RISC DISC 1/RISC_DISC_1.iso/pd_share/apps/image/!Image/Docs/P4

Text File | 1994-03-31 | 5.6 KB | 133 lines

Loading and Saving
------------------

Image can load and save various file formats :

    Sprite       - 1, 2, 4, 8, 16 and 32 bpp   (not CMYK)
    Clear files
    JPEG files
    PBMPlus      - PPM and PGM only, not PBM   (only saves P6, raw full colour)
    GIF files    - Memory Permitting           (load only)

It can also request ChangeFSI to convert pictures for it, thus it can load
many different picture formats. See below for details.

Where files contain more than one picture, the first picture is
loaded.

To load a picture simply drag the file to any of Image's windows -
Image will then attempt to load the picture into its current input
image bank (see 'BankCont').

To save an image simply click MENU over the Image Display Window. This
brings up a save window which contains the usual icons, plus icons to
select the output format :

   Sprite     - Saves the current display sprite (the sprite used
                to display in the display window). This will be
                an old style sprite of 1, 2, 4 or 8 bpp (depending
                on the options set in the configuration window
                (See 'Configuration').
   Sprite 32k - This saves the image as an old style 16 bpp sprite
                (as used by Computer Concepts' Colour Cards)
   16 bpp     - Saves a new style 16 bpp sprite. (actually 15 bpp)
   32 bpp     - Saves a new style 32 bpp sprite. (actually 24 bpp)
   Clear      - Saves a clear file.
   JPEG       - Saves a JPEG file (see below)

In all but the 'Sprite' option the image saved is that which resides
in the current input image bank.

The masking information is generally lost when saving a picture,
except when saving 'Sprite' and the mask option in the configuration
window is set - or when saving '32 bpp' when the masking information
will be preserved as far as Image is concerned (other applications
will ignore this information).

In order to maintain full information (full colour information and
full masking information) you should save as a 32 bpp sprite. Note,
however, many current (Risc OS 3.1) applications will not be able to
read this sprite.


Sprite Palettes
---------------

When loading a sprite, Image can (if requested) load the palette (if
available) which goes with the palette into a special 'User Palette' which
can be used for the display sprite (see 'Configuration').

The palette will automatically be loaded if the auto loading option is set in
the configuration window - you can override this by holding down SHIFT when
loading the file.

Alternatively, you can force the palette to be loaded into the user palette
by holding down CTRL when loading the sprite.


When saving a sprite the palette associated with it (either user palette or a
grey scale palette) will be saved with the sprite.


Saving JPEG files
-----------------

Next to the JPEG option is a writable icon for specifying what 'quality'
value to pass to the JPEG compressor.

The quality value lets you trade off compressed file size against quality of
the reconstructed image: the higher the quality setting, the larger the JPEG
file, and the closer the output image will be to the original input.
Normally you want to use the lowest quality setting (smallest file) that
decompresses into something visually indistinguishable from the original
image. For this purpose the quality setting should be between 50 and 95; the
default of 75 is often about right. If you see defects at a quality of 75,
then go up 5 or 10 counts at a time until you are happy with the output
image. (The optimal setting will vary from one image to another.)

A quality of 100 will generate a quantization table of all 1's, eliminating
loss in the quantization step (but there is still information loss in
subsampling, as well as roundoff error). This setting is mainly of interest
for experimental purposes. Quality values above about 95 are NOT recommended
for normal use; the compressed file size goes up dramatically for hardly any
gain in output image quality.

In the other direction, quality values below 50 will produce very small files
of low image quality. Settings around 5 to 10 might be useful in preparing an
index of a large image library, for example.

JPEG loading and saving are handled by software from the 'Independent JPEG
Group's  JPEG software' which has been altered to provide support for Image's
own use.



ChangeFSI loading
-----------------

If you have a copy of ChangeFSI you can get Image to use it for loading file
formats that Image can't handle itself.

In fact if 'ChangeFSI' has been 'seen' by the filer then you need do nothing
to use this facility, any file dragged to Image that Image can't handle
itself will be passed to ChangeFSI to convert, and then Image will load the
converted version.

You can also 'teach' Image about the location of 'ChangeFSI' so that it is
not necessary to make sure that the filer has 'seen' it. To do this simply
drag the '!ChangeFSI' application to any of Image's windows, then using the
configuration window save your configuration, and the location of ChangeFSI
will be saved as part of that configuration.

The configuration window contains an icon which shows the current location
known for ChangeFSI and also an option icon marked 'New'. This option is used
to tell Image whether the version of ChangeFSI you have is new sprite aware.
If it is (ChangeFSI version 0.95 or later) then this option will cause
ChangeFSI to use a 32 bpp sprite for transfer, otherwise a PPM file is used.
If in doubt as to which version of ChangeFSI you have, you should not select
this option. The advantage of setting this option is that loading will be
faster.

ChangeFSI is supplied with RISC OS 3.1 on the RISC OS 3 support disc.