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<h3 class="sectionHead"><span class="titlemark">3.1 </span> <a
id="x20-850003.1"></a>Introduction</h3>
<!--l. 2--><p class="noindent" ><a name="nip_label_sec:oper"></a>
<!--l. 4--><p class="indent" > This chapter explains how to write image processing
operations using the VIPS image I/O (input-output) system.
For background, you should probably take a look at <span
class="cmsy-10">§</span><a
href="vipsmanualse7.html#x13-360002.1">2.1<!--tex4ht:ref: sec:appl --></a>.
This is supposed to be a tutorial, if you need detailed
information on any particular function, use the on-line
UNIX manual pages.
<!--l. 10--><p class="noindent" >
<h4 class="subsectionHead"><span class="titlemark">3.1.1 </span> <a
id="x20-860003.1.1"></a>Why use VIPS?</h4>
<!--l. 12--><p class="noindent" >If you use the VIPS image I/O system, you get a number of
benefits:
<dl class="description"><dt class="description">
<span
class="ptmb7t-">Threading</span> </dt><dd
class="description">If your computer has more than one CPU,
the VIPS I/O system will automatically split your
image processing operation into separate threads
(provided you use PIO, see below). You should get
an approximately linear speed-up as you add more
CPUs. </dd><dt class="description">
<span
class="ptmb7t-">Pipelining</span> </dt><dd
class="description">Provided you use PIO (again, see below),
VIPS can automatically join operations together. A
sequence of image processing operations will all
execute together, with image data flowing through
the processing pipeline in small pieces. This makes
it possible to perform complex processing on very
large images with no need to worry about storage
management.
</dd><dt class="description">
<span
class="ptmb7t-">Composition</span> </dt><dd
class="description">Because VIPS can efficiently compose
image processing operations, you can
implement your new operation in small, reusable,
easy-to-understand pieces. VIPS already has a lot
of these: many new operations can be implemented
by simply composing existing operations.
</dd><dt class="description">
<span
class="ptmb7t-">Large files</span> </dt><dd
class="description">Provided you use PIO and as long as the
underlying OS supports large files (that is, files
larger than 2GB), VIPS operations can work on
files larger than can be addressed with 32 bits on a
plain 32-bit machine. VIPS operations only see 32
bit addresses; the VIPS I/O system transparently
maps these to 64 bit operations for I/O. Large file
support is included on most machines after about
1998.
</dd><dt class="description">
<span
class="ptmb7t-">Abstraction</span> </dt><dd
class="description">VIPS operations see only arrays of
numbers in native format. Details of representation
(big/little endian, VIPS/TIFF/JPEG file format,
etc.) are hidden from you.
</dd><dt class="description">
<span
class="ptmb7t-">Interfaces</span> </dt><dd
class="description">Once you have your image processing
operation implemented, it automatically appears in
all of the VIPS interfaces. VIPS comes with a GUI
(<span
class="pcrr7t-">nip2</span>), a UNIX command-line interface (<span
class="pcrr7t-">vips</span>)
and a C++ and Python API.
</dd><dt class="description">
<span
class="ptmb7t-">Portability</span> </dt><dd
class="description">VIPS operations can be compiled on most
unixes, Mac OS X and Windows NT, 2000 and XP
without modification. Mostly.
</dd></dl>
<!--l. 59--><p class="noindent" >
<h4 class="subsectionHead"><span class="titlemark">3.1.2 </span> <a
id="x20-870003.1.2"></a>I/O styles</h4>
<!--l. 61--><p class="noindent" >The I/O system supports three styles of input-output.
<dl class="description"><dt class="description">
<span
class="ptmb7t-">Whole-image I/O (WIO)</span> </dt><dd
class="description">This style is a largely a
left-over from VIPS 6.x. WIO image-processing
operations have all of the input image given to
them in a large memory array. They can read
any of the input pels at will with simple pointer
arithmetic.
</dd><dt class="description">
<span
class="ptmb7t-">Partial-image I/O (PIO)</span> </dt><dd
class="description">In this style operations only
have a small part of the input image available to
them at any time. When PIO operations are joined
together into a pipeline, images flow through them
in small pieces, with all the operations in a pipeline
executing at the same time.
</dd><dt class="description">
<span
class="ptmb7t-">In-place</span> </dt><dd
class="description">The third style allows pels to be read and
written anywhere in the image at any time, and
is used by the VIPS in-place operations, such
as <span class="obeylines-h"><span class="verb"><span
class="pcrr7t-">im_fastline()</span></span></span>. You should only use it
for operations which would just be impossibly
inefficient to write with either of the other two
styles.
</dd></dl>
<!--l. 85--><p class="indent" > WIO operations are easy to program, but slow and
inflexible when images become large. PIO operations are
harder to program, but scale well as images become larger,
and are automatically parallelized by the VIPS I/O
system.
<!--l. 89--><p class="indent" > If you can face it, and if your algorithm can be expressed
in this way, you should write your operations using PIO.
Whichever you choose, applications which call your
operation will see no difference, except in execution
speed.
<!--l. 93--><p class="indent" > If your image processing operation performs no
coordinate transformations, that is, if your output image is
the same size as your input image or images, and if
each output pixel depends only upon the pixel at the
corresponding position in the input images, then you
can use the <span class="obeylines-h"><span class="verb"><span
class="pcrr7t-">im_wrapone()</span></span></span> and <span class="obeylines-h"><span class="verb"><span
class="pcrr7t-">im_wrapmany()</span></span></span>
operations. These take a simple buffer-processing operation
supplied by you and wrap it up as a full-blown PIO
operation. See <span
class="cmsy-10">§</span><a
href="vipsmanualse15.html#x22-930003.3.1">3.3.1<!--tex4ht:ref: sec:wrapone --></a>.
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