Computer and Machine Vision

Transcription

1 Computer and Machine Vision Introduction to Continuous Camera Capture, Sampling, Encoding, Decoding and Transport January 22, 2014 Sam Siewert

2 Video Camera Fundamentals Overview Introduction to Codecs (Encode/Decode for Digital Media) Digital Sampling Color Models Color: Luminance/Chrominance Digital Video Encoding Pixel (POINT) Frame (XY Pixel Map) Sequence of Frames (Group of Pictures) Digital Media Systems Encoding, Transport, Decoding MPEG Basics and Standards Sam Siewert 2

3 CCD Frame Grabbers Bt878 Micro-programmable NTSC Capture Luminance and Chrominance ADC Channels YPrPb (YCrCb) We Will See how Y is Gray, and PrPb are Red and Blue Bands that are Sub-sampled CCD Collects Charge Over Time Through Exposure to Photons (Light field) Integration Time Charge is Read Out Through ADC Channels Sam Siewert 3

4 Analog NTSC Signal for Camera As Seen on Agilent MSO Composite Luminance and Chorma Signals at 6MhZ Interlaced ODD/EVEN line raster Sam Siewert 4

5 Rev-A Computational Photometer Board Dual TI NTSC Decoders and FTDI Uplink Plugs Into DE0, DE2i and DE4 Terasic Boards GPIO Header(s) Can Uplink to DE2i or any Embedded Linux Board Via USB 2.0 Reference Startix State Machine, FIFO, and Transforms Sam Siewert 5

6 Example NTSC Digital Output As Seen on Texas Instruments Decoder Digital Output for YCrCb by Intronix Logic Analyzer Clock, Data, Control at Logic Levels 6 MhZ NTSC decode, Sampled at 10MhZ or Higher Sam Siewert 6

7 Real-Time Video Codecs and Tools Codec = Compression, Decompression (Encode/Decode) Basic Inputs to Encoder (Original Uncompressed Data) POINT or Pixel: RGB and YCrCb Color Encoding MACRO BLOCK: Portion of a frame (e.g. 8x8 pixels) FRAME: XY Spatial Map of Pixels (Aspect Ratio, Resolution) SEQUENCE: Group of Pictures (GoP) I-Frame Single Frame Compression (Start) Forward/Backward Difference Images Uncompressed Images Portable Pixmap (PPM) Leverage HW and SW MPEG Encoder/Decoder VLC Media Player ffmpeg - Sam Siewert 7

8 Color Considerations 1. Physics Visible Spectrum 2. Physiology What the Human Eye is Sensitive to (Rods and Cones) and what the Brain Perceives 3. Psychology What Looks Good 4. Technology Palette and Methods of Display and Projection of Light Sam Siewert 8

9 POINT or PIXEL: RGB Color Model RGB, 24-bit, 8 bits [0-255] for each color band (x, y, z) sampled Each Pixel is a 3-D Vector in RGB Space, Opponent Colors Blue Cyan Magenta White Black Green Red Yellow Sam Siewert 9

10 Discussion What Does Eye See? Color Models (E.g. CIE Rec 709) RGB Cube HSV - Hue/Saturation/Value Hue Similarity to R, G, Y, B Saturation Color vs. Brightness Value Low=Black, High=Color Luminance (Candela/Square-Meter) Light Passing Through Area Forming a Solid Angle in A Direction Candela (Photonic Power )= Watts/Steradian More Precise than Brightness Chrominance ( CrCb or UV in YCrCb or YUV) U=Blue Luminance (Y) V=Red - Luminance (Y) Wavelength Spectrum - ROYGBIV RGB Cube HSV Cylinder/Cone Sam Siewert 10

11 YUV ITU-R BT.601 Component Video Standard Y is Luma UV is Color information designed so that BW TV (CRT) will still display grayscale image Used by NTSC, PAL, SECAM (Standard Definition TV) RGB to YUV Conversion Y = (0.299 * R) + (0.587 * G) + (0.114 * B) U = -(0.147 * R) - (0.289 * G) + (0.436 * B) V = (0.615 * R) - (0.515 * G) - (0.100 * B) YUV to RGB Conversion R = Y V G = Y (0.394 * U) (0.581 * V) B = Y + (2.032 * U) Sam Siewert 11

