Remove artifact of PDF images

Question

I have a PDF figure created with Export[] in Mathematica 11.0.1.0 on Mac OSX

The figure has these improper white-lines when I zoom-out and make the image smaller (The size it will appear in the document).

I have seen a previous post Avoiding white lines inside filled area in RegionPlot exported as PDF or PS but this does not resolve this issue.

The code used to generate the image is:

f21fps = Show[SmoothDensityHistogram[data21fps,
   PlotRange -> {{0, 200}, {0, 200}}, 
   ColorFunction -> "SunsetColors",
   FrameLabel -> {Text[
      Style["t2-t1", FontSize -> 14, Italic],
       FormatType -> StandardForm], 
     Text[Style["t1", FontSize -> 14, 
       Italic], FormatType -> StandardForm]},
   PlotLabel -> "FPS", LabelStyle -> {FontFamily -> "Times", 14},
   PlotLegends -> 
    BarLegend[Automatic, LegendFunction -> f, 
     LegendMarkerSize -> 368]
   , Mesh -> 0
   ], Plot[x, {x, 0, 200}], ImageSize -> 400, PlotRangePadding -> 0]
 Export["f21fps.pdf",f21fps];

where data21fps is a list of {x,y} values. How could I avoid the undesired white lines when reducing the size of the pdf image?

EDITED 1

Due to comment of @march, I am trying to rasterize the plot but not the axes as it is explained in the post Rasterized image together with vector-graphics axes

which uses this code to make it

rasterizeBackground[g_, res_: 450] := 
 Show[Rasterize[
    Show[g, PlotRangePadding -> 0, ImagePadding -> 0, 
     ImageMargins -> 0, LabelStyle -> Opacity[0], 
     FrameTicksStyle -> Opacity[0], FrameStyle -> Opacity[0], 
     AxesStyle -> Opacity[0], TicksStyle -> Opacity[0], 
     PlotRangeClipping -> False], ImageResolution -> res] /. 
   Raster[data_, rect_, rest__] :> 
    Raster[data, 
     Transpose@OptionValue[AbsoluteOptions[g, PlotRange], PlotRange], 
     rest], Sequence @@ Options[g], Sequence @@ Options[g, PlotRange]]

I have been testing it but gives me error

f21fpsR = rasterizeBackground[f21fps]
Rule::argr: Rule called with 1 argument; 2 arguments are expected.

How could I fix this and make it work with my image?

Thank you very much

Answer 1

To fix the error you get when using rasterizeBackground, add the following definitions (remove the old one):

rasterizeBackground[g_Graphics, res_: 450] := 
 Show[Rasterize[
    Show[g, PlotRangePadding -> 0, ImagePadding -> 0, 
     ImageMargins -> 0, PlotLabel -> None, FrameTicks -> None, 
     Frame -> None, Axes -> None, Ticks -> None, 
     PlotRangeClipping -> False], ImageResolution -> res] /. 
   Raster[data_, rect_, rest__] :> 
    Raster[data, 
     Transpose@OptionValue[AbsoluteOptions[g, PlotRange], PlotRange], 
     rest], Sequence @@ Options[g], 
  Sequence @@ Options[g, PlotRange]] 

rasterizeBackground[g_, res_: 450] := 
  ReplacePart[
    g,
    # -> rasterizeBackground[Extract[g, #], res] & 
     /@ Position[g, Graphics[__]]
  ]

The issue with rasterizeBackground is that it works under the assumption that it's argument is the Graphics expression without any wrappers. This assumption is no longer fulfilled when you use PlotLegends, as this causes the result to have head Legended.

The above definition fixes this by searching for anything of the form Graphics[__] and replacing it by a rasterized version (using a modified version of the original rasterizeBackground) inside the original expression.

Update

Fully working example: The below code produces the expected result for me (fresh kernel, MMA 11.1). I basically copied your code and created some sample data.

(*some random data*)
data21fps = 200 {#1 #2, Sin[#1 + #2]^2} & @@@ RandomReal[{0, 1}, {100000, 2}];
(*the original plot*)
f21fps = Show[
  SmoothDensityHistogram[data21fps, PlotRange -> {{0, 200}, {0, 200}},
    ColorFunction -> "SunsetColors", 
   FrameLabel -> {Text[Style["t2-t1", FontSize -> 14, Italic], 
      FormatType -> StandardForm], 
     Text[Style["t1", FontSize -> 14, Italic], 
      FormatType -> StandardForm]}, PlotLabel -> "FPS", 
   LabelStyle -> {FontFamily -> "Times", 14}, 
   PlotLegends -> 
    BarLegend[Automatic, LegendFunction -> "Panel", 
     LegendMarkerSize -> 368], Mesh -> 0], Plot[x, {x, 0, 200}], 
  ImageSize -> 400, PlotRangePadding -> 0]

(*the fixed version of rasterizeBackground for Graphics expressions*)
rasterizeBackground[g_Graphics, res_: 450] := 
 Show[Rasterize[
    Show[g, PlotRangePadding -> 0, ImagePadding -> 0, 
     ImageMargins -> 0, PlotLabel -> None, FrameTicks -> None, 
     Frame -> None, Axes -> None, Ticks -> None, 
     PlotRangeClipping -> False], ImageResolution -> res] /. 
   Raster[data_, rect_, rest__] :> 
    Raster[data, 
     Transpose@OptionValue[AbsoluteOptions[g, PlotRange], PlotRange], 
     rest], Sequence @@ Options[g], 
  Sequence @@ Options[g, PlotRange]] 

(*the extension for wrapped Graphics expressions*)
rasterizeBackground[g_, res_: 450] := 
 ReplacePart[
  g, # -> rasterizeBackground[Extract[g, #], res] & /@ 
   Position[g, Graphics[__]]]

(*test it*)
rasterizeBackground[f21fps]

rasterizeBackground[Column@{Panel[f21fps], "Works anywhere"}, 10]

Update 2

As noted in the comments, the original version of rasterizeBackground has some issues and produced a white bar at the top of the plot range. The reason for this was that labels, frames, etc. were only hidden (with Opacity[0]) instead of removed. The updated version (see above) now simply removes all these features using None.