Different image brightness when exporting same plot to PDF and PNG

Question

I noticed that exporting a plot to PDF results in increased brightness relative to exporting the same plot to PNG.

Here are screenshots of a PDF and a PNG obtained from the same plot. As you can see, the picture above looks brighter than the one below.

Question. Is there a way to export to PDF while having the resulting picture look exactly as the corresponding PNG export?

I am using Mathematica 12.2.0 on a Mac (running macOS Monterey 12.5.1). To generate the plot and export the images I am using the code below.

SetDirectory[NotebookDirectory[]];

sphere[u_, v_] := {Cos[v] Cos[u], Cos[v] Sin[u], Sin[v]}

plot = ParametricPlot3D[
   Evaluate[sphere[u, v]], {u, -Pi, Pi}, {v, -Pi/2, Pi/2}, 
   PlotPoints -> 200, Mesh -> {30, 5}, MeshStyle -> Opacity[0.8], 
   PlotTheme -> {"Classic"}, Ticks -> None];

Export["sphere.pdf", plot];

Export["sphere.png", plot];

EDIT Aug 27, 2022. On my machine, exporting the PDF as suggested in Alexey Popkov's answer still results in a picture that looks brigther than the corresponding PNG (both with Preview and Adobe Acrobat Reader). If, on the other hand, I export the PDF by means of the Preview's export command, then the resulting PDF appears identical to the original PNG. Needless to say, I am puzzled.

EDIT Aug 29, 2022. The problem does not occur on the Windows computer in my office. So perhaps this is a macOS-related issue.

Answer 1

Let us compare what is exported to PDF and PNG (I'm using Mathematica 12.3.1 on Windows 10 x64):

Export["sphere.pdf", plot];
Export["sphere.png", plot];
imgPDF = FirstCase[Import["sphere.pdf", "PageGraphics"], 
   r_Raster :> ImageReflect@Image[r], Automatic, -1];
imgPNG = Import["sphere.png"];
Information /@ {imgPDF, imgPNG}

We see that the main difference is in raster size of the images. Also, the image embedded in PDF has a transparency channel. Let us try to reproduce the image we obtained from PDF with PNG export:

Export["sphere.png", plot, RasterSize -> 1080, Background -> None];
imgPNG2 = Import["sphere.png"];

Information /@ {imgPDF, imgPNG2}

Excepting the resolution, other characteristics are identical. Let us check the ImageDifference:

ImageDifference[imgPDF, imgPNG2]

We see that the images are still different, probably due to different rendering methods used on the rasterization stage. Let us compare them more closely:

Image[ImageTake[#, {338, 443}, {228, 313}], 
   ImageSize -> 200] & /@ {imgPDF, imgPNG2}

Apparenlty, the renderings are quite different. I'm not sure which options allow to reproduce the image embedded in PDF. If you like it more, you could simply export it as PNG:

Export["sphere_from_PDF.png", imgPDF]

If you like the PNG variant more, you can use Rasterize with the corresponding settings to generate Image, and then export it as PDF:

Export["sphere_as_in_PNG.pdf", 
  img = Rasterize[plot, RasterSize -> 360, Background -> White]];

Now get the image back from PDF and compare with the original:

imgPDF2 = 
  FirstCase[Import["sphere_as_in_PNG.pdf", "PageGraphics"], 
   r_Raster :> Image[r], Automatic, -1];
ImageData[ImageDifference[img, imgPDF2]] // Abs // Max

0.

They are absolutely identical. Hence any visible differences are due to different rendering algorithms used by your image- and PDF-viewers. Here is a screenshot with the two images rendered on Windows 10 by Microsoft Photos and Adobe Acrobat Reader respectively: