What do you mean by

Bitmap Images ??

There are different ways to describe images, corresponding roughly to `analog' and `digital'. An analog description (best example: PostScript) describes the image precisely in terms of shapes and lines and coordinates and colors. A digital description (e.g., JPEG, PNG) treats the image as a rectangular array of points (pixels) and describes the color and brightness of each pixel.

ANALOG (not bitmap)

The ``analog'' method of describing an image might be like this:

Draw a circle, r=6cm, in the middle of the page using a red line of thickness 3mm, fill the interior with solid yellow. Superimpose a solid blue pentagon, R=12mm, over the topmost point of the circle. Superimpose in black letters (1-inch Times-Roman font) ``My Picture'', centered over the bottom of the circle.

Here is PostScript code which corresponds to the description above:


%!
1 1 0 setrgbcolor                       % yellow
306 396 170 0 360 arc fill              % 6cm~170pt
1 0 0 setrgbcolor 9 setlinewidth        % red.  3mm~9pt
306 396 170 0 360 arc stroke
0 0 1 setrgbcolor                       % blue
gsave 306 566 translate 0 48 moveto     % 17mm~48pt
5 { 72 rotate 0 48 lineto } repeat      % pentagon
fill grestore
/Times-Roman findfont 72 scalefont setfont
0 setgray 140 205 moveto (My Picture) show

With such a description it is possible to move and magnify the image, and show it accurately at any resolution. However, such descriptions must be interpreted by software if they are to be displayed or printed. PostScript printers have built-in software to interpret code like that above. On computers, ghostview interprets PostScript and renders it for display on a computer monitor. (Another image-description language is fig, and xfig interprets fig images, displaying them and allowing them to be manipulated. The file mypicture.fig has the code for our sample picture, in Fig format.)

BITMAP (digital)

Computer monitors and printers display images & text as a rectangular array of ``dots'' or ``pixels'' (look closely with a magnifying glass if you need to!). So an analog image (above) has to be bitmapped -- mapped onto a discrete rectangular array of units each taking one of a discrete number of colors -- in order to be displayed or printed.

Images stored in bitmapped form -- PNG or JPEG, for example -- are thus in a form which is instantly displayable on a computer monitor. But here are some drawbacks of a bitmap description of an image:

it cannot be reverse-mapped to analog form
true magnification of the image is not possible
the image cannot gain sharpness on a high-resolution printer

Magnifying a bitmap image just make bigger blockier pixels (right,1).
Or, software might cheat and smooth the boundaries between blocks, but this doesn't really restore the detail of the original image (right,2). "Anti-aliasing" techniques can do a better job of appearing to smooth the jags, but still cannot restore lost information.

The same sorts of issues arise when deciding, from a web page, whether to print the PostScript form of a document rather than the PNG version which appears on the monitor.

PDF documents, fortunately, can deliver all the quality of a PostScript document, and PDF-displaying software (Adobe Acrobat Reader) is free and available for almost all kinds of computers; it makes viewing and printing a document easy.