image (MATLAB Function Reference)

Display Image object

image(C)
image(x,y,C)
image(...,'PropertyName',PropertyValue,...)
image('PropertyName',PropertyValue,...) Formal synatx - PN/PV only
handle = image(...)

Description

image creates an Image graphics object by interpreting each element in a matrix as an index into the Figure's colormap or directly as RGB values, depending on the data specified.

The image function has two forms:

A high-level function that calls newplot to determine where to draw the graphics objects and sets the following Axes properties:
XLim and YLim to enclose the Image
Layer to top to place the Image in front of the tick marks and grid lines
YDir to reverse
View to [0 90]
A low-level function that adds the Image to the current Axes without calling newplot. The low-level function argument list can contain only property name/property value pairs.

You can specify properties as property name/property value pairs, structure arrays, and cell arrays (see the set and get reference pages for examples of how to specify these data types).

image(C)

displays matrix C as an Image. Each element of C specifies the color of a rectangular segment in the Image.

image(x,y,C)

where x and y are two-element vectors, specifies the range of the x- and y-axis labels, but produces the same Image as image(C). This can be useful, for example, if you want the axis tick labels to correspond to real physical dimensions represented by the image.

image(x,y,C,'PropertyName',PropertyValue,...)

is a high-level function that also specifies property name/property value pairs. This syntax calls newplot before drawing the Image.

image('PropertyName',PropertyValue,...)

is the low-level syntax of the image function. It specifies only property name/property value pairs as input arguments.

handle = image(...)

returns the handle of the Image object it creates. You can obtain the handle with all forms of the image function.

Remarks

Image data can be either indexed or true color. An indexed image stores colors as an array of indices into the Figure colormap. A true color image does not use a colormap; instead, the color values for each pixel are stored directly as RGB triplets. In MATLAB , the CData property of a truecolor Image object is a three-dimensional (m-by-n-by-3) array. This array consists of three m-by-n matrices (representing the red, green, and blue color planes) concatenated along the third dimension.

The imread function reads image data into MATLAB arrays from graphics files in various standard formats, such as TIFF. You can write MATLAB image data to graphics files using the imwrite function. imread and imwrite both support a variety of graphics file formats and compression schemes.

When you read image data into MATLAB using imread, the data is stored as an array of 8-bit integers. This is a much more efficient storage method than the double-precision (64-bit) floating-point numbers that MATLAB typically uses. However, it is necessary for MATLAB to interpret 8-bit image data differently from 64-bit data. This table summarizes these differences:


Image Type	Double-precision Data (double array)	8-bit Data (uint8 array)
indexed (colormap)	Image is stored as a two-dimensional (`m`-by-`n`) array of integers in the range [1, `length(colormap)`]; colormap is an `m`-by-3 array of floating-point values in the range [0, 1]	Image is stored as a two-dimensional (`m`-by-`n`) array of integers in the range [0, 255]; colormap is an `m`-by-3 array of floating-point values in the range [0, 1]
truecolor (RGB)	Image is stored as a three-dimensional (`m`-by-`n`-by-3) array of floating-point values in the range [0, 1]	Image is stored as a three-dimensional (`m`-by-`n`-by-3) array of integers in the range [0, 255]

Indexed images

In an indexed image of class double, the value 1 points to the first row in the colormap, the value 2 points to the second row, and so on. In a uint8 indexed image, there is an offset; the value 0 points to the first row in the colormap, the value 1 points to the second row, and so on. The uint8 convention is also used in graphics file formats, and enables 8-bit indexed images to support up to 256 colors. Note that when you read in an indexed image with imread, the resulting image array is always of class uint8. (The colormap, however, is of class double; see below.)

If you want to convert a uint8 indexed image to double, you need to add 1 to the result. For example:

X64 = double(X8) + 1;

To convert from double to uint8, you need to first subtract 1, and then use round to ensure all the values are integers:

X8 = uint8(round(X64 - 1));

The order of the operations must be as shown in these examples, because you cannot perform mathematical operations on uint8 arrays.

