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在Octave中加载图像并转成灰度图时，发现rgb2gray()函数不存在，上网搜了一下，stackoverflow上给出了相关解释，如下：
The problem is that you didn‘t load your packages. When you type pkg list you can find which ones are loaded by an asterisk in front of their names. Load a package with
pkg load image.
Having to load a packages is that thing that most users find strange but if you compare with other languages, such as Python, Perl, or C++, would you expect them to
import, use, or #include every libraries available in the system by default? See
Octave‘s FAQ for more details.
原来Octave缺省状态下缺少相关包，比如图像处理、信号处理方面的包，需要我们手动安装。
去sourceforge上下载相关包，放在Octave的搜索路径上，在Octave的命令行中执行安装命令：
pkg install -forge package_name
rgb2gray()函数需要安装image包。
附一下sourceforge上Octave的相关包：
These packages are meant for current versions of Octave. See the unmaintainedsection for information on older versions.
The BSLTL package is a free collection of OCTAVE/MATLAB routines for working with the biospeckle laser technique
Digital Communications, Error Correcting Codes (Channel Code), Source Code functions, Modulation and Galois Fields
Computer-Aided Control System Design (CACSD) Tools for GNU Octave, based on the proven SLICOT Library
divand performs an n-dimensional variational analysis (interpolation) of arbitrarily located observations
The Octave-Forge Doctest package finds specially-formatted blocks of example code within documentation files
Monte Carlo simulation, options pricing routines, financial manipulation, plotting functions and additional date manipulation tools
The Octave-FITS package provides functions for reading, and writing FITS (Flexible Image Transport System) files
Collection of routines to export data produced by Finite Elements or Finite Volume Simulations in formats used by some visualization programs
This package provides functions for generating HTML pages that contain the help texts for a set of functions
The Octave-forge Image Aquisition package provides functions to capture images from connected devices
Low level I/O functions for serial, i2c, parallel, tcp, gpib, vxi11 and usbtmc interfaces
The interval package for real-valued interval arithmetic allows one to evaluate functions over subsets of their domain
Routines for calculating the time-evolution of the level-set equation and extracting geometric information from the level-set function
A package implementing tools to compute spectral decompositions of irregularly-spaced time series
The Large Time/Frequency Analysis Toolbox (LTFAT) is a Matlab/Octave toolbox for working with time-frequency analysis, wavelets and signal processing
Library with functions useful for numerical computation in classical mechanics and structural analysis
Create and manage triangular and tetrahedral meshes for Finite Element or Finite Volume PDE solvers
Collection of routines for the creation, and manipulation of Non-Uniform Rational B-Splines (NURBS), based on the NURBS toolbox by Mark Spink
This package allows to do boolean operations with polygons using the Greiner-Hormann algorithm
A wrapper for ffmpeg‘s libavformat and libavcodec, implementing addframe, avifile, aviinfo and aviread
Some packages have been renamed or merged over time. When thishappens, a dummy transitional package is created -- empty anddependent on its new package. After some time, those are removedas well. The following is a list of such packages:
The Octave Forge project is a location to develop multiple packages for GNUOctave. With time, some packages become unmaintained, usually because the originalauthor no longer requires it and no one steps up to take its position. Without amaintainer or active user base, and as Octave evolves, we do not know how usablethey are, so we can no longer honestly recommend their use.
They are still stored and release tarballs can be downloaded from ourFile Release System(same for the very oldmonolithic releases) .However, many of them have accumulated bug fixes and new features in themercurial orSVNrepositories, it simply happens that no maintainer has prepared a release.Because of this, we encourage users to check the development sources, not the releasedtarballs. Furthermore,bug reports for those packagescontinue to be welcome, and anyone wishing to take up the position of maintainer, please contactus through themaintainers mailing list.
The complete list of unmaintained packages is:
参考：
[1]：http://octave.sourceforge.net/
[2]：http://octave.sourceforge.net/packages.php
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原文地址：http://blog.csdn.net/garfielder007/article/details/51355572
