Bitmap
bitmap images
You probably have heard before that bitmap images are grids of pixels, where
- resolution: the number of horizontal and vertical pixels. E.g. 1920x1080 is 1920 pixels wide and 1080 high
- color depth: the number of bits used to indicate the color of a single pixel
for a good introduction to bit maps see: See Bourke, Paul. n.d. ‘A Beginners Guide to Bitmaps’. Accessed 2 January 2018. http://paulbourke.net/dataformats/bitmaps/.
Color depth
(use microscope)
- 1-bit color (21 = 2 colors): monochrome, often black and white, compact Macintoshes, Atari ST.
- 2-bit color (22 = 4 colors): CGA, gray-scale early NeXTstation, color Macintoshes, Atari ST.
- 3-bit color (23 = 8 colors): many early home computers with TV displays, including the ZX Spectrum and BBC Micro
- 4-bit color (24 = 16 colors): as used by EGA and by the least common denominator VGA standard at higher resolution, color Macintoshes, Atari ST, Commodore 64, Amstrad CPC.
- 5-bit color (25 = 32 colors): Original Amiga chipset
- 6-bit color (26 = 64 colors): Original Amiga chipset
- 8-bit color (28 = 256 colors): most early color Unix workstations, VGA at low resolution, Super VGA, color Macintoshes, Atari TT, Amiga AGA chipset, Falcon030, Acorn Archimedes.
- 12-bit color (212 = 4096 colors): some Silicon Graphics systems, Color NeXTstation systems, and Amiga systems in HAM mode.
- 8-bit color: A very limited but true direct color system, there are 3 bits (8 possible levels) for each of the R and G components, and the two remaining bits in the byte pixel to the B component (four levels), enabling 256 (8 × 8 × 4) different colors.
- Grey scale (8-bit)
- True color (24-bit): Usually, true color is defined to mean 256 shades of red, green, and blue, for a total of 224, or alternately 2563, or 16,777,216 color variations. The human eye can discriminate up to ten million colors
converting resolution
with Imagemagick
to 100px wide
convert myimage.jpg -resize 100x output.jpg
to 100px hight
convert myimage.jpg -resize x100 output.jpg
to 100x100px (image will loose its proportion; hence the "!" so imagemagick is sure that you wanna do that:)
convert myimage.jpg -resize 100x100! output.jpg
converting color (bit depth)
convert an image into monochrome (1 bit depth) image: 2color values (b/w)
convert myimage.jpg -colorspace gray -depth 1 out.png
(2 bit depth) image: 4 color values
convert myimage.jpg -colorspace gray -depth 2 out.png
(3 bit depth) image: 8 color values
convert myimage.jpg -colorspace gray -depth 3 out.png
(4 bit depth) image: 16 color values
convert myimage.jpg -colorspace gray -depth 4 out.png
(8 bit depth) image: 256 color values
convert myimage.jpg -colorspace gray -depth 4 out.png
How can we be certain about this? How do can we see the grid that forms a bitmap image?
- magnify any digital screen
- a Plain-text bitmap: where you can read each pixel: X PixMap (XPM)
X PixMap (XPM) is a plain-text bitmap image format, where each color is assigned a character, which is mapped onto the text space.
convert myimage.jpg out.xpm
/* XPM */
static char *output[] = {
/* columns rows colors chars-per-pixel */
"50 73 2 1 ",
" c black",
". c white",
/* pixels */
" ",
" ",
" ",
" ",
" ................ ",
" ...... . .. . . ....... ",
" .. . . ... ",
" . . .. . . .. ",
" .. . . . . . . . ",
" . . . . . . . .. ",
" . . . . . . . ",
" ... .. . .. ",
" ... .. . ... ",
" .. . ",
" . .. ",
" .. . ",
" . .. ",
" ... . . ",
" . . . ",
" .. . ",
" .. . . . . ",
" . . . ",
" .. . ..... . ",
" . .. .. . ",
" . . .. . . ",
" . .. . .. ",
" .. . . . ..... ",
" . ... . . . ",
" . . .... .. . .. ",
" . .. .... . . ",
" . . . . .... ",
" . . . .. ... ",
" . . . .. ",
" .. . . . . ",
" . . .. . . ",
" . .. .. .. .. ",
" .. ... ... . ",
" .. .. ",
" . . . . . ",
" . . .. . ",
" . . .... . . ",
" . .. ... . .. .. ",
" .. . .. ..... . .. . ... . ",
" . ... .. .... .... . ",
" . ..... ...... ...... . ",
" .. . ..... . ......... ... . ",
" .. ......... .. . ...... . ",
" . .. .................... ",
" . .. . ............... . ",
" . .. .. ............. . ",
" . .. . . ..... . ",
" ... . ... . .. ...... . ",
" ... . ............. . ",
" .. . .. . ",
" ... .. ",
" ... . . .. ",
" .. . . ",
" .. . ",
" .... .. ",
" ..... .... ",
" ..... ",
" ",
" . ",
" .. . ... ",
" ... .. .. ... .. .. ...... ... ",
" ............................ .. ",
" ...... ................. ... . ",
" ...... ... ... ..... .... .. . ",
" .. ",
" . ",
" ",
" ",
" "
};