resolution
FPS