by bob “the bird” fiske & anita “the snail” cost
TRANSCRIPT
![Page 1: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/1.jpg)
By Bob “The Bird” Fiske & Anita “The Snail” Cost
![Page 2: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/2.jpg)
Bitmaps & the brightness grayscale Color pictures are red, green & blue Storing bitmaps as files Compressing bitmap data
![Page 3: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/3.jpg)
A bitmap is not a bit of a map or maybe it is. It might be a map bit but not in the
computer world A bitmap is an image made up of tiny
little pixels, not to be confused with pixies. Pixels are tiny dots of color within a
computer image
![Page 4: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/4.jpg)
A grayscale image is what you might call a “black and white” picture
The grayscale determines the brightness of each pixel Each pixel is a number between 0 and 255.
This represents a scale of brightness going from black (0) to white (255) and shades of gray (the numbers in between).
![Page 5: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/5.jpg)
First row of a bitmap picture:
A lower down row in the picture:
0 0 1 6 18 20 20 20 20 20 24 24 24 24
148 152 200 210 210 210 210 150 150 100 78 60 60 60
![Page 6: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/6.jpg)
To get a color image, you combine 3 primary colors. The commonly used colors are RGB (red, green and
blue). When we show a color picture on the computer,
we are combining 3 black and white images By sending them through the 3 color channels (RGB).
![Page 7: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/7.jpg)
How do they do it? In essence, you make 3 black and white images of the picture filtered through a red filter, filtered through a green filter, and filtered through a blue filter.
Red image Green image Blue image
![Page 8: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/8.jpg)
Red channel Green channel Blue channel
Combine to create…
![Page 9: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/9.jpg)
![Page 10: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/10.jpg)
Each grayscale pixel uses 1 byte of storage This is the same as 8 bits
A bit stores either a 1 or a 0 8 bits are necessary to encode the numbers 0 to 255 8 bits = 1 byte
If I have 1000 pixels, that is 1 kilobyte (KB) Technically, a KB is actually 1024 bytes
Computer math is funny. Ha ha!
![Page 11: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/11.jpg)
If I have a color image that has 245 pixels per row, and 200 rows The number of pixels is:
245 x 200 = 49,000 pixels But it's a color image, so in fact it's 3 grayscale
images:3 x 49,000 = 147,000 pixels
And each pixel uses 1 byte. So I'm using 147,000 bytes.
How many kilobytes?147,000 / 1024 = 143.5546875 KB
144 KB!?! And that is a small bitmap image!
![Page 12: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/12.jpg)
A Bitmap image (known as a BMP) uses full memory storage for each pixel
There are many schemes for compressing this data into a smaller file
Compression compacts the data before it stores it to a file.
![Page 13: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/13.jpg)
JPEG is one type of compression scheme. JPEG is a "lossy" compression
It changes the original data.
Another compression scheme that is very widely used is GIF. GIF is a "lossless" compression
It preserves the original data.
A much older compression is the 256-color bitmap This method compresses the color scale to save on file
space
![Page 14: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/14.jpg)
How many colors? With an RGB image, each pixel consists of 8 bits
times 3 For the 3 color images That’s a total of 24 bits for each color pixel
The total colors that can be produced by a single pixel in this system is: Red channel: 0 to 255 (256 colors) Green channel: 0 to 255 (256 colors) Blue channel: 0 to 255 (256 colors)
(See the next slide)
![Page 15: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/15.jpg)
The total number of combinations is256 x 256 x 256
So, the total number of possible colors is256 x 256 x 256 = 16,777,216
![Page 16: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/16.jpg)
There is an older bitmap encoding that doesn’t use 16 million colors
Instead, it uses only 256 colors This makes it possible to use only a single pixel image
plane Instead of the separate Red, Green and Blue image planes
combined into a single picture
There is a tradeoff, however: The 256 colors are actually a color-and-brightness scale
at the same time. This means that you don’t get the full range of colors You also don’t get the full range of brightness levels
![Page 17: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/17.jpg)
GIF compression compresses the image like this: Run Length Encoding
With this scheme, the data can be packaged tightly, using less file space
Yet it can also be perfectly unpacked That restores the original image data.
![Page 18: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/18.jpg)
In run length compression, you count the number of repeated pixels. Start with this:
After compression, end with this:2:0s, 1:1, 1:6, 1:18, 5:20s ,4:24s1:148, 1:152, 1:200, 4:210s, 2:150s, 1:100, 1:78, 3:60s
This is lossless encoding The original data can be brought back
It works best with “geometric” images These are non-photographic images that have large regions
consisting of the same color value
0 0 1 6 18 20 20 20 20 20 24 24 24 24148 152 200 210 210 210 210 150 150 100 78 60 60 60
![Page 19: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/19.jpg)
See how this geometric image uses the same color pixels over and over?
This is ideal for saving space using GIF encoding
![Page 20: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/20.jpg)
Oh yeah, I forgot. When I converted my bitmap to GIF, it also did a color compression Storing each 24-bit pixel as an 8-bit pixel This is the same as my 256-color bitmap.
![Page 21: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/21.jpg)
JPEG uses a sophisticated mathematical compression based on Fourier analysis. So it's hard to describe what is happening to the data. As if I could! I’m not that nerdy!
Suffice it to say, the while the data is being compressed, it is being changed: Errors are introduced.
![Page 22: By Bob “The Bird” Fiske & Anita “The Snail” Cost](https://reader036.vdocuments.site/reader036/viewer/2022062315/5697bfe01a28abf838cb369b/html5/thumbnails/22.jpg)
Does anybody complain about the JPEG errors? In photographic images (that don't actually contain
sharp edges), the errors are not noticeable. In "geometric images" that contain lots of straight lines
and regions of uniform color, the errors become noticeable They appear as "edge echoes".
Photographic Geometric