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31.1 Loading and Saving Images

The first step in most image processing tasks is to load an image into Octave. This is done using the imread function, which uses the GraphicsMagick library for reading. This means a vast number of image formats is supported. The imwrite function is the corresponding function for writing images to the disk.

In summary, most image processing code will follow the structure of this code

 
I = imread ("my_input_image.img");
J = process_my_image (I);
imwrite ("my_output_image.img", J);

Function File: [img, map, alpha] = imread (filename)

Read images from various file formats.

The size and numeric class of the output depends on the format of the image. A color image is returned as an MxNx3 matrix. Grey-level and black-and-white images are of size MxN. The color depth of the image determines the numeric class of the output: "uint8" or "uint16" for grey and color, and "logical" for black and white.

See also: imwrite, imfinfo.

Function File: imwrite (img, filename, fmt, p1, v1, …)
Function File: imwrite (img, map, filename, fmt, p1, v1, …)

Write images in various file formats.

If fmt is missing, the file extension (if any) of filename is used to determine the format.

The parameter-value pairs (p1, v1, …) are optional. Currently the following options are supported for JPEG images

Quality

Sets the quality of the compression. The corresponding value should be an integer between 0 and 100, with larger values meaning higher visual quality and less compression.

See also: imread, imfinfo.

Built-in Function: val = IMAGE_PATH ()
Built-in Function: old_val = IMAGE_PATH (new_val)

Query or set the internal variable that specifies a colon separated list of directories in which to search for image files.

It is possible to get information about an image file on disk, without actually reading it into Octave. This is done using the imfinfo function which provides read access to many of the parameters stored in the header of the image file.

Function File: info = imfinfo (filename)
Function File: info = imfinfo (url)

Read image information from a file.

imfinfo returns a structure containing information about the image stored in the file filename. The output structure contains the following fields.

Filename

The full name of the image file.

FileSize

Number of bytes of the image on disk

FileModDate

Date of last modification to the file.

Height

Image height in pixels.

Width

Image Width in pixels.

BitDepth

Number of bits per channel per pixel.

Format

Image format (e.g., "jpeg").

LongFormat

Long form image format description.

XResolution

X resolution of the image.

YResolution

Y resolution of the image.

TotalColors

Number of unique colors in the image.

TileName

Tile name.

AnimationDelay

Time in 1/100ths of a second (0 to 65535) which must expire before displaying the next image in an animated sequence.

AnimationIterations

Number of iterations to loop an animation (e.g., Netscape loop extension) for.

ByteOrder

Endian option for formats that support it. Is either "little-endian", "big-endian", or "undefined".

Gamma

Gamma level of the image. The same color image displayed on two different workstations may look different due to differences in the display monitor.

Matte

true if the image has transparency.

ModulusDepth

Image modulus depth (minimum number of bits required to support red/green/blue components without loss of accuracy).

Quality

JPEG/MIFF/PNG compression level.

QuantizeColors

Preferred number of colors in the image.

ResolutionUnits

Units of image resolution. Is either "pixels per inch", "pixels per centimeter", or "undefined".

ColorType

Image type. Is either "grayscale", "indexed", "truecolor", or "undefined".

View

FlashPix viewing parameters.

See also: imread, imwrite.


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