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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[MNT]: Revamp `draw_path_collection()` API","articleBody":"### Summary\n\nI am opening this issue in order to discuss the possibility of enhancing the API of `draw_path_collection()` for the long term. This was initially discussed in [this and the following comments](https://github.com/matplotlib/matplotlib/pull/29044#issuecomment-2656133496) by @anntzer and @timhoffm.\n\nAdding/ vectorizing new attributes in collections requires changing the signature of `draw_path_collection()`. This causes incompatibility with third party backends, such as `mplcairo` , because they still use the older signature.\n\nTo workaround this in #29044 (that added the `hatchcolors` argument) a provisional API was used, which relies on making an empty call to `draw_path_collection()` to infer the signature. This current API is temporary and requires a rewrite.\n\n### Proposed fix\n\nA solution proposed by @anntzer in [this comment](https://github.com/matplotlib/matplotlib/pull/29044#issuecomment-2656991777) is to use an API similar to `draw_path()` - send parameters through a dataclass and use getters to retrieve the required parameters, analogous to `GraphicsContextBase` in `draw_path()`. \nWe could call this dataclass something like `VectorizedGraphicsContextBase` . It is similar to `GraphicsContextBase`,  but each attribute is vectorized. A simple draft of the proposed solution is shown below.\n\n### Why is this change needed?\n\nThis would cause a one time break for third party backends. However, in the long term, it would become much simpler to add / vectorize new attributes in collections, without completely breaking third party backends. For example, it would greatly simplify #27937, that aims to vectorize the `hatch` attribute.\n\n### Code Draft\n\n```python\n# backend_bases.py\nclass VectorizedGraphicsContextBase:\n    def __init__(self):\n        # vectorized attributes\n        self._facecolor = [(0.0, 0.0, 0.0, 1.0)]\n        self._rgb = [(0.0, 0.0, 0.0, 1.0)]\n        self._linewidth = [1]\n        ...\n\n        # non-vectorized attributes\n        self._cliprect = None\n        self._clippath = None\n        ...\n```\n\n```python\n# collections.py\n\n# `vgc` is an instance of VectorizedGraphicsComtextBase\ndef draw(self, renderer):\n    ...\n    transform, offset_trf, offsets, paths = self._prepare_points()\n\n    vgc = renderer.new_vgc()\n    self._set_gc_clip(vgc)\n\n    vgc._facecolor = self.get_facecolor()\n    vgc._rgb = self.get_edgecolor()\n    vgc._linewidth = self.get_linewidth()\n    ...\n\n    renderer.draw_path_collection(vgc, transform.frozen(), paths, self.get_transforms(),\n                                  offsets, offset_trf)\n```\n\n```python\n# backend_bases.py\ndef draw_path_collection(self, vgc, master_transform, paths, all_transforms,\n                         offsets, offset_trans):\n    path_ids = self._iter_collection_raw_paths(\n        master_transform, paths, all_transforms\n    )\n\n    for xo, yo, path_id, gc, rgbFace in self._iter_collection(\n        vgc, list(path_ids), offsets, offset_trans\n    ):\n        path, transform = path_id\n\n        if xo != 0 or yo != 0:\n            transform = transform.frozen()\n            transform.translate(xo, yo)\n        self.draw_path(gc, path, transform, rgbFace)\n\n\ndef _iter_collection(self, vgc, path_ids, offsets, offset_trans):\n    Npaths = len(path_ids)\n    Noffsets = len(offsets)\n    N = max(Npaths, Noffsets)\n    Nfacecolors = len(vgc._facecolor)\n    Nedgecolors = len(vgc._rgb)\n    Nlinewidths = len(vgc._linewidth)\n\n    pathids = cycle_or_default(path_ids)\n    toffsets = cycle_or_default(offset_trans.transform(offsets), (0, 0))\n    fcs = cycle_or_default(vgc._facecolor)\n    ecs = cycle_or_default(vgc._rgb)\n    lws = cycle_or_default(vgc._linewidth)\n\n    gc = self.new_gc()\n    # attributes that are not vectorized\n    gc._clip_path = vgc._clip_path\n    gc._clip_rect = vgc._clip_rect\n    ...\n\n    for pathid, (xo, yo), fc, ec, lw in itertools.islice(\n        zip(pathids, toffsets, fcs, ecs, lws), N\n    ):\n        if not (np.isfinite(xo) and np.isfinite(yo)):\n            continue\n\n        if Nedgecolors:\n            if Nlinewidths:\n                gc.set_linewidth(lw)\n            if len(ec) == 4 and ec[3] == 0.0:\n                gc.set_linewidth(0)\n            else:\n                gc.set_foreground(ec)\n        if fc is not None and len(fc) == 4 and fc[3] == 0:\n            fc = None\n        ...\n\n        yield xo, yo, pathid, gc, fc\n\n    gc.restore()\n```","author":{"url":"https://github.com/r3kste","@type":"Person","name":"r3kste"},"datePublished":"2025-09-11T10:01:47.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/30547/matplotlib/issues/30547"}