tiles option leads to rendering artefacts

[Klaus-Tockloth]

The usage of the tiles option (e.g. --tiles 2) leads to rendering artefacts.

If the aim is to create one large image (e.g. 25000x25000 pixels) the tiles option isn't necessary at all. mapnik has (as far as I know) no size limit. It seems that the problem comes in with the resize function (which seems to be limited [16384?]):

...
    # for layer processing we need to create the Map object
    m = mapnik.Map(100, 100)  # temporary size, will be changed before output
    mapnik.load_map_from_string(m, style_xml.encode("utf-8"), False, style_path)
    m.srs = proj_target.params()
...
    # export image
    m.aspect_fix_mode = mapnik.aspect_fix_mode.GROW_BBOX
    m.resize(size[0], size[1])
    m.zoom_to_box(bbox)

A possible solution to avoid the rendering artefacts (if the aim is to create one large image) is to avoid tiling and resizing.

...
    # for layer processing we need to create the Map object
    m = mapnik.Map(size[0], size[1])
    mapnik.load_map_from_string(m, style_xml.encode("utf-8"), False, style_path)
    m.srs = proj_target.params()
...
    # export image
    m.aspect_fix_mode = mapnik.aspect_fix_mode.GROW_BBOX
    m.zoom_to_box(bbox)
...

[Zverik]

Thanks for finding the issue, Klaus. I don't remember exactly, but this resizing fixed some issue with resolution or density, I don't know. Maybe size may be not available before the style is loaded.

If you need tiles from a mapnik stylesheet, I recommend using polytiles — it supports rendering in metatiles and all that.