Image encodings CSC 1040 Computing with Images 1
- Slides: 22
Image encodings CSC 1040 - Computing with Images 1
Last time: Color basics Additive primaries • • Start with black background, • • use a hard edged brush of large diameter and set mode to “Difference” • • Apply red, green, blue Subtractive primaries Start with white background, use a hard edged brush of large diameter and set mode to “Darken” Apply cyan, yellow, magenta CSC 1040 - Computing with Images 2
y=9 red=108 green=86 Color: (108, 86, 142) Position: (12, 9) x = 12 CSC 1040 - Computing with Images 3 blue
Today: Pixel encodings Bitmap 1 bit Grayscale 8 bits CSC 1040 - Computing with Images RGB Color 3 colors: red, green, blue 8 bits/color 24 bits 4
1 byte = 8 bits How much can we encode in 8 bits? • Let’s walk it through. – If we have one bit, we can represent two patterns: 0 and 1. – If we have two bits, we can represent four patterns: 00, 01, 10, and 11. – If we have three bits, we can represent eight patterns: 000, 001, 010, 011, 100, 101, 110, 111 CSC 1040 - Computing with Images 5
In n bits, we can have n 2 patterns – In 8 bits, we can have 28 patterns, or 256 – Values can be encoded as numbers from 0. . 255 CSC 1040 - Computing with Images 6
Encoding RGB • Each component color (red, green, and blue) is encoded as a single byte • Colors go from (0, 0, 0) to (255, 255) – If all three components are the same, the color is in greyscale • (50, 50) at (2, 2) – (0, 0, 0) (at position (1, 2) in example) is black – (255, 255) is white CSC 1040 - Computing with Images 7
Is that enough? • We’re representing color in 24 (3 * 8) bits. – That’s 16, 777, 216 (224) possible colors – Our eye can discern millions of colors - so probably pretty close • Some graphics systems support 32 bits per pixel – Maybe more pixels for color, or an additional 8 bits to represent 256 levels of translucence Remember: An image that uses 24 bit color is 24 times the size of a bitmap image of the same resolution CSC 1040 - Computing with Images 8
File Size File size is usually measured in: bits, bytes, etc: • • • Byte = 8 bits Kilobyte = 1024 bytes (= 210 bytes) Megabyte = 1024 KB (= 210 KB or 220 bytes ) Gigabyte = 1024 MB (= 210 MB or 230 bytes) Terabyte = 1024 GB (= 210 GB or 240 bytes) … CSC 1040 - Computing with Images 9
Image size • Image size: – Pixels (eg: 300 x 200) – Inches/centimeters (eg: 3 x 2 inches) • Resolution: #pixels/inch (eg: 100 pixels/inch) CSC 1040 - Computing with Images 10
Increasing Image Print Size • Image size: – Pixels (eg: 300 x 200) – Inches/centimeters (eg: 33 xx 2 inches) CSC 1040 - Computing with Images 6 x 4 11
Increasing Image Print Size • Image size: Same pixels, NO Resampling – Pixels (eg: 300 x 200) – Inches/centimeters (eg: 33 xx 2 inches) • Resolution: #pixels/inch 6 x 4 (eg: 100 pixels/inch) 50 CSC 1040 - Computing with Images 12
Decreasing Image Print Size • Image size: – Pixels (eg: 300 x 200) – Inches/centimeters (eg: 33 xx 2 inches) 1. 5 x 1 • Resolution: #pixels/inch (eg: 100 pixels/inch) 200 CSC 1040 - Computing with Images 13
Changing the Number of Pixels (Resampling image) • Image size: – Pixels 300 x 200) 300 x 200 150 x 100 – Inches/centimeters (eg: 3 x 2 inches) • Resolution: #pixels/inch (eg: 100 pixels/inch) 50 CSC 1040 - Computing with Images 14
Photoshop Image Size Menu Actual pixels Print size Lock aspect ratio Changes the actual pixels CSC 1040 - Computing with Images 15
Pixel encodings Bitmap 1 bit Grayscale 8 bits CSC 1040 - Computing with Images RGB Color 3 colors: red, green, blue 8 bits/color 24 bits 16
RGB Color File Size Example: Image size: 300 x 200 pixels • Total pixels = 300 x 200 pixels = 60, 000 pixels RGB color image 24 bits (3 bytes)/pixel • File size = 60, 000 x 3 bytes (3 bytes/pixel ) = 180, 000 …. . since there are 1024 bytes/KB: = 175. 8 KB CSC 1040 - Computing with Images 17
Grayscale File Size Example: Image size: 300 x 200 pixels • Total pixels = 300 x 200 pixels = 60, 000 pixels Grayscale image 8 bits (1 byte)/pixel • File size = 60, 000 bytes (1 byte/pixel ) …. . since there are 1024 bytes/KB: = 58. 6 KB CSC 1040 - Computing with Images 18
Bitmap File Size Example: Image size: 300 x 200 pixels • Total Pixels = 300 x 200 pixels = 60, 000 pixels Bitmap image 1 bit/pixel • File size = 60, 000 bits (1 bit/pixel ) …. . Since there are 8 bits/byte: • File size = 60, 000 / 8 bytes = 7500 bytes …. . since there are 1024 bytes/KB: = 7. 3 KB CSC 1040 - Computing with Images 19
Comparing file sizes for 300 x 200 image: Bitmap 1 bit 7. 3 KB Grayscale 8 bits (1 byte) 58. 6 KB CSC 1040 - Computing with Images RGB Color 3 colors: red, green, blue 24 bits (3 bytes) 175. 8 KB 20
Size of images: More Examples Type of image Bits/pixel 320 x 240 image 640 x 480 image 1024 x 768 image Bitmap 1 76, 800 bits =9, 600 bytes =9. 375 KB 307, 200 bits =38, 400 =37. 5 KB 786, 432 bits =98, 304 bytes =96 KB Grayscale 8 (1 byte) 614, 400 bits =76, 800 bytes =75 KB 2, 457, 600 bits = 307, 200 bytes =300 KB 6291456 bits 786, 432 bytes 768 KB RGB color 24 (3 bytes) 225 KB 900 KB 2, 304 KB =2. 25 MB 32 bit color (4 bytes) 32 (4 bytes) 300 KB 1200 KB 3, 072 KB = 3 MB CSC 1040 - Computing with Images 21
Next Time: Image Compression • Goal: Use fewer bytes to encode the same or similar information • Reduce redundancy • Take advantage of human visual system – don’t bother differentiating things that are unnoticeable • Lose unimportant details CSC 1040 - Computing with Images 22
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