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COMPUTER GRAPHICS

COMPUTER GRAPHICS. CS 482 – FALL 2014. SEPTEMBER 29, 2014. IMAGE MANIPULATION. COMPRESSION COMPOSITING. COMPRESSION. JPEG.

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COMPUTER GRAPHICS

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  1. COMPUTER GRAPHICS CS 482 – FALL 2014 SEPTEMBER 29, 2014 IMAGE MANIPULATION • COMPRESSION • COMPOSITING

  2. COMPRESSION JPEG 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% CS 482 – FALL 2014 SEPTEMBER 29, 2014: IMAGE MANIPULATION PAGE 138

  3. COMPRESSION -784 -49 -784 -49 16 0 0 11 0 0 -16 -16 10 -164 -164 0 0 0 16 0 -1 -1 -16 24 -16 0 0 40 0 0 51 -19 0 0 -19 0 0 61 0 0 12 0 0 0 0 0 0 12 0 0 0 0 0 0 14 0 0 19 0 0 0 0 28 0 0 0 0 0 0 56 0 0 0 0 60 0 0 0 55 0 -164 -12 -12 -164 14 0 0 0 0 13 9 16 137 137 9 0 0 0 0 24 -1 40 -21 -1 -21 0 0 0 57 0 0 0 69 0 0 0 56 0 0 0 0 0 14 0 0 0 0 0 17 0 0 0 22 0 0 0 29 0 0 0 51 0 0 0 0 0 0 0 0 87 0 0 0 0 80 0 62 0 0 0 18 -16 -1 -16 -1 0 0 22 0 0 -1 -21 -1 -21 37 56 0 0 0 0 1 48 48 68 1 0 0 109 0 0 0 0 103 -9 -9 0 0 0 0 77 0 0 0 24 0 35 0 0 0 0 0 0 0 55 0 0 64 0 0 0 0 0 0 81 0 0 0 104 0 0 0 113 0 0 0 0 92 0 0 0 -19 0 -19 49 0 0 0 64 0 0 0 11 78 11 0 0 0 87 0 0 103 0 0 -9 -9 0 121 0 0 0 23 0 0 23 120 0 0 0 101 0 0 72 0 0 0 0 92 0 0 0 0 0 0 95 0 0 0 98 0 0 0 112 0 0 0 100 0 0 0 0 0 0 103 0 0 0 0 0 0 99 JPEG’S ZIG-ZAG RUN-LENGTH ENCODING 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. Step 4: Quantize the array to reduce its size in memory.  Step 5: Use zigzag pattern to yield a vector; use run-length encoding to compress it for transmitting. = CS 482 – FALL 2014 SEPTEMBER 29, 2014: IMAGE MANIPULATION PAGE 139

  4. COMPRESSION GIF The Graphics Interchange Format (GIF) applies Lempel-Ziv-Welch (LZW) compressionto 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.. CS 482 – FALL 2014 SEPTEMBER 29, 2014: IMAGE MANIPULATION PAGE 140

  5. COMPRESSION PNG 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. CS 482 – FALL 2014 SEPTEMBER 29, 2014: IMAGE MANIPULATION PAGE 141

  6. COMPOSITING FOREGROUND AND BACKGROUND To combine images that have been rendered separately, compositing techniques are used. Background A matte painting of the hallway corridor down which the actual actor ran during the scene. Actor The actor is filmed, without makeup, conducting the scene. Reference Sphere A chrome sphere is filmed on the actual set in the actor’s approximate position in order to derive lighting direction. CS 482 – FALL 2014 SEPTEMBER 29, 2014: IMAGE MANIPULATION PAGE 142

  7. COMPOSITING ILLUMINATION Diffuse Reflection Combines the main light source and the back light to approximate light behavior as it hits microscopically rough surfaces. Specular Reflection Expresses wetness of character’s skin by approximating how scene light’s would reflect off a smooth surface. CS 482 – FALL 2014 SEPTEMBER 29, 2014: IMAGE MANIPULATION PAGE 143

  8. COMPOSITING MATERIAL PROPERTIES Reflection Reflection of the non-light objects surrounding the character, perhaps even including character self-reflection. Color The various components of the character’s body have different colors mapped to them. CS 482 – FALL 2014 SEPTEMBER 29, 2014: IMAGE MANIPULATION PAGE 144

  9. COMPOSITING RADIANCE Subsurface Scattering When light enters a translucent object (in this case, skin), it bounces around somewhat and then exits. Ambient Illumination The contribution of light bouncing around in all directions within the character’s environment is simulated. CS 482 – FALL 2014 SEPTEMBER 29, 2014: IMAGE MANIPULATION PAGE 145

  10. COMPOSITING CAMERA TRICKS Z-Depth Pass The distance from the camera of each surface point is determined to mask for color correction and apply depth defocus. Fresnel Reflection Highlights around the border of surfaces are adjusted by determining the surface angle with respect to the camera. CS 482 – FALL 2014 SEPTEMBER 29, 2014: IMAGE MANIPULATION PAGE 146

  11. COMPOSITING FINISHING TOUCHES Vein Matte Veins 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 CS 482 – FALL 2014 SEPTEMBER 29, 2014: IMAGE MANIPULATION PAGE 147

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