Monochrome bitmaps on a full colour display?

The Nokia 6610 display can support up to 12-bit colour so why bother with monochrome bitmaps?

Well, using a PIC microcontroller, even with 24C256 I2C eeprom (256kb) attached, doesn't provide a massive amount of memory for displaying full colour bitmaps. Even a simple 256 colour image (8-bit palette, similar to a GIF) uses one byte per pixel (the Nokia supports modes that use up to 2-bytes per pixel). The display is 131x131 (often listed as 128 by 128 but this allows for non-viewable pixels around the edge of the display) so a GIF-type image would take a whopping 17,161 bytes (around 16kb).
Given that the PIC18F2455 has 24kb of program space (it was tempting to write "only 24kb" but that's doing the chip a disservice!) coding a single full-colour image uses up 3/4 of the available space. Add in USB support, using the Oshonsoft PIC18 compiler and you've filled the program memory without actually writing any of your own code!
Also, dragging data back from the eeprom chip inbetween drawing pixels on the screen means that displaying the full colour image actually takes quite a long time.

Compare this to a displaying a monochrome bitmap.
First off, pixel data can be packed into packets of 8-bits - since each pixel takes on the colour of either "foreground" or "background" (a one or a zero can be used to represent either state). This means that a full screen image takes up (131 x 131)/8 = 2,146 bytes.
For images that have large areas of the same colour, the "bitmap" data could be written as "number of pixels before pixel changes colour". This can reduce the amount of memory used quite drastically.

Below is a screenshot of a VB app thrown together which loads a monochrome bitmap image and outputs two byte arrays - one an array of pixel data using 1=foreground colour, 0=background colour, the other an array listing the number of pixels in a (vertical) row before the pixel colour changes:



The image loaded was 43x87 pixels.
This can be represented as (43x88)/8 = 473 bytes.
(the image size has to be rounded up to 88 so that it is a multiple of 8, allowing the data to be packed into byte-sized packets).

Below is the byte array used to store the golfer bitmap data:

0x00, 0x00, 0x00, 0xE0, 0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00, 0x00, 0xF0, 0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00, 0x00, 0x98, 0x07, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00, 0x00, 0x8C, 0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00, 0x00, 0x06, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00, 0x00, 0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00, 0x80, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00, 0xC0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00, 0x60, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00, 0x30, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00, 0x1F, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0xC0, 0x0F, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0xE0, 0x0F, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0xBF, 0x00, 0xF0, 0x1F, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xF8, 0xFF, 0x00, 0xF8, 0x3F, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xFC, 0xFF, 0x00, 0xF8, 0x7F, 0x06, 0x00, 0x00, 0x00, 0x00, 0x00, 0xFE, 0xFF, 0x00, 0xF8, 0xFF, 0x1F, 0x00, 0x00, 0x00, 0x00, 0x00, 0xFE, 0x9F, 0x00, 0xF0, 0xFF, 0x3F, 0x00, 0x00, 0x00, 0x00, 0x80, 0xFF, 0x83, 0x00, 0xF0, 0xFF, 0x7F, 0x00, 0x00, 0x00, 0x00, 0xC0, 0xFF, 0x81, 0x00, 0xD8, 0xFF, 0xFF, 0x00, 0x00, 0x00, 0x00, 0xF0, 0xFF, 0x81, 0x00, 0x0C, 0xFC, 0xFF, 0x03, 0x00, 0x00, 0x00, 0xF8, 0xFF, 0x80, 0x00, 0x06, 0xF8, 0xFF, 0x0F, 0x00, 0x00, 0x00, 0xFC, 0x7F, 0x80, 0x00, 0x03, 0xF8, 0xFF, 0x1F, 0x00, 0x00, 0x00, 0xFE, 0x3F, 0x80, 0x80, 0x01, 0xFC, 0xFF, 0xFF, 0x00, 0x00, 0x00, 0xFF, 0x0F, 0x80, 0xC0, 0x00, 0xFE, 0xFF, 0xFF, 0x03, 0x00, 0xE0, 0xFF, 0x0F, 0x80, 0x60, 0x00, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x03, 0x80, 0x38, 0xC0, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x00, 0x80, 0x7C, 0xE0, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x7F, 0x00, 0xA0, 0x7E, 0xF8, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x1F, 0x00, 0xF0, 0xFE, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x0F, 0x00, 0xF8, 0xFE, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x03, 0x00, 0xFC, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xF8, 0xFF, 0xFF, 0xFF, 0xDF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x3E, 0xC0, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFE, 0xC3, 0x7F, 0xFE, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xBF, 0xFC, 0xFF, 0x3F, 0xF8, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x9F, 0xF8, 0xFF, 0x1F, 0xF0, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x8F, 0x80, 0xFF, 0x0F, 0x80, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x80, 0x00, 0xFE, 0x07, 0x00, 0xFE, 0xFF, 0xFF, 0xFF, 0xFF, 0x3F, 0x80, 0x00, 0xF8, 0x01, 0x00, 0xFC, 0xFF, 0xFF, 0xFF, 0xFF, 0x01, 0x80, 0x00, 0x00, 0x00, 0x00, 0xF0, 0xFF, 0xFF, 0xFF, 0x03, 0x00, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0xFE, 0x7F, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0xF0, 0x03, 0x00, 0x00, 0x00, 0x80

But changing the method of recording data as "number of same coloured pixels" can be recorded as

29, 5, 53, 1, 28, 6, 53, 1, 27, 2, 2, 4, 52, 1, 26, 2, 3, 3, 53, 1, 25, 2, 5, 1, 54, 1, 24, 2, 61, 1, 23, 2, 62, 1, 22, 2, 63, 1, 21, 2, 64, 1, 20, 2, 65, 1, 16, 5, 66, 1, 14, 6, 67, 1, 13, 7, 59, 7, 1, 1, 12, 9, 54, 13, 11, 11, 52, 14, 11, 12, 2, 2, 46, 15, 11, 18, 44, 12, 2, 1, 12, 18, 41, 11, 5, 1, 12, 19, 39, 11, 6, 1, 11, 2, 1, 18, 36, 13, 6, 1, 10, 2, 6, 16, 33, 13, 7, 1, 9, 2, 8, 17, 30, 13, 8, 1, 8, 2, 9, 18, 28, 13, 9, 1, 7, 2, 9, 22, 24, 12, 11, 1, 6, 2, 9, 25, 19, 15, 11, 1, 5, 2, 9, 58, 13, 1, 3, 3, 8, 58, 15, 1, 2, 5, 6, 58, 14, 1, 1, 1, 1, 6, 4, 58, 15, 4, 1, 67, 15, 5, 1, 65, 16, 70, 3, 34, 1, 74, 1, 5, 8, 74, 1, 9, 4, 9, 2, 61, 1, 1, 2, 20, 5, 58, 2, 1, 3, 18, 7, 56, 3, 1, 7, 13, 11, 49, 7, 1, 9, 10, 14, 45, 9, 1, 11, 6, 17, 39, 14, 1, 36, 30, 21, 1, 41, 14, 32, 1, 44, 6, 37

This uses just 231 bytes to store the entire image.
And here it is drawn back on the Nokia display (code includes a yet-to-be-resolved bug, but you can see the bitmap image displayed clearly on the LCD).



So why use monochrome images when you have a full-colour display?
When working with PIC microcontrollers or limited memory space, because you can get an image size down from a ridiculous 16kb down to just 231!