今天，我们来讲解Matlab中图片读取函数imread的使用，即Matlab如何读取图片。然后，我们就可以处理图片，比如大小重定义，图片去噪，图片加噪，图片分割，图片运算等。大致说一下imread函数的存储原理，将图片读取后存放为一个三维矩阵，由三个二维矩阵组成，分别存放图片中的rgb三个色彩。二维矩阵的行号列号表示图片的横纵像素点，每个二维矩阵元素对应的值即为rgb，三个二维矩阵刚好组成一个rgb颜色，即可存储图片。
imread函数的基本用法就是给定图片的名称、路径、格式。假设我们有一个0827.jpg图片，放置于D:\MATLAB\R2016a\mine目录下
- 名称：0827
- 格式：jpg
- 路径：该图片在硬盘中的位置，D:\MATLAB\R2016a\mine
imread实际使用中，我们可以把图片放于Matlab的m文件同目录下
下面是实际使用的几个例子，以供参考：
%%
%清理工作区与命令区
clc
clear
close all
%%
%图片读取
I1 = imread('0827','jpg');
I2 = imread('0827.jpg');
I3 = imread('D:\MATLAB\R2016a\mine\0827.jpg');
figure(1)
imshow(I1)
上述的figure(1)表示作图，imshow(I1)表示把I1变量的矩阵输出到图片中。实际操作中，如果原图过大，可以使用imresize函数重新定义下图片的大小即可。
大致的用法就上面几种了，关于官方给的help文件中的介绍如下，感兴趣的可以看看，Matlab在图片读取中的bit信息相关吧。不感兴趣的可以略过。
>> help imread
imread Read image from graphics file.
A = imread(FILENAME,FMT) reads a grayscale or color image from the file
specified by the string FILENAME. FILENAME must be in the current
directory, in a directory on the MATLAB path, or include a full or
relative path to a file.
The text string FMT specifies the format of the file by its standard
file extension. For example, specify 'jpg' for Graphics Interchange
Format files. To see a list of supported formats, with their file
extensions, use the IMFORMATS function. If imread cannot find a file
named FILENAME, it looks for a file named FILENAME.FMT.
The return value A is an array containing the image data. If the file
contains a grayscale image, A is an M-by-N array. If the file contains
a truecolor image, A is an M-by-N-by-3 array. For TIFF files containing
color images that use the CMYK color space, A is an M-by-N-by-4 array.
See TIFF in the Format-Specific Information section for more
information.
The class of A depends on the bits-per-sample of the image data,
rounded to the next byte boundary. For example, imread returns 24-bit
color data as an array of uint8 data because the sample size for each
color component is 8 bits. See the Remarks section for a discussion of
bitdepths, and see the Format-Specific Information section for more
detail about supported bitdepths and sample sizes for a particular
format.
[X,MAP] = imread(FILENAME,FMT) reads the indexed image in FILENAME into
X and its associated colormap into MAP. Colormap values in the image
file are automatically rescaled into the range [0,1].
[...] = imread(FILENAME) attempts to infer the format of the file
from its content.
[...] = imread(URL,...) reads the image from an Internet URL.
Remarks
Bitdepth is the number of bits used to represent each image pixel.
Bitdepth is calculated by multiplying the bits-per-sample with the
samples-per-pixel. Thus, a format that uses 8-bits for each color
component (or sample) and three samples per pixel has a bitdepth of 24.
Sometimes the sample size associated with a bitdepth can be ambiguous:
does a 48-bit bitdepth represent six 8-bit samples or three 16-bit
samples? The following format-specific sections provide sample size
information to avoid this ambiguity.
Format-Specific Information (Listed Alphabetically by Format)
BMP -- Windows Bitmap
Supported Compression Output
Bitdepths None RLE Class Notes
---------------------------------------------------------
1-bit x - logical
4-bit x x uint8
8-bit x x uint8
16-bit x - uint8 1 sample/pixel
24-bit x - uint8 3 samples/pixel
32-bit x - uint8 3 samples/pixel (1 byte padding)
CUR -- Cursor File
Supported Compression Output
Bitdepths None Compressed Class
--------------------------------------------------
1-bit x - logical
4-bit x - uint8
8-bit x - uint8
Special syntaxes:
[...] = imread(...,IDX) reads in one image from a multi-image icon or
cursor file. IDX is an integer value that specifies the order that the
image appears in the file. For example, if IDX is 3, imread reads the
third image in the file. If you omit this argument, imread reads the
first image in the file.
