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<h2><a name="adding-code-writing-new-file-format">Adding Code for
Writing a New File Format</a></h2>
<p>Note: Despite the wide variety of displays and file formats <i>xv</i>
deals with, internally it only manipulates <i>either </i>8-bit
colormapped images <i>or</i> 24-bit RGB images. Your <tt>Write()</tt>
routine must be prepared to take either sort of image, and
convert it (if necessary) to the image format that your file
format dictates.</p>
<p>If you haven't already done so (if/when you created the <tt>Load()</tt>
function):</p>
<p>Make a copy of <tt>xvpbm.c</tt> , calling it something
appropriate. For the rest of this appendix, mentally replace the
string ' <tt>xvpbm.c</tt> ' with the name of your new file.</p>
<p>Edit the <tt>Makefile</tt> and/or the <tt>Imakefile</tt> so
that your new module will be compiled. In the <tt>Makefile</tt> ,
add " <tt>xvpbm.o</tt> " to the " <tt>OBJS = ...</tt>
" macro definition. In the <tt>Imakefile</tt> , add " <tt>xvpbm.o</tt>
" to the end of the " <tt>OBJS1 = ...</tt> " macro
definition, and " <tt>xvpbm.c"</tt> to the end of the <tt>"SRCS1
= ..."</tt> macro definition.</p>
<p>Edit the new module.</p>
<p>You'll need to <tt>#include "xv.h"</tt> , of course.</p>
<p>The module should have one externally callable function that
does the work of writing the file. The function is called with a
large number of arguments, described below. The function should
return '0' if everything succeeded, and '-1' on failure.</p>
<blockquote>
<pre><font size="2">/*******************************************/
int WritePBM(fp,pic,ptype,w,h,rmap,gmap,bmap,numcols,colorstyle,raw,comment)
FILE *fp;
byte *pic;
int ptype, w,h;
byte *rmap, *gmap, *bmap;
int numcols, colorstyle, raw;
char *comment;
/*******************************************/</font></pre>
</blockquote>
<dl>
<dt><font size="2" face="Courier New">file *fp;</font></dt>
<dd>This is a pointer to an already- <tt>fopen()</tt> 'd
stream. Your function should neither open nor close this
stream, as that all gets handled elsewhere in <tt>xvdir.c</tt>
.</dd>
<dt> </dt>
<dt><font size="2" face="Courier New">byte *pic;</font></dt>
<dd>This points to the image data that will be written. In
the case of a <tt>PIC8</tt> image, <tt>pic</tt> will
point to a <tt>w*h</tt> long array of bytes, one byte per
pixel, starting at the top-left, and proceeding in normal
scan-line order. There is no padding of any sort at the
end of the lines.</dd>
<dt> </dt>
<dd>In the case of a <tt>PIC24</tt> image, <tt>pic</tt> will
point to a <tt>w*h*3</tt> long array of bytes. There are
three bytes per pixel, in red, green, blue order. The
pixels start at the top-left, and proceed in normal scan
line order. There is no padding of any sort at the end of
the lines.</dd>
<dt> </dt>
<dt><font size="2" face="Courier New">int ptype, w, h;</font></dt>
<dd>These variables describe the format of <tt>pic</tt> . <tt>ptype</tt>
can be set to either <tt>PIC8</tt> or <tt>PIC24</tt> . <tt>w</tt>
and <tt>h</tt> are the width and height of the image, in
pixels.</dd>
<dt> </dt>
<dt><font size="2" face="Courier New">byte *rmap, *gmap,
*bmap;<br>
int numcols;</font></dt>
<dd>These pointers point to the colormap associated with the
image. They are only relevant when <i>ptype</i> is PIC8,
meaning that <i>pic</i> is an 8-bit per pixel colormapped
image. These arrays will each be <tt>numcols</tt> entries
long, with a maximum length of 256. Do not attempt to
access entries >= <tt>numcols</tt> , as the colormaps
are <i>not necessarily</i> 256 entries long. You are
guaranteed that pixel values found in <tt>pic</tt> will
be within the range [0..numcols-1], so you don't have to
check each pixel value. Also, do not attempt to access
these arrays at all if <tt>ptype</tt> is PIC24, as these
pointers will probably be NULL in that case.</dd>
<dt> </dt>
<dt><font size="2" face="Courier New">int colorstyle;</font></dt>
<dd>The <i>Colors</i> choice selected in the <i>xv save</i>
window. It can be either <tt>F_FULLCOLOR</tt> , <tt>F_GREYSCALE</tt>
, or <tt>F_BWDITHER</tt> . It will <i>not</i> be <tt>F_REDUCED</tt>
. If the user selects <b>Reduced Color</b> in the <i>xv
save</i> window, the appropriate image will be computed,
and you'll be given that image, and <i>colorstyle</i>
will be set to <tt>F_FULLCOLOR</tt> .</dd>
<dt> </dt>
<dd>Likewise, if the user has selected <b>B/W Dithered</b> in
the <i>xv save</i> window, an appropriate black-and-white
image will have been generated before your <tt>Write()</tt>
routine is called, so you won't have to worry about that.
Such an image will be a <tt>PIC8</tt> image, with a
2-entry colormap. It is up to you to decide which of the
two colors should be written as <i>black</i>, and which
should be written as <i>white</i>. You should do this by
comparing the values of <font size="2"><tt>MONO(rmap[0],gmap[0],bmap[0])</tt></font>
and <font size="2"><tt>MONO(rmap[1],gmap[1],bmap[1])</tt></font>.
