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COMPRESSION CS 482 – FALL 2015 JPEG OCTOBER 6, 2015: IMAGE MANIPULATIONPAGE 139 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. UNCOMPRESS ED COMPRESSED TO 33% COMPRESSED TO 22% COMPRESSED TO 9%
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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
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