The `image`, rgbimage, and rgbalpha plotting styles all project a uniformly sampled grid of data values onto a plane in either a 2D or 3D. The input data may be an actual bitmapped image, perhaps converted from a standard format such as PNG, or a simple array of numerical values.
This figure illustrates generation of a heat map from an array of scalar values. The current palette is used to map each value onto the color assigned to the corresponding pixel.
plot '-' matrix with image
5 4 3 1 0
2 2 0 0 1
0 0 0 1 0
0 1 2 4 3
e
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Each pixel (data point) of the input 2D image will become a rectangle or parallelipiped in the plot. The coordinates of each data point will determine the center of the parallelipiped. That is, an M x N set of data will form an image with M x N pixels. This is different from the pm3d plotting style, where an M x N set of data will form a surface of (M-1) x (N-1) elements. The scan directions for a binary image data grid can be further controlled by additional keywords. See `binary general keywords flipx`, `center`, and `rotate`.
Image data can be scaled to fill a particular rectangle within a 2D plot coordinate system by specifying the x and y extent of each pixel. See `binary general keywords dx` and `dy`. To generate the figure at the right, the same input image was placed multiple times, each with a specified dx, dy, and origin. The input PNG image of a building is 50x128 pixels. The tall building was drawn by mapping this using `dx=0.5 dy=1.5`. The short building used a mapping `dx=0.5 dy=0.35`.
The `image` style handles input pixels containing a grayscale or color palette value. Thus 2D plots (`plot` command) require 3 columns of data (x,y,value), while 3D plots (`splot` command) require 4 columns of data (x,y,z,value).
The rgbimage style handles input pixels that are described by three separate values for the red, green, and blue components. Thus 5D data (x,y,r,g,b) is needed for `plot` and 6D data (x,y,z,r,g,b) for `splot`. The individual red, green, and blue components are assumed to lie in the range [0:255].
The rgbalpha style handles input pixels that contain alpha channel (transparency) information in addition to the red, green, and blue components. Thus 6D data (x,y,r,g,b,a) is needed for `plot` and 7D data (x,y,z,r,g,b,a) for `splot`. The r, g, b, and alpha components are assumed to lie in the range [0:255].