Image ¶
- Title
- Image Element
- Dependencies
- Matplotlib
- Backends
- Matplotlib
- Bokeh
import numpy as np
import holoviews as hv
hv.extension('matplotlib')
Image represents a regularly sampled 2D grid of an underlying
continuous space
of intensity values, which will be colormapped on plotting. The grid of intensity values may be specified as a
NxM
sized array of values along with a bounds, but it may also be defined through explicit and regularly spaced x/y-coordinate arrays of shape
M
and
N
respectively. The two most basic supported constructors of an Image therefore include:
Image((X, Y, Z))
where
X
is a 1D array of shape
M
,
Y
is a 1D array of shape
N
and
Z
is a 2D array of shape
NxM
, or equivalently:
Image(Z, bounds=(x0, y0, x1, y1))
where
Z
is a 2D array of shape
NxM
defining the intensity values and the bounds define the (left, bottom, right, top) edges of four corners of the grid. Other gridded formats which support declaring of explicit x/y-coordinate arrays such as xarray are also supported. See the
Gridded Datasets
user guide for all the other accepted data formats.
ls = np.linspace(0, 10, 200)
xx, yy = np.meshgrid(ls, ls)
bounds=(-1,-1,1,1) # Coordinate system: (left, bottom, right, top)
img = hv.Image(np.sin(xx)*np.cos(yy), bounds=bounds)
img
Slicing, sampling, etc. on an
Image
all operate in this continuous space, whereas the corresponding operations on a
Raster
work on the raw array coordinates.
img + img[-0.5:0.5, -0.5:0.5]
Notice how, because our declared coordinate system is continuous, we can slice with any floating-point value we choose. The appropriate range of the samples in the input numpy array will always be displayed, whether or not there are samples at those specific floating-point values. This also allows us to index by a floating value, since the
Image
is defined as a continuous space it will snap to the closest coordinate, to inspect the closest coordinate we can use the
closest
method:
closest = img.closest((0.1,0.1))
points = hv.Points([closest])
print('The value at position %s is %s' % (closest, img[0.1, 0.1]))
img * points.opts(color='black', marker='x', s=60)
We can also easily take cross-sections of the Image by using the sample method or collapse a dimension using the
reduce
method:
img.sample(x=0) + img.reduce(x=np.mean)
The constructor of
Image
attempts to validate the input data by ensuring it is regularly sampled. In some cases, your data may be not be regularly sampled to a sufficiently high precision in which case you qill see an exception recommending the use of
QuadMesh
instead. If you see this message and are sure that the
Image
element is appropriate, you can set the
rtol
value in the constructor to allow a higher deviation in sample spacing than the default of
10e-6
. Alternatively, you can set this globally using
hv.config.image_rtol
as described in the
Installing and Configuring
user guide.
One additional way to create Image objects is via the separate ImaGen library, which creates parameterized streams of images for experiments, simulations, or machine-learning applications.
For full documentation and the available style and plot options, use
hv.help(hv.Image).
Download this notebook from GitHub (right-click to download).