Whichever value is smaller is the darker of the two, and
should be written as <i>black</i>.</dd>
<dt> </dt>
<dt><font size="2" face="Courier New">int raw;</font></dt>
<dd>This is a value passed in specifically for the <tt>WritePBM()</tt>
function, as PBM has two closely-related subformats (raw,
and ascii) both of which are written by this one
function. Your function won't need this, nor should it be
passed in to your function.</dd>
<dt> </dt>
<dt><font size="2" face="Courier New">char *comment;</font></dt>
<dd>This will point to a zero-terminated character string
which contains the comments that should be written into
the image file. Note that this string can be of <i>any</i>
length, and it may contain any number of lines (separated
by '<tt>\n</tt>' characters). If your image format
supports comments in the file, you should write this
information to the file. If it doesn't, you should just
ignore this variable. Also, this variable may be <tt>NULL</tt>,
(signifying 'no comments'), in which case it should not
be used.</dd>
</dl>
<p>You may pass more parameters, since you're going to be adding
the call to this function later on. For example, in my PBM code,
I pass one more parameter, 'raw' (whether to save the file as
'raw' or 'ascii') to handle two very similar formats. (Rather
than having to write <tt>WritePBMRaw</tt>() and <tt>WritePBMAscii</tt>()
functions.)</p>
<p>Write the function as you deem appropriate. See <tt>xvpbm.c</tt>
for an example of a <tt>Write()</tt> routine that writes
different formats for 1-bit per pixel images, 8-bit per pixel
images, and 24-bit per pixel images, based on <tt>ptype</tt> and <tt>colorstyle</tt>
.</p>
<p>Note: If your file format can only handle 8-bit images, and <tt>ptype</tt>
is set to <tt>PIC24</tt> , you will have to call <tt>Conv24to8()</tt>
to convert the 24-bit image into an 8-bit colormapped image that
you can write to the file. See <tt>xvgifwr.c</tt> for an example
of how this is done.</p>
<p>That done, edit ' <tt>xv.h</tt> ' and add a function
declaration for your new function. Search for ' <tt>WritePBM()</tt>
' in the file for a sample declaration to copy.</p>
<p>Also find the block that begins with:</p>
<blockquote>
<pre><font size="2">#define F_GIF 0
#define F_JPEG ( 0 + F_JPGINC)</font></pre>
</blockquote>
<p>and add a definition for your format. Note that it'll be
easiest to tack it on at the end.</p>
<p>These numbers <i>must</i> be contiguous, as they are used as
indices into the <tt>fmtMB</tt> menu in <tt>xvdir.c</tt> .</p>
<p>Edit '<tt>xvdir.c</tt>'. This is the module that controls the <i>xv
save</i> window.</p>
<p>Add another format name, in the appropriate position, to the <tt>saveFormats[]</tt>
string array.</p>
<p>In the function <tt>DoSave()</tt> , find the following block:</p>
<blockquote>
<pre><font size="2">switch (fmt) {
case F_GIF:
rv = WriteGIF(fp, thepic, ptype, w, h, rp, gp, bp, nc, col,
picComments);
break;
case F_PM:
rv = WritePM (fp, thepic, ptype, w, h, rp, gp, bp, nc, col,
picComments);
break;</font></pre>
</blockquote>
<p>and add a case for your function.</p>
<p>Finally, if your format has a common 3 or 4 letter filename
suffix (like, ".gif", ".jpg", etc.), you
should modify the <tt>changeSuffix()</tt> routine in <tt>xvdir.c</tt>
so that it recognizes your suffix, and puts your suffix on when
someone selects your format.</p>
<p>And It's just that easy!</p>
<h3><a name="writing-complex-formats">Writing Complex Formats</a></h3>
<p>Okay, maybe it's not <i>that</i> easy...</p>
<p>If your format requires some additional information to specify
how the file should be saved (such as the 'quality' setting in
JPEG, or position/size parameters in PostScript), then your task
is somewhat more difficult. You'll have to create some sort of
pop-up dialog box to get the additional information that you
want. You'll also have to change the way your <tt>Write()</tt>
function gets called (as it will now get called from your pop-up
dialog box). (Though, if you only feel like doing a quick hack,
you can probably just use the <tt>GetStrPopUp()</tt> function to
get a one-line character string from the user, and avoid the
complication of writing your own dialog box.)</p>
<p>This is not recommended for anyone who doesn't understand Xlib
programming. Frankly, it's not recommended for those who <i>do</i>,
either, but they at least stand some chance of success.</p>
<p>The more adventurous types who wish to pursue this should take
a look at the <tt>xvjpeg.c</tt> code, which implements an
extremely simple pop-up dialog. A considerably more complicated
dialog box is implemented in <tt>xvps.c</tt>. In addition to
writing a module like these for your format, you'll also have to
add the appropriate hooks to the <tt>DoSave()</tt> function (in <tt>xvdir.c</tt>)
and the <tt>HandleEvent()</tt> function (in <tt>xvevent.c</tt> ).
'<tt>grep PS *.c</tt>' will be helpful in finding places where
the <tt>xvps.c</tt> module is called. </p>
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