When you write an indexed image using imwrite, MATLAB automatically converts the values if necessary.

Colormaps

Colormaps in MATLAB are alway m-by-3 arrays of double-precision floating-point numbers in the range [0, 1]. In most graphics file formats, colormaps are stored as integers, but MATLAB does not support colormaps with integer values. imread and imwrite automatically convert colormap values when reading and writing files.

True Color Images

In a truecolor image of class double, the data values are floating-point numbers in the range [0, 1]. In a truecolor image of class uint8, the data values are integers in the range [0, 255].

If you want to convert a truecolor image from one data type to the other, you must rescale the data. For example, this call converts a uint8 truecolor image to double:

RGB64 = double(RGB8)/255;

This statement converts a double truecolor image to uint8:

RGB8 = uint8(round(RGB*255));

The order of the operations must be as shown in these examples, because you cannot perform mathematical operations on uint8 arrays.

When you write a truecolor image using imwrite, MATLAB automatically converts the values if necessary.

Object Hierarchy

Setting Default Properties

You can set default Image properties on the Axes, Figure, and Root levels:

set(0,'DefaultImageProperty',PropertyValue...)
set(gcf,'DefaultImageProperty',PropertyValue...)
set(gca,'DefaultImageProperty',PropertyValue...)

Where Property is the name of the Image property and PropertyValue is the value you are specifying. Use set and get to access Image properties.

The following table lists all Image properties and provides a brief description of each. The property name links take you to an expanded description of the properties.


Property Name	Property Description	Property Value
Data Defining the Object
`CData`	The image data	Values: matrix or m-by-n-by-3 array Default: enter `image;axis image ij` and see
`CDataMapping`	Specify the mapping of data to colormap	Values: `scaled`, `direct` Default: `direct`
`XData`	Control placement of image along x-axis	Values: `[min max]` Default: `[1 size(CData,2)]`
`YData`	Control placement of image along y-axis	Values: `[min max]` Default: `[1 size(CData,1)]`
Controlling the Appearance
Clipping	Clipping to Axes rectangle	Values: `on`, `off` Default: `on`
EraseMode	Method of drawing and erasing the image (useful for animation)	Values: `normal`, `none`, `xor`, `background` Default: `normal`
`SelectionHighlight`	Highlight image when selected (`Selected` property set to `on`)	Values: `on`, `off` Default: `on`
`Visible`	Make the image visible or invisible	Values: `on`, `off` Default: `on`
Controlling Access to Objects
`HandleVisibility`	Determines if and when the the Line's handle is visible to other functions	Values: `on`, `callback`, `off` Default: `on`
`HitTest`	Determine if image can become the current object (see the Figure `CurrentObject` property)	Values: `on`, `off` Default: `on`
General Information About the Image
`Children`	Image objects have no children	Values: `[]` (empty matrix)
`Parent`	The parent of an image object is always an Axes object	Value: Axes handle
`Selected`	Indicate whether image is in a "selected" state.	Values: `on`, `off` Default: `on`
`Tag`	User-specified label	Value: any string Default: `''` (empty string)
`Type`	The type of graphics object (read only)	Value: the string `'image'`
`UserData`	User-specified data	Values: any matrix Default: `[]` (empty matrix)
Properties Related to Callback Routine Execution
`BusyAction`	Specify how to handle callback routine interruption	Values: `cancel`, `queue` Default: `queue`
`ButtonDownFcn`	Define a callback routine that executes when a mouse button is pressed on over the image	Values: string Default: empty string
`CreateFcn`	Define a callback routine that executes when an image is created	Values: string Default: empty string
`DeleteFcn`	Define a callback routine that executes when the image is deleted (via `close` or `delete`)	Values: string Default: empty string
`Interruptible`	Determine if callback routine can be interrupted	Values: `on`, `off` Default: `on` (can be interrupted)
`UIContextMenu`	Associate a context menu with the image	Values: handle of a Uicontrextmenu

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