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Fix ImagePlot subpixel cases when zoomed and panned to an edge #199

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Fix ImagePlot subpixel cases when zoomed and panned to an edge #199

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24 changes: 18 additions & 6 deletions chaco/image_plot.py
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Original file line number Diff line number Diff line change
Expand Up @@ -284,10 +284,17 @@ def _calc_zoom_coords(self, px, py, plot_width, plot_height,
yparams = (1, 3)
for pos_index, size_index in (xparams, yparams):
if subimage_index[pos_index] == subimage_index[pos_index+2]-1:
# xcoords lie inside the same pixel, so set the subimage
# coords to be the width of the image
subimage_coords[pos_index] = plot_dimensions[pos_index]
subimage_coords[size_index] = plot_dimensions[size_index]
# coords lie inside the same pixel, so can clamp subimage_coords
# to the plot bounds
clamped_min = max(subimage_coords[pos_index],
plot_dimensions[pos_index])

si_max = subimage_coords[pos_index] + subimage_coords[size_index]
plot_max = plot_dimensions[pos_index] + plot_dimensions[size_index]
clamped_max = min(si_max, plot_max)

subimage_coords[pos_index] = clamped_min
subimage_coords[size_index] = clamped_max - clamped_min
elif subimage_index[pos_index] == subimage_index[pos_index+2]-2:
# coords span across a pixel boundary. Find the scaling
# factor of the virtual (and potentially large) subimage
Expand All @@ -300,12 +307,17 @@ def _calc_zoom_coords(self, px, py, plot_width, plot_height,
# entire screen, since we are only straddling one pixel boundary.
# The formula for calculating the new origin can be worked out
# on paper.
#
# Panning to near an edge so only one transition is visible
# causes an attempt to scale up the subimage_coords, so we
# guard against that case too.
extent = subimage_coords[size_index]
pixel_extent = extent/2 # we are indexed into two pixels
origin = subimage_coords[pos_index]
scale = float(2 * plot_dimensions[size_index] / extent)
subimage_coords[size_index] *= scale
subimage_coords[pos_index] = origin + (1-scale)*pixel_extent
if scale < 1:
subimage_coords[size_index] *= scale
subimage_coords[pos_index] = origin + (1-scale)*pixel_extent

subimage_index = map(int, subimage_index)

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