Save ArrayPlot in vector format as PDF?

Question

ClearAll[spiral];
spiral[n_?OddQ] := 
  Nest[With[{d = Length@#, l = #[[-1, -1]]}, 
     Composition[Insert[#, l + 3 d + 2 + Range[d + 2], -1] &, 
       Insert[#\[Transpose], l + 2 d + 1 + Range[d + 1], 
          1]\[Transpose] &, 
       Insert[#, l + d + Range[d + 1, 1, -1], 1] &, 
       Insert[#\[Transpose], 
          l + Range[d, 1, -1], -1]\[Transpose] &][#]] &, {{1}}, (n - 
      1)/2];

spiral[351] // PrimeQ // Boole // ArrayPlot
Export["Color351.pdf", %]

Code is from other post on the forum, credit to the OP. The question is, the exported PDF does not seem to have saved the graph in a lossless vector format.

This is what I get from the adobe acrobat:

If you go back and look at the picture from MMA10.2 X64 (win7), we can zoom in/out, the graph is still very sharp, in high resolution.

If there a way to save/export it with much higher quality?

Thanks.

Answer 1

I suspect that the fuzziness you see might not be caused by rasterization, but rather by antialiasing applied to the ArrayPlot markers. Anti-aliasing makes the edges of small graphical elements fuzzy to avoid jaggedness, and it is typically a good thing that generates visually pleasing results. In your case, however, the very small plot markers present in a default sized ArrayPlot get antialiased to this shape, when looking at the PDF generated by your code under maximum zoom:

Note that this effect is not visible in MMA if I zoom in to the plot, as the plot is evidently redrawn at the appropriate dimensions after zooming, and the effects of anti-aliasing are not as prominent on larger features.

---

I was able to get crisper results simply by requesting a larger output from ArrayPlot, and then exporting that:

spiral[351] // PrimeQ // Boole // ArrayPlot[#, ImageSize -> Large] &
Export["Color351Large.pdf", %]

Here is the a blown-up view of a similar portion of the graph from the exported "Color351Large.pdf":