Sysquake Pro – Table of Contents
Sysquake for LaTeX – Table of Contents
Image I/O
This section describes functions which offer support for reading and writing image files. Formats supported include PNG and JPEG.
Calerga gratefully acknowledges the following contributions: PNG encoding and decoding are based on libpng; and JPEG encoding and decoding are based on the work of the Independent JPEG Group.
Functions
imageread
Read an image file.
Syntax
A = imageread(fd) A = imageread(fd, options)
Description
imageread(fd) reads a PNG or JPEG file from file descriptor fd and returns it as an array whose first dimension is the image height and second dimension the image width. Grayscale images have a third dimension equal to 1 (i.e. plain matrices). Color images have a third dimension equal to 3; fist plane is the red component, second plane the green component, and third plane the blue component. By default, the result is a uint8 or uint16 array whose type matches the pixel format in the image files. Value range from 0 for black to 255 or 65535 for maximum intensity.
The file descriptor is usually obtained by opening a file with fopen in binary mode (text mode, with end-of-line translation, would produce garbage or cause a decoding error).
A second argument can specify special options to modify the result. Options are usually created with function imagereadset, or given directly as named arguments.
Example
fd = fopen('image.png', 'r');
im = imageread(fd);
fclose(fd);
See also
imagereadset
Options for image input.
Syntax
options = imagereadset options = imagereadset(name1=value1, ...) options = imagereadset(name1, value1, ...) options = imagereadset(options0, name1=value1, ...) options = imagereadset(options0, name1, value1, ...)
Description
imagereadset(name1,value1,...) creates the option argument used by imageread. Options are specified with name/value pairs, where the name is a string which must match exactly the names in the table below. Case is significant. Options which are not specified have a default value. The result is a structure whose fields correspond to each option. Without any input argument, imagereadset creates a structure with all the default options. Note that imageread also interpret the lack of an option argument, or the empty array [], as a request to use the default values.
When its first input argument is a structure, imagereadset adds or changes fields which correspond to the name/value pairs which follow.
Here is the list of permissible options. Currently they are used only when reading PNG files.
| Name | Default | Meaning |
|---|---|---|
| Scale16 | false | convert 16-bit images to uint8 |
| StripAlpha | true | ignore the alpha channel |
Named arguments can be given directly to imageread without calling explicitly imageset.
Examples
Default options:
imagereadset Scale16: false StripAlpha: true
Reading a PNG file with alpha channel (an RGB+alpha image is an array of size [height, width, 4]):
fd = fopen('image.png');
im = imageread(fd, StripAlpha=false);
fclose(fd);
See also
imageset
Options for image output.
Syntax
options = imageset options = imageset(name1=value1, ...) options = imageset(name1, value1, ...) options = imageset(options0, name1=value1, ...) options = imageset(options0, name1, value1, ...)
Description
imageset(name1,value1,...) creates the option argument used by imagewrite. Options are specified with name/value pairs, where the name is a string which must match exactly the names in the table below. Case is significant. Options which are not specified have a default value. The result is a structure whose fields correspond to each option. Without any input argument, imageset creates a structure with all the default options. Note that imagewrite also interpret the lack of an option argument, or the empty array [], as a request to use the default values.
When its first input argument is a structure, imageset adds or changes fields which correspond to the name/value pairs which follow.
Here is the list of permissible options:
| Name | Default | Meaning |
|---|---|---|
| Type | 'PNG' | 'PNG' or 'JPG'/'JPEG' |
| Quality | 80 | JPEG quality (0=worst,100=best) |
| Progressive | false | true to permit progressive decoding |
Named arguments can be given directly to imagewrite without calling explicitly imageset.
Examples
Default options:
imageset Type: 'png' Quality: 80 Progressive: false
Writing the contents of array A into a small, low-quality JPEG file:
fd = fopen('A.jpg', 'w');
imagewrite(fd, A, Type='JPG', Quality=20);
fclose(fd);
See also
imagewrite
Write an image file.
Syntax
imagewrite(fd, A) imagewrite(fd, A, options)
Description
imagewrite(fd,A) writes array A to a PNG file specified by file descriptor fd. Image A is an array whose first dimension is the image height and second dimension the image width. Grayscale images have their third dimension equal to 1 (i.e. they are plain matrices). Color images have a third dimension equal to 3; fist plane is the red component, second plane the green component, and third plane the blue component. In both cases, value range is 0 for black to 1 for maximum intensity. Values outside this range are clipped.
imagewrite(fd,A,options) uses structure options to specify image file options. Options are usually created with function imageset, or given directly as named arguments; they include the file type.
The file descriptor is usually obtained by opening a file with fopen in binary mode (text mode, with end-of-line translation, would produce a corrupted image file).
Example
Write the image contained in the matrix im to a file "image.png", using the default options.
fd = fopen('image.png', 'w');
imagewrite(fd, im);
fclose(fd);
Write the same image as a JPEG file.
fd = fopen('image.jpg', 'w');
imagewrite(fd, im, Type='JPEG');
fclose(fd);