reduce PNG resolution when pixel width or height of the image

gets too close to CairoGraphics limits

ocitysmap/__init__.py

@@ -483,6 +483,14 @@ SELECT ST_AsText(ST_LongestLine(
             except configparser.NoOptionError:
                 dpi = OCitySMap.DEFAULT_RENDERING_PNG_DPI
 
+            w_px = int(layoutlib.commons.convert_mm_to_dots(config.paper_width_mm, dpi))
+            h_px = int(layoutlib.commons.convert_mm_to_dots(config.paper_height_mm, dpi))
+
+            if w_px > 25000 or h_px > 25000:
+                LOG.warning("%d DPI to high for this paper size, using 72dpi instead" % dpi)
+                dpi = layoutlib.commons.PT_PER_INCH
+
+                
             # As strange as it may seem, we HAVE to use a vector
             # device here and not a raster device such as
             # ImageSurface. Because, for some reason, with

ocitysmap/layoutlib/commons.py

@@ -32,5 +32,8 @@ def convert_pt_to_dots(pt, dpi = PT_PER_INCH):
 def convert_mm_to_pt(mm):
     return mm / MM_PER_INCH * PT_PER_INCH
 
+def convert_mm_to_dots(mm, dpi = PT_PER_INCH):
+    return float(convert_mm_to_pt(mm) * dpi) / PT_PER_INCH
+
 def convert_pt_to_mm(pt):
     return float(pt) * MM_PER_INCH / PT_PER_INCH