[A,MAP,ALPHA] = imread(...) returns the AND mask for the resource,
which can be used to determine transparency information. For cursor
files, this mask may contain the only useful data.
GIF -- Graphics Interchange Format
Supported Output Class
---------------------------
1-bit logical
2-to-8 bit uint8
Special syntaxes:
[...] = imread(...,IDX) reads in one or more frames from a multiframe
(i.e., animated) GIF file. IDX must be an integer scalar or vector of
integer values. For example, if IDX is 3, imread reads the third image
in the file. If IDX is 1:5, only the first five frames are returned.
[...] = imread(...,'Frames',IDX) is the same as the syntax above except
that IDX can be 'all'. In this case, all of the frames are read and
returned in the order that they appear in the file.
Note: Because of the way GIF files are structured, all of the frames
must be read when a particular frame is requested. Consequently, it is
much faster to specify a vector of frames or 'all' for IDX than to call
imread in a loop when reading multiple frames from the same GIF file.
HDF -- Hierarchical Data Format
Supported Raster image Raster image Output
Bitdepths with colormap without colormap Class Notes
------------------------------------------------------------
8-bit x x uint8
24-bit - x uint8 3 samples/pixel
Special Syntaxes:
[...] = imread(...,REF) reads in one image from a multi-image HDF file.
REF is an integer value that specifies the reference number used to
identify the image. For example, if REF is 12, imread reads the image
whose reference number is 12. (Note that in an HDF file the reference
numbers do not necessarily correspond with the order of the images in
the file. You can use IMFINFO to match up image order with reference
number.) If you omit this argument, imread reads the first image in
the file.
ICO -- Icon File
See CUR.
JPEG -- Joint Photographic Experts Group
Note: imread can read any baseline JPEG image as well as JPEG images
with some commonly used extensions.
Supported Compression Output
Bitdepths Lossy Lossless Class Notes
--------------------------------------------------------
8-bit x x uint8 Grayscale or RGB
12-bit x x uint16 Grayscale
16-bit - x uint16 Grayscale
36-bit x x uint16 RGB(Three 12-bit samples/pixel)
JPEG 2000 - Joint Photographic Experts Group 2000
Supported Compression Output
Bitdepths Lossy Lossless Class
(per sample)
----------------------------------------------------------
1-bit x x logical
2- to 8-bit x x uint8, int8
9- to 16-bit x x uint16, int16
Note: Indexed JPEG 2000 images are not supported. Only JP2 compatible
color spaces are supported for JP2/JPX files. By default, all image
channels are returned in the order they are stored in the file.
Special Syntaxes
[...] = imread(..., 'Param1', value1, 'Param2', value2, ...) uses
parameter-value pairs to control the read operation.
Parameter name Value
-------------- -----
'ReductionLevel' A non-negative integer specifying the reduction in
the resolution of the image. For a reduction
level 'L', the image resolution is reduced by a
factor of 2^L. The default value is 0 implying
no reduction. The reduction level is limited by
the total number of decomposition levels as
provided by 'WaveletDecompositionLevels' field
in the structure returned from IMFINFO function.
'PixelRegion' {ROWS, COLS}. imread returns the sub-image
specified by the boundaries in ROWS and COLS.
ROWS and COLS must both be two-element vectors
that denote the 1-based indices [START STOP]. If
'ReductionLevel' is greater than 0, then ROWS and
COLS are coordinates in the reduced-sized image.
'V79Compatible' A logical value. If true, the image returned is
transformed to gray-scale or RGB as consistent with
previous versions of imread (MATLAB 7.9 [R2009b]
and earlier). Use this option to transform YCC
images into RGB. The default is false.
