How to correctly export image for grid of plot in high resolution?

Question

For example, I have a grid of plot stored in variable plots

   plots = Grid[{{Grid[
     Table[ListDensityPlot[{{1, 1, 1, 1}, {1, 2, 1, 2}, {1, 1, 3, 
         1}, {1, 2, 1, 4}}, Mesh -> All], {i, 2}, {j, 2}]], 
    Plot[x, {x, 0, 1}, PlotLabel -> "yyy"]}}]

and it is looked like in frontend

If I export it to jpg like

Export["test.jpg", plots]

The image looking is exactly the same as it shows in mathematica frontend. However, the resolution is low.

But I want high resolution, however,

Export["test.jpg", plots, ImageResolution -> 600]

gives

So my questions is

• why ImageResolution affect the looking?
• how to export plots as high resolution jpg, while keep the same visual looking in frontend?

---

update

I found even Jens' trick will fail for some complicated grid of graphics. For example, try below

plot = Plot[x, {x, 0, 1}, Frame -> True, AspectRatio -> 1];
plots = Grid[{{Grid[Table[plot, {i, 2}, {j, 2}]], 
     Plot[x, {x, 0, 1}, PlotLabel -> "yyy"]}}];
newplots = 
 Grid[{{Show[plot, ImageSize -> 200, AspectRatio -> 2], plots}}]

and Jens' method

h = First@ImportString[ExportString[newplots, "PDF"]];
Rasterize[h, ImageResolution -> 300]

You will notice significant difference.

After tested many solutions, I found the most stable way to "export" to file and keep exactly the same looking as in front end and keep high resolution is using "save sections as" PDF for the whole cell. You can confirm that this is not equivalent to Export["..pdf",...] using newplots as test. Though an undesirable thing is that the output file will have cell tag.

However, this operation can not batch processing, if we want to save a bunches of files.

So I am wondering, Mathematica do have a mechanism which can keep all the same looking when saving cell, no matter how complicated the graphics is ( grid, even grid of grid). There must be a underlining command that can mimic "save section as" PDF. Does anyone know the command to mimic "save section as" PDF, so we can do batch processing?

Answer 1

There are (at least) two problems with Mathematica that come into play here. A solution that addresses them both is to wrap all your plots with toPDF as follows, before exporting them:

toPDF[plot_] := 
 First@ImportString[
   ExportString[
    Show[plot, 
     Prolog -> {Opacity[0], Texture[{{{0, 0, 0, 0}}}], 
       VertexTextureCoordinates -> {{0, 0}, {1, 0}, {1, 1}}, 
       Polygon[{{0, 0}, {.1, 0}, {.1, .1}}]}], "PDF"]]


plots = Grid[{{Grid[
      Table[
        toPDF@
        ListDensityPlot[{{1, 1, 1, 1}, {1, 2, 1, 2}, {1, 1, 3, 1}, {1,
            2, 1, 4}}, Mesh -> All], {i, 2}, {j, 2}]], 
     toPDF@Plot[x, {x, 0, 1}, PlotLabel -> "yyy"]}}];

Export["test.jpg", plots, ImageResolution -> 600]

(* ==> "test.jpg" *)

The result looks the same as in the notebook. The first issue I'm addressing with toPDF is the incorrect scaling of the ticks and frame for which I suggested the PDF export and re-import trick here. The second issue is that this trick doesn't work properly with the DensityPlot output, but this can be fixed by triggering automatic rasterization upon export. I do this with another earlier hack by adding an invisible textured polygon to each plot.

Answer 2

You may use the ImageSize option for Graphics; both of these functions have the same options as Graphics.

The issue is that the default value for ImageSize is Automatic. This fits the graphic to the size of its the container. In Grid this resizes the graphics when they go through the export process.

You may add any setting to the option for the the graphics. ImageSize -> Small or any other setting will work.

plots = 
 Grid[{
  {Grid[Table[
    ListDensityPlot[{{1, 1, 1, 1}, {1, 2, 1, 2}, {1, 1, 3, 1}, {1, 2, 1, 4}}, 
      Mesh -> All, 
      ImageSize -> Small], 
    {i, 2}, {j, 2}]], 
   Plot[x, {x, 0, 1}, 
     PlotLabel -> "yyy", 
     ImageSize -> Small]}
  }];

Export["test.jpg", plots, ImageResolution -> 600]

Hope this helps.