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Commitf4ac6fb

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jklymakMeeseeksDev[bot]
authored and
MeeseeksDev[bot]
committed
Backport PR#10947: DOC: add tutorial explaining imshow *origin* and *extent*
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‎doc/_static/mpl.css

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"""
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*origin* and *extent* in `~.Axes.imshow`
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========================================
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:meth:`~.Axes.imshow` allows you to render an image (either a 2D array
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which will be color-mapped (based on *norm* and *cmap*) or and 3D RGB(A)
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array which will be used as-is) to a rectangular region in dataspace.
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The orientation of the image in the final rendering is controlled by
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the *origin* and *extent* kwargs (and attributes on the resulting
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`~.AxesImage` instance) and the data limits of the axes.
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The *extent* kwarg controls the bounding box in data coordinates that
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the image will fill specified as ``(left, right, bottom, top)`` in
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**data coordinates**, the *origin* kwarg controls how the image fills
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that bounding box, and the orientation in the final rendered image is
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also affected by the axes limits.
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.. hint:: Most of the code below is used for adding labels and informative
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text to the plots. The described effects of *origin* and *extent* can be
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seen in the plots without the need to follow all code details.
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For a quick understanding, you may want to skip the code details below and
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directly continue with the discussion of the results.
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"""
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importnumpyasnp
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importmatplotlib.pyplotasplt
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frommatplotlib.gridspecimportGridSpec
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defindex_to_coordinate(index,extent,origin):
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"""Return the pixel center of an index."""
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left,right,bottom,top=extent
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hshift=0.5*np.sign(right-left)
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left,right=left+hshift,right-hshift
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vshift=0.5*np.sign(top-bottom)
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bottom,top=bottom+vshift,top-vshift
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iforigin=='upper':
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bottom,top=top,bottom
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return {
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"[0, 0]": (left,bottom),
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"[M', 0]": (left,top),
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"[0, N']": (right,bottom),
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"[M', N']": (right,top),
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}[index]
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defget_index_label_pos(index,extent,origin,inverted_xindex):
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"""
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Return the desired position and horizontal alignment of an index label.
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"""
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ifextentisNone:
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extent=lookup_extent(origin)
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left,right,bottom,top=extent
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x,y=index_to_coordinate(index,extent,origin)
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is_x0=index[-2:]=="0]"
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halign='left'ifis_x0^inverted_xindexelse'right'
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hshift=0.5*np.sign(left-right)
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x+=hshift* (1ifis_x0else-1)
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returnx,y,halign
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defget_color(index,data,cmap):
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"""Return the data color of an index."""
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val= {
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"[0, 0]":data[0,0],
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"[0, N']":data[0,-1],
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"[M', 0]":data[-1,0],
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"[M', N']":data[-1,-1],
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}[index]
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returncmap(val/data.max())
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deflookup_extent(origin):
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"""Return extent for label positioning when not given explicitly."""
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iforigin=='lower':
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return (-0.5,6.5,-0.5,5.5)
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else:
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return (-0.5,6.5,5.5,-0.5)
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defset_extent_None_text(ax):
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ax.text(3,2.5,'equals\nextent=None',size='large',
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ha='center',va='center',color='w')
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defplot_imshow_with_labels(ax,data,extent,origin,xlim,ylim):
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"""Actually run ``imshow()`` and add extent and index labels."""
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im=ax.imshow(data,origin=origin,extent=extent)
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# extent labels (left, right, bottom, top)
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left,right,bottom,top=im.get_extent()
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ifxlimisNoneortop>bottom:
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upper_string,lower_string='top','bottom'
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else:
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upper_string,lower_string='bottom','top'
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ifylimisNoneorleft<right:
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port_string,starboard_string='left','right'
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inverted_xindex=False
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else:
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port_string,starboard_string='right','left'
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inverted_xindex=True
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bbox_kwargs= {'fc':'w','alpha':.75,'boxstyle':"round4"}
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ann_kwargs= {'xycoords':'axes fraction',
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'textcoords':'offset points',
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'bbox':bbox_kwargs}
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ax.annotate(upper_string,xy=(.5,1),xytext=(0,-1),
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ha='center',va='top',**ann_kwargs)
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ax.annotate(lower_string,xy=(.5,0),xytext=(0,1),
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ha='center',va='bottom',**ann_kwargs)
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ax.annotate(port_string,xy=(0,.5),xytext=(1,0),
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ha='left',va='center',rotation=90,
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**ann_kwargs)
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ax.annotate(starboard_string,xy=(1,.5),xytext=(-1,0),
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ha='right',va='center',rotation=-90,
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**ann_kwargs)
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ax.set_title('origin: {origin}'.format(origin=origin))
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# index labels
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forindexin ["[0, 0]","[0, N']","[M', 0]","[M', N']"]:
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tx,ty,halign=get_index_label_pos(index,extent,origin,
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inverted_xindex)
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facecolor=get_color(index,data,im.get_cmap())
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ax.text(tx,ty,index,color='white',ha=halign,va='center',
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bbox={'boxstyle':'square','facecolor':facecolor})
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ifxlim:
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ax.set_xlim(*xlim)
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ifylim:
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ax.set_ylim(*ylim)
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defgenerate_imshow_demo_grid(extents,xlim=None,ylim=None):
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N=len(extents)
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fig=plt.figure(tight_layout=True)
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fig.set_size_inches(6,N* (11.25)/5)
