COMPUTER GRAPHICSCS 482 – FALL 2015

OCTOBER 6, 2015IMAGE MANIPULATION• COMPRESSION• COMPOSITING

COMPRESSION

CS 482 – FALL 2015

JPEG

OCTOBER 6, 2015: IMAGE MANIPULATION PAGE 2

THE JOINT PHOTOGRAPHIC EXPERT GROUP (JPEG), A VIDEO

STANDARDS ORGANIZATION, HAS DEVELOPED

A TECHNIQUE FOR

COMPRESSING STILL IMAGES BY EXPLOITING THE

LOCALIZED COHERENCE OF MOST IMAGES.

UNCOMPRESSED

COMPRESSED TO 33%

COMPRESSED TO 22%

COMPRESSED TO 9%

COMPRESSION

CS 482 – FALL 2015

JPEG’S ZIG-ZAG RUN-LENGTH ENCODING

OCTOBER 6, 2015: IMAGE MANIPULATION PAGE 3

-49 00 0-12 00 0

-16 00 09 00 0

-1 00 0-1 00 0

0 00 00 00 0

-1 00 00 00 0

-1 00 00 00 0

1 00 00 00 0

0 00 00 00 0

JPEG USES A ZIGZAG PATTERN TO IMPLEMENT ITS RUN-LENGTH ENCODING OF PIXEL INFORMATION, THUS TAKING ADVANTAGE OF BOTH THE HORIZONTAL AND THE VERTICAL COHERENCE OF MOST IMAGES.

STEP 1: SPLIT THE IMAGE INTO ITS RED, GREEN, AND BLUE PLANES.

STEP 2: BREAK EACH COLOR PLANE INTO 8X8 BLOCKS OF PIXELS.

STEP 3: APPLY DCT TO EACH 8X8 BLOCK.

-784 00 0

-164 00 0

-164 00 0

137 00 0

-16 00 0-21 00 0

-19 00 00 00 0

-16 00 0-19 00 0

-21 00 011 00 0

48 00 0-9 00 0

-9 00 023 00 0

STEP 4: QUANTIZE THE ARRAY TO REDUCE ITS SIZE IN MEMORY. 16 11

12 1214 1314 17

10 1614 1916 2422 29

24 4028 5640 5751 87

51 6160 5569 5680 62

18 2224 3549 6472 92

37 5655 6478 8795 98

68 10981 104103 121112 100

103 77113 92120 101103 99

-49 00 0-12 00 0

-16 00 09 00 0

-1 00 0-1 00 0

0 00 00 00 0

-1 00 00 00 0

-1 00 00 00 0

1 00 00 00 0

0 00 00 00 0

-784 00 0

-164 00 0

-164 00 0

137 00 0

-16 00 0-21 00 0

-19 00 00 00 0

-16 00 0-19 00 0

-21 00 011 00 0

48 00 0-9 00 0

-9 00 023 00 0

STEP 5: USE ZIGZAG PATTERN TO YIELD A VECTOR; USE RUN-LENGTH ENCODING TO COMPRESS IT FOR TRANSMITTING.

=

COMPRESSION

CS 482 – FALL 2015

GIF

OCTOBER 6, 2015: IMAGE MANIPULATION PAGE 4

THE GRAPHICS INTERCHANGE FORMAT (GIF) APPLIES LEMPEL-ZIV-WELCH (LZW) COMPRESSION TO AN IMAGE, SEEKING RECURRING SEQUENCES OF

COLORS.FOR EXAMPLE, THE FOLLOWING

SEQUENCE OF 33 COLORED PIXELS:

OCCURS AT SEVERAL PLACES WITHIN THE IMAGE.THE MOST COMMON LENGTHY

SEQUENCES ARE REPLACED WITH SPECIAL CODES, POTENTIALLY

REDUCING THE IMAGE FILE’S SIZE SIGNIFICANTLY.UNLIKE JPEG, GIF IS A LOSSLESS

COMPRESSION TECHNIQUE, BUT IT HAS THE DISADVANTAGE OF ONLY

WORKING WITH IMAGES CONTAINING NO MORE THAN 256 DIFFERENT

COLORS.