PBM -- Portable Bitmap
Supported Raw ASCII (Plain) Output
Bitdepths Binary Encoded Class
----------------------------------------
1-bit x x logical
PCX -- Windows Paintbrush
Supported Output
Bitdepths Class Notes
----------------------------------------------
1-bit logical Grayscale only
8-bit uint8 Grayscale or indexed
24-bit uint8 RGB (8-bit samples)
PGM -- Portable Graymap
Supported Raw ASCII (Plain) Output
Bitdepths Binary Encoded Class
------------------------------------------------
up to 16-bit x - uint8
Arbitrary - x
PNG -- Portable Network Graphics
Supported Output
Bitdepths Class Notes
-------------------------------------------
1-bit logical Grayscale only
2-bit uint8 Grayscale only
4-bit uint8 Grayscale only
8-bit uint8 Grayscale or Indexed
16-bit uint16 Grayscale or Indexed
24-bit uint8 RGB (Three 8-bit samples/pixel)
48-bit uint16 RGB (Three 16-bit samples/pixel)
Special Syntaxes:
[...] = imread(...,'BackgroundColor',BG) composites any transparent
pixels in the input image against the color specified in BG. If BG is
'none', then no compositing is performed. Otherwise, if the input image
is indexed, BG should be an integer in the range [1,P] where P is the
colormap length. If the input image is grayscale, BG should be a value
in the range [0,1]. If the input image is RGB, BG should be a
three-element vector whose values are in the range [0,1]. The string
'BackgroundColor' may be abbreviated.
If the ALPHA output argument is used (see below), then BG defaults to
'none' if not specified by the user. Otherwise, if the PNG file
ontains a background color chunk, that color is used as the default
value for BG. If ALPHA is not used and the file does not contain a
background color chunk, then the default value for BG is 1 for indexed
images; 0 for grayscale images; and [0 0 0] for RGB images.
[A,MAP,ALPHA] = imread(...) returns the alpha channel if one is
present; otherwise ALPHA is []. If 'BackgroundColor' is specified by
the user then ALPHA is []. Note that MAP may be empty if the file
contains a grayscale or truecolor image.
PPM -- Portable Pixmap
Supported Raw ASCII (Plain) Output
Bitdepths Binary Encoded Class
------------------------------------------------
up to 16-bit x - uint8
Arbitrary - x
RAS -- Sun Raster
Supported Output
Bitdepths Class Notes
----------------------------------------------------
1-bit logical Bitmap
8-bit uint8 Indexed
24-bit uint8 RGB (8-bit samples)
32-bit uint8 RGB with Alpha (8-bit samples)
TIFF -- Tagged Image File Format
NOTE: Images with a YCbCr photometric interpretation are converted to
the RGB colorspace.
Special Syntaxes:
A = imread(...) returns color data that uses the RGB, CIELAB, ICCLAB,
or CMYK color spaces. If the color image uses the CMYK color space, A
is an M-by-N-by-4 array.
[...] = imread(..., 'Param1', value1, 'Param2', value2, ...) uses
parameter-value pairs to control the read operation. There are three
different parameters you can use:
Parameter name Value
-------------- -----
'Index' A positive integer specifying which image to read in
a multi-image TIFF file. For example, if 'Index' is
3, imread reads the third image in the file.
'Info' A structure array; the output of IMFINFO. When
reading images from a multi-image TIFF file, passing
the output of IMFINFO as the 'Info' parameter helps
imread locate the images in the file more quickly.
'PixelRegion' {ROWS, COLS}. imread returns the sub-image
specified by the boundaries in ROWS and COLS. ROWS
and COLS must be either two- or three-element
vectors. If two elements are provided, they denote
the 1-based indices [START STOP]. If three elements
are provided, the indices [START INCREMENT STOP]
allow image downsampling.
XWD -- X Window Dump
Supported Output
Bitdepths ZPixmaps XYBitmaps XYPixmaps Class
--------------------------------------------------
1-bit x - x logical
8-bit x - - uint8
Please read the file libtiffcopyright.txt for more information.
Example:
imdata = imread('ngc6543a.jpg');
See also imfinfo, imwrite, imformats, fread, image, double, uint8.
imread 的参考页
© 版权声明
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