12 POINT: R, G, or B band only vs. Balance R G B Balanced Y = 0.3R G B Sam Siewert 12

13 POINT: YCrCb RGB An Alternative to RGB is YUV, Where Y is Luminance and CrCb is Chrominance The following 2 sets of formulae are taken from information from Keith Jack's excellent book "Video Demystified" (ISBN ). RGB to YCrCb Conversion (For Computers with RGB [0-255]) Y = (0.257 * R) + (0.504 * G) + (0.098 * B) + 16 Cr = (0.439 * R) - (0.368 * G) - (0.071 * B) Cb = -(0.148 * R) - (0.291 * G) + (0.439 * B) YCrCb to RGB Conversion R = 1.164(Y - 16) (Cr - 128) G = 1.164(Y - 16) (Cr - 128) (Cb - 128) B = 1.164(Y - 16) (Cb - 128) In both these cases, you have to clamp the output values to keep them in the [0-255] range. Sam Siewert 13

14 POINT: YCrCb 4:4:4 24-bit Format For every Y sample in a scan-line, there is also one CrCb sample Each Y (Y7:Y0), Cr (Cr7:Cr0), & Cb (Cb7:Cb0) Sample is 8 bits No compression between RGB and YCrCb 4:4: ,480 76,799 = Y, Cr, and Cb sample = Y sample only Sam Siewert 14

15 POINT: YCrCb 4:2:2 16-bit Format For every 2 Y samples in a scan-line, there is one CrCb sample bit to 32 bit 76,480 76,799 = Y, Cr, and Cb sample = Y sample only Pixel-0 = Y7:Y0 0, Cb7:Cb0 0 ; Pixel-1 = Y7:Y0 1, Cr7:Cr0 0 Pixel-2 = Y7:Y0 2, Cb7:Cb0 1 ; Pixel-3 = Y7:Y0 3, Cr7:Cr0 1 Pixel-4 = Y7:Y0 4, Cb7:Cb0 2 ; Pixel-5 = Y7:Y0 5, Cr7:Cr0 2 Sam Siewert 15

16 FRAME: XY Pixel Maps FRAME Resolution Computer Graphics Resolutions VGA = 640x480 SVGA=800x600 NTSC = Standard Defintion, 720x480 High Definition (Progressive full frame or Interlaced) 720i/p = 1280x i/p = 1920x1080 FRAME Aspect Ratios X to Y Ratio NTSC = 3:2, 720x480 (240) HD 720 = 16:9, 1280x720 (80) HD 1080 = 16:9, 1920x1080 (120) 2K = 17:9, 2048x1080 (120) FRAME Rates 60i, NTSC = odd/even, or p, Cinema = 24 fps (motion blur) 60p, HDTV Progressive Modes Sam Siewert 16

17 Display and Camera Resolutions Red Epic 645 9K: 9334x7000, Red Epic K: 28000x9334 Sam Siewert 17

18 SEQUENCE: Series of Frames in Encoded Group of Pictures I-Frame Initial Frame in GoP, Compression Within Frame Only P-Frame Predicted Frame B-Frame Bi-Directional Interpolated Frame (Differences Between Last I- Frame and Next P-Frame or I-Frame) Sam Siewert 18

19 Uncompressed Streams Used Only in Digital Cinema and Computer / Machine Vision True Color (RGB, 8-bits Each), YCrCb (16-bits Each Pixel) Uncompressed Lossless or Lossy I-Frame Only MJPEG ( JPEG 2000 ( ) Not Practical for Digital Video Transport Standard Definition 720x480 30fps Requires 30MB/sec High Definition 720p (1280x720x3 = 30fps = 80MB/sec High Defintion 1080p (1920x1080x3 = 30fps = 178MB/sec DCI 2K, 2048x1080x3 24fps = 152MB/sec DCI 4K, 4096x2160x3 24fps = 607.5MB/sec Sam Siewert 19

20 MPEG2 Encode/Decode Introduction Sam Siewert 20

21 Group of Pictures: High Level View Sam Siewert 21

22 MPEG-2: Order Of Operators POINT (Pixel) Encoding Macro-Block Lossy Intra-Frame Compression Motion-Based Compression in Group of Pictures Sam Siewert 22