COMPRESSION

CS 482 – FALL 2015

PNG

OCTOBER 6, 2015: IMAGE MANIPULATION PAGE 5

THE PORTABLE NETWORK GRAPHICS (PNG) STANDARD ATTEMPTS TO USE SPATIAL COHERENCE TO REDUCE THE RANGE OF PIXEL COLOR VALUES

BEFORE COMPRESSING.THIS IS ACCOMPLISHED BY SUBTRACTING SOME COMBINATION OF THE COLORS IN THE PIXELS TO THE NORTH, WEST, AND NORTHWEST OF THE PIXEL BEING

“FILTERED”.PNG THEN APPLIES A “DEFLATION” ALGORITHM, SIMILAR

TO GIF’S LZW ALGORITHM, TO COMPRESS THE MOST COMMON LENGTHY PATTERNS OF COLOR SEQUENCES.

FINALLY, PNG COMPRESSES EVEN FURTHER BY APPLYING HUFFMAN ENCODING TO THE RESULTS OF THE DEFLATION.

COMPOSITING

CS 482 – FALL 2015

FOREGROUND AND BACKGROUND

OCTOBER 6, 2015: IMAGE MANIPULATION PAGE 6

TO COMBINE IMAGES THAT HAVE BEEN RENDERED SEPARATELY, COMPOSITING TECHNIQUES ARE USED.

BACKGROUNDA MATTE PAINTING OF THE HALLWAY CORRIDOR DOWN WHICH THE ACTUAL ACTOR RAN DURING

THE SCENE.

REFERENCE SPHEREA CHROME SPHERE IS FILMED ON THE

ACTUAL SET IN THE ACTOR’S APPROXIMATE POSITION IN ORDER TO DERIVE LIGHTING

DIRECTION.

ACTORTHE ACTOR IS FILMED, WITHOUT MAKEUP, CONDUCTING THE SCENE.

COMPOSITING

CS 482 – FALL 2015

ILLUMINATION

OCTOBER 6, 2015: IMAGE MANIPULATION PAGE 7

DIFFUSE REFLECTIONCOMBINES THE MAIN LIGHT

SOURCE AND THE BACK LIGHT TO APPROXIMATE LIGHT BEHAVIOR AS IT HITS MICROSCOPICALLY ROUGH

SURFACES.

SPECULAR REFLECTIONEXPRESSES WETNESS OF

CHARACTER’S SKIN BY APPROXIMATING HOW SCENE

LIGHT’S WOULD REFLECT OFF A SMOOTH SURFACE.

COMPOSITING

CS 482 – FALL 2015

MATERIAL PROPERTIES

OCTOBER 6, 2015: IMAGE MANIPULATION PAGE 8

REFLECTIONREFLECTION OF THE NON-LIGHT

OBJECTS SURROUNDING THE CHARACTER, PERHAPS EVEN

INCLUDING CHARACTER SELF-REFLECTION.

COLORTHE VARIOUS

COMPONENTS OF THE CHARACTER’S BODY HAVE

DIFFERENT COLORS MAPPED TO THEM.

COMPOSITING

CS 482 – FALL 2015

RADIANCE

OCTOBER 6, 2015: IMAGE MANIPULATION PAGE 9

AMBIENT ILLUMINATIONTHE CONTRIBUTION OF LIGHT

BOUNCING AROUND IN ALL DIRECTIONS WITHIN THE

CHARACTER’S ENVIRONMENT IS SIMULATED.

SUBSURFACE SCATTERINGWHEN LIGHT ENTERS A

TRANSLUCENT OBJECT (IN THIS CASE, SKIN), IT BOUNCES AROUND SOMEWHAT AND

THEN EXITS.

COMPOSITING

CS 482 – FALL 2015

CAMERA TRICKS

OCTOBER 6, 2015: IMAGE MANIPULATION PAGE 10

Z-DEPTH PASSTHE DISTANCE FROM THE

CAMERA OF EACH SURFACE POINT IS DETERMINED TO MASK FOR COLOR CORRECTION AND

APPLY DEPTH DEFOCUS.

FRESNEL REFLECTIONHIGHLIGHTS AROUND THE BORDER OF SURFACES ARE ADJUSTED BY DETERMINING THE SURFACE ANGLE WITH RESPECT TO THE CAMERA.

COMPOSITING

CS 482 – FALL 2015

FINISHING TOUCHES

OCTOBER 6, 2015: IMAGE MANIPULATION PAGE 11

VEIN MATTEVEINS ON THE TORSO, NEAR THE EYES,

AND ON THE NECK AND HEAD ARE KEPT IN DIFFERENT CHANNELS TO

PROVIDE SEPARATE CONTROL MECHANISMS TO THE ARTIST.

FINAL COMPOSITE