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gs=GridSpec(N,5,figure=fig)
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columns= {'label': [fig.add_subplot(gs[j,0])forjinrange(N)],
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'upper': [fig.add_subplot(gs[j,1:3])forjinrange(N)],
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'lower': [fig.add_subplot(gs[j,3:5])forjinrange(N)]}
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x,y=np.ogrid[0:6,0:7]
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data=x+y
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fororiginin ['upper','lower']:
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forax,extentinzip(columns[origin],extents):
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plot_imshow_with_labels(ax,data,extent,origin,xlim,ylim)
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forax,extentinzip(columns['label'],extents):
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text_kwargs= {'ha':'right',
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'va':'center',
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'xycoords':'axes fraction',
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'xy': (1,.5)}
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ifextentisNone:
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ax.annotate('None',**text_kwargs)
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ax.set_title('extent=')
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else:
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left,right,bottom,top=extent
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text= ('left: {left:0.1f}\nright: {right:0.1f}\n'+
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'bottom: {bottom:0.1f}\ntop: {top:0.1f}\n').format(
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left=left,right=right,bottom=bottom,top=top)
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ax.annotate(text,**text_kwargs)
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ax.axis('off')
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returncolumns
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###############################################################################
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#
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# Default extent
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# --------------
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#
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# First, let's have a look at the default `extent=None`
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generate_imshow_demo_grid(extents=[None])
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###############################################################################
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#
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# Generally, for an array of shape (M, N), the first index runs along the
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# vertical, the second index runs along the horizontal.
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# The pixel centers are at integer positions ranging from 0 to ``N' = N - 1``
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# horizontally and from 0 to ``M' = M - 1`` vertically.
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# *origin* determines how to the data is filled in the bounding box.
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#
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# For ``origin='lower'``:
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#
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# - [0, 0] is at (left, bottom)
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# - [M', 0] is at (left, top)
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# - [0, N'] is at (right, bottom)
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# - [M', N'] is at (right, top)
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#
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# ``origin='upper'`` reverses the vertical axes direction and filling:
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#
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# - [0, 0] is at (left, top)
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# - [M', 0] is at (left, bottom)
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# - [0, N'] is at (right, top)
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# - [M', N'] is at (right, bottom)
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#
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# In summary, the position of the [0, 0] index as well as the extent are
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# influenced by *origin*:
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#
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# ====== =============== ==========================================
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# origin [0, 0] position extent
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# ====== =============== ==========================================
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# upper top left ``(-0.5, numcols-0.5, numrows-0.5, -0.5)``
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# lower bottom left ``(-0.5, numcols-0.5, -0.5, numrows-0.5)``
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# ====== =============== ==========================================
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#
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# The default value of *origin* is set by :rc:`image.origin` which defaults
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# to ``'upper'`` to match the matrix indexing conventions in math and
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# computer graphics image indexing conventions.
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#
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#
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# Explicit extent
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# ---------------
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#
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# By setting *extent* we define the coordinates of the image area. The
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# underlying image data is interpolated/resampled to fill that area.
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#
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# If the axes is set to autoscale, then the view limits of the axes are set
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# to match the *extent* which ensures that the coordinate set by
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# ``(left, bottom)`` is at the bottom left of the axes! However, this
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# may invert the axis so they do not increase in the 'natural' direction.
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#
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extents= [(-0.5,6.5,-0.5,5.5),
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(-0.5,6.5,5.5,-0.5),
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(6.5,-0.5,-0.5,5.5),
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(6.5,-0.5,5.5,-0.5)]
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columns=generate_imshow_demo_grid(extents)
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set_extent_None_text(columns['upper'][1])
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set_extent_None_text(columns['lower'][0])
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###############################################################################
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#
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# Explicit extent and axes limits
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# -------------------------------
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#
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# If we fix the axes limits by explicity setting `set_xlim` / `set_ylim`, we
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# force a certain size and orientation of the axes.
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# This can decouple the 'left-right' and 'top-bottom' sense of the image from
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# the orientation on the screen.
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#
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# In the example below we have chosen the limits slightly larger than the
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# extent (note the white areas within the Axes).
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#
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# While we keep the extents as in the examples before, the coordinate (0, 0)
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# is now explicitly put at the bottom left and values increase to up and to
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# the right (from the viewer point of view).
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# We can see that:
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#
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# - The coordinate ``(left, bottom)`` anchors the image which then fills the
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# box going towards the ``(right, top)`` point in data space.
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# - The first column is always closest to the 'left'.
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# - *origin* controls if the first row is closest to 'top' or 'bottom'.
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# - The image may be inverted along either direction.
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# - The 'left-right' and 'top-bottom' sense of the image may be uncoupled from
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# the orientation on the screen.
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generate_imshow_demo_grid(extents=[None]+extents,
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xlim=(-2,8),ylim=(-1,6))

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