MATLAB Function Reference

imread - Read image from graphics file

Synopsis

Description

A = imread(filename, fmt) reads a grayscale or color image from the file specified by the string filename. If the file is not in the current directory, or in a directory on the MATLAB^® path, specify the full pathname.

The text string fmt specifies the format of the file by its standard file extension. For example, specify 'gif' 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 Remarks for a discussion of bitdepths, and see Format-Specific Information for more detail about supported bitdepths and sample sizes for a particular format.

[X, map] = imread(...) 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. The URL must include the protocol type (e.g., http://).

See the format-specific sections for additional syntaxes.

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, four 12-bit samples, or three 16-bit samples? The following format-specific sections provide sample size information to avoid this ambiguity.

Format-Specific Information

The following sections provide information about the support for specific formats, listed in alphabetical order by format name. These sections include information about format-specific syntaxes, if they exist. The following is a list of links to the various sections.

• BMP — Windows^® Bitmap

• CUR — Cursor File

• GIF — Graphics Interchange Format

• HDF4 — Hierarchical Data Format

• ICO — Icon File

• JPEG — Joint Photographic Experts Group

• PBM — Portable Bitmap

• PCX — Windows^® Paintbrush

• PGM — Portable Graymap

• PNG — Portable Network Graphics

• PPM — Portable Pixmap

• RAS — Sun™ Raster

• TIFF — Tagged Image File Format

• XWD — X Window Dump

BMP — Windows^® Bitmap

The following table lists the supported bitdepths, compression, and output classes for BMP data.

CUR — Cursor File

The following table lists the supported bitdepths, compression, and output classes for Cursor files and Icon files.

The following are format-specific syntaxes for Cursor files and Icon files.

[...] = imread(..., idx)    CUR or ICO 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(...)    CUR or ICO returns the AND mask for the resource, which can be used to determine the transparency information. For cursor files, this mask may contain the only useful data.

Note   By default, Microsoft® Windows® cursors are 32-by-32 pixels. MATLAB pointers must be 16-by-16. You will probably need to scale your image. If you have Image Processing Toolbox™, you can use the imresize function.

GIF — Graphics Interchange Format

The following table lists the supported bitdepths, compression, and output classes for GIF files.

The following are format-specific syntaxes for GIF files.

[...] = imread(..., idx)    GIF 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, imread returns only the first five frames.

[...] = imread(..., 'frames', idx)    GIF is the same as the syntax above except that idx can be 'all'. In this case, all the frames are read and returned in the order that they appear in the file.

Note   Because of the way that GIF files are structured, all 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.

HDF4 — Hierarchical Data Format

The following table lists the supported bitdepths, compression, and output classes for HDF4 files.

The following are format-specific syntaxes for HDF4 files.

[...] = imread(..., ref)    HDF4 reads in one image from a multi-image HDF4 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 HDF4 file the reference numbers do not necessarily correspond to the order of the images in the file. You can use imfinfo to match image order with reference number.) If you omit this argument, imread reads the first image in the file.

ICO — Icon File

See CUR – Cursor File

JPEG — Joint Photographic Experts Group

imread can read any baseline JPEG image as well as JPEG images with some commonly used extensions. The following table lists the supported bitdepths, compression, and output classes for JPEG files.

PBM — Portable Bitmap

The following table lists the supported bitdepths, compression, and output classes for PBM files.

PCX — Windows^® Paintbrush

The following table lists the supported bitdepths, compression, and output classes for PCX files.

PGM — Portable Graymap

The following table lists the supported bitdepths, compression, and output classes for PGM files.

PNG — Portable Network Graphics

The following table lists the supported bitdepths, compression, and output classes for PNG data.

The following are format-specific syntaxes for PNG files.

[...] = imread(...,'BackgroundColor',BG)   PNG composites any transparent pixels in the input image against the color specified in BG. If BG is 'none', then no compositing is performed. If the input image is indexed, BG must be an integer in the range [1,P] where P is the colormap length. If the input image is grayscale, BG should be an integer 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.

[A, map, alpha] = imread(...)   PNG returns the alpha channel if one is present; otherwise alpha is []. Note that map may be empty if the file contains a grayscale or truecolor image.

If the alpha output argument is specified, BG defaults to 'none', if not specified by the user. Otherwise, if the PNG file contains 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 truecolor images.

PPM — Portable Pixmap

The following table lists the supported bitdepths, compression, and output classes for PPM files.

RAS — Sun™ Raster

The following table lists the supported bitdepths, compression, and output classes for RAS files.

TIFF — Tagged Image File Format

imread supports the following TIFF capabilities:

• Any number of samples-per-pixel

• CCITT group 3 and 4 FAX, Packbits, JPEG, LZW, Deflate, ThunderScan compression, and uncompressed images

• Logical, grayscale, indexed color, truecolor and hyperspectral images

• RGB, CMYK, CIELAB, ICCLAB color spaces

• Data organized into tiles or scanlines

The following table lists the supported bit/sample and corresponding output classes for TIFF files.

The following are format-specific syntaxes for TIFF files.

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(..., idx) reads in one image from a multi-image TIFF file. idx is an integer value that specifies the order in which 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.

[...] = imread(..., 'PixelRegion', {ROWS, COLS}) returns the subimage specified by the boundaries in ROWS and COLS. For tiled TIFF images, imread reads only the tiles that encompass the region specified by ROWS and COLS, improving memory efficiency and performance. 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.

For TIFF files, imread can read color data represented in the RGB, CIELAB, or ICCLAB color spaces. To determine which color space is used, look at the value of the PhotometricInterpretation field returned by imfinfo. Note, however, that if a file contains CIELAB color data, imread converts it to ICCLAB before bringing it into the MATLAB workspace. 8- or 16-bit TIFF CIELAB-encoded values use a mixture of signed and unsigned data types that cannot be represented as a single MATLAB array.

XWD — X Window Dump

The following table lists the supported bitdepths, compression, and output classes for XWD files.

Class Support

For most image file formats, imread uses 8 or fewer bits per color plane to store image pixels. The following table lists the class of the returned array for the data types used by the file formats.

Note   For indexed images, imread always reads the colormap into an array of class double, even though the image array itself may be of class uint8 or uint16.

Examples

This example reads the sixth image in a TIFF file.

[X,map] = imread('your_image.tif',6);

This example reads the fourth image in an HDF4 file.

info = imfinfo('your_hdf_file.hdf');
[X,map] = imread('your_hdf_file.hdf',info(4).Reference);

This example reads a 24-bit PNG image and sets any of its fully transparent (alpha channel) pixels to red.

bg = [255 0 0];
A = imread('your_image.png','BackgroundColor',bg);

This example returns the alpha channel (if any) of a PNG image.

[A,map,alpha] = imread('your_image.png');

This example reads an ICO image, applies a transparency mask, and then displays the image.

[a,b,c] = imread('your_icon.ico'); 
% Augment colormap for background color (white).
b2 = [b; 1 1 1]; 
% Create new image for display. 
d = ones(size(a)) * (length(b2) - 1); 
% Use the AND mask to mix the background and
% foreground data on the new image
d(c == 0) = a(c == 0); 
% Display new image
image(uint8(d)), colormap(b2)

See Also

double, fread, image, imfinfo, imformats, imwrite, uint8, uint16

Bit-Mapped Images for related functions

importdata

imwrite