By Bob “The Bird” Fiske & Anita “The Snail” Cost

Bitmaps & the brightness grayscale Color pictures are red, green & blue Storing bitmaps as files Compressing bitmap data

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

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).

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

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).

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

Red channel Green channel Blue channel

Combine to create…

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!

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!

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.

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

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)

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

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

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.

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

See how this geometric image uses the same color pixels over and over?

This is ideal for saving space using GIF encoding

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.

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.

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