sunterlib/scsh/image-info -- Extracting Vital Stats from Images

Loose port of small parts from Marco Schmidt's public domain ImageInfo 1.3
Java program.  The port supports gif, jpeg, png images and extracts 
* the width and height in pixels, 
* maybe the physical width and height in dpi,
* the (unreliable) colour depth (#bits / pixel),
* a symbolic format id: GIF87A, GIF89A, JPEG, PNG.
All numbers are integers.  The colour depth may deviate from the actual
value in multi-part gifs.  (It is taken from the global header, but
local palettes may reset, typically increase, that value.)

The original code, which does much more, is at
http://www.geocities.com/marcoschmidt.geo/image-info.html

Typical use:  in web-authoring, generate the width and height tags of 
inline images programmatically, like so:

;; ,open image-info
(receive (w h) (with-input-from-file "pix/herb.jpg"
                 (lambda () (image-dimension (current-input-port))))
  `(img ((src "pix/herb.jpg")
         (width ,w) (height ,h)
         (alt "Enter!"))))

--> (img ((src "pix/herb.jpg") (width 632) (height 474) (alt "Enter!")))

                                  *

Procedures

exported by structure IMAGE-INFO.  IMAGE-DIMENSION is the convenient
user interface for the most common usage:  getting pixel width and
height.  Some procedures refer to ``b.s.''s.  These are described at
bottom.


(image-dimension img) --> [w h]

Synopsis:  Extract the (numeric) width W and height H (in pixels) from
the byte source (input port, file descriptor, byte-vector, b.s.(*))
IMG -- or signal an error.

                                  *
                                
(get-image-info bs) --> info | #f

Synopsis:  Extract information on the image represented by b.s.(*) BS
and return an image-info record or #f.  (Don't believe in the colour
depth for gifs.)

                                  *

(image:info-format info) --> format | #f
Get the symbolic format id from image-info record INFO.

(image:info-depth info) --> depth | #f
Get the colour DEPTH (#bits/pixel) from image-info record INFO.

(image:info-width/pixel info) --> w | #f
(image:info-height/pixel info) --> h | #f 
Get the pixel width W resp. height H from image-info record INFO.

(image:info-width/dpi info) --> width | #f
(image:info-height/dpi info) --> height | #f 
Get the physical width W resp. height H in dots per inch from INFO.

                                 *

(*) Creating and accessing ``byte streams''

Byte-streams, or b.s.s for short, are random-access lazy byte sources.
The image-info project doesn't commit to the current implementation,
i.e. create and access b.s.s by the procedures below or blame yourself.

(inport->byte-stream in) --> bs

Synopsis:  Convert the input port (or file descriptor) IN to a
b.s. BS.  (The BS will read from IN as needed.  It doesn't duplicate
or close IN.) 

                                 *

(byte-vector->byte-stream bv) --> bs

Synopsis: Convert the byte-vector bv to a b.s. BS.

                                 *

(segment-byte-stream bs start end) --> [bv bs']

Synopsis:  Segment the b.s. BS into the first [start:end) bytes in the
byte-vector BV and the following [end:*) bytes in the b.s. BS', or get
the little that's there up to eof.  BV may be short, BV and BS' may be
empty.

                                oOo