René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Speeding up contours with (non-affine) transforms","articleBody":"For the Astropy project, we provide astronomy-specific transforms that users can use in various Matplotlib methods/artists, including e.g. ``Axes.contour``. In fact, we have a sub-class of Axes called [WCSAxes](http://docs.astropy.org/en/stable/visualization/wcsaxes/index.html) which until now just inherited the default ``contour`` and ``contourf``. However, the complexity of the transforms means that there is a small overhead (\u003e1-10ms) for every call to the transform that is unrelated to the number of elements to transform.\r\n\r\nIn Matplotlib, when drawing contours with a transform:\r\n```\r\nax.contour(..., transform=...)\r\n```\r\neach individual path in the contour is passed to ``transform_path_non_affine``. This is done here:\r\n\r\nhttps://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/collections.py#L191\r\n\r\nHowever, this means that in cases where a contour map has say 1000 individual lines (not that uncommon with some astronomy images), drawing a contour map can take \u003e10s.\r\n\r\nI found a solution in astropy/astropy#7568 - override ``contour`` and call matplotlib's ``contour`` without the transform, then modify the vertices in-place afterwards by constructing a single array with all the vertices and transforming it in one go and splitting it up again. The key function is:\r\n\r\nhttps://github.com/astropy/astropy/pull/7568/files#diff-421f6873fdd2d8a90f39f3050534b0c3R139\r\n\r\nThis dramatically sped up drawing contours in our case. The reason I'm opening this issue (as suggested by @dopplershift on Gitter) is that this solution is not specific to Astropy. In fact, this could be copied almost as-is into Matplotlib if there was interest. In our case, there is basically no downside to doing this performance-wise even when there are very few contour lines, and I think this is generally true for this approach.\r\n\r\nFeel free to close this if you'd rather not port this over to Matplotlib though, as there may be subtleties I am not aware of!","author":{"url":"https://github.com/astrofrog","@type":"Person","name":"astrofrog"},"datePublished":"2018-06-20T18:56:52.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":16},"url":"https://github.com/11464/matplotlib/issues/11464"}