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### 3.1 Introduction

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 §2.1. This is supposed to be a tutorial, if you need detailed information on any particular function, use the on-line UNIX manual pages.

#### 3.1.1 Why use VIPS?

If you use the VIPS image I/O system, you get a number of benefits:

Threading
  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.
Pipelining
  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.
Composition
  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.
Large files
  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.
Abstraction
  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.
Interfaces
  Once you have your image processing operation implemented, it automatically appears in all of the VIPS interfaces. VIPS comes with a GUI (nip2), a UNIX command-line interface (vips) and a C++ and Python API.
Portability
  VIPS operations can be compiled on most unixes, Mac OS X and Windows NT, 2000 and XP without modification. Mostly.

#### 3.1.2 I/O styles

The I/O system supports three styles of input-output.

Whole-image I/O (WIO)
  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.
Partial-image I/O (PIO)
  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.
In-place
  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 im_fastline(). You should only use it for operations which would just be impossibly inefficient to write with either of the other two styles.

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.

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.

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 im_wrapone() and im_wrapmany() operations. These take a simple buffer-processing operation supplied by you and wrap it up as a full-blown PIO operation. See §3.3.1.

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