How tiny is tinyexr, and how big is OpenEXR?

Blog post about all of this: https://aras-p.info/blog/2025/11/22/OpenEXR-vs-tinyexr/

tinyexr is an excellent simple library for loading and saving OpenEXR files. It has one big advantage, in that it is very simple to start using: just one source file to compile and include! However, it also has some downsides, namely that not all features of OpenEXR are supported (for example, it can't do PXR24, B44/B44A, DWAA/DWAB, HTJ2K compression modes), and performance might be behind the official library. It probably can't do some of more exotic EXR features either (e.g. "deep" images), but I'll ignore those for now.

But how large and how complex to use is the "official" OpenEXR library, anyways?

I do remember that a decade ago it was quite painful to build it, especially on anything that is not Linux. However these days (2025), that seems to be much simpler: it uses a CMake build system, and either directly vendors or automatically fetches whatever dependencies it needs, unless you really ask it to "please don't do this".

It is not exactly a "one source file" library though. However, I noticed that OpenUSD vendors OpenEXR "Core" library, builds it as a single C source file, and uses their own "nanoexr" wrapper around the API; see pxr/imaging/plugin/hioOpenEXR/OpenEXR. So I took that, adapted it to more recent OpenEXR versions (theirs uses 3.2.x, I updated to 3.4.4).

And then I wrote a tiny app that reads an EXR file, and writes it back as downsampled EXR (so this includes both reading & writing parts of an EXR library). And compared how large is the binary size between tinyexr and OpenEXR, as well as their respective source code sizes and EXR read performance.

Actual process was:

• Take OpenEXR source repository (v3.4.4, 2025 Nov),
  • Take only the src/lib/OpenEXRCore and external/deflate folders from it.
  • openexr_config.h, compression.c, internal_ht.cpp have local changes! Look for LOCAL CHANGE comments.
• Take OpenJPH (https://github.com/aous72/OpenJPH) source code, used 0.25.3 (2025 Nov), put under external/OpenJPH.
• Take openexr-c.c, openexr-c.h, OpenEXRCoreUnity.h from the OpenUSD repository. They were for OpenEXR v3.2, and needed some adaptations for later versions.
• Take tinyexr source repository (v1.0.12, 2025 Mar).

Results

Library	Binary size, KB	Source size, KB	read+write time, s	Notes
tinyexr 1.0.12	251	726	6.55
OpenEXR 3.2.4	2221	8556	2.19
OpenEXR 3.3.5	826	3831	1.68	Removed giant DWAA/DWAB lookup tables.
OpenEXR 3.4.3	1149	5373	1.68	Added HTJ2K compression (via OpenJPH).
OpenEXR 3.4.4	649	3216	1.65	Removed more B44/DWA lookup tables.
+ no HTJ2K	370	1716		Above, with HTJ2K/OpenJPH compiled out.
+ no DWA	318			Above, and with DWAA/DWAB compiled out.
+ no B44	305			Above, and with B44/B44A compiled out.
+ no PXR24	303			Above, and with PXR24 compiled out.

Notes:

• Hardware is Windows, Ryzen 5950X, compiler Visual Studio 2022 (17.14), Release build.
• This compares both tinyexr and OpenEXR in fully single-threaded mode. Tinyexr has threading capabilities, but it spins up and shuts down a whole thread pool for each processed image, which is a bit "meh"; and while OpenEXRCore can be threaded (and using full high level OpenEXR library does use it that way), the "nanoexr" wrapper I took from USD codebase does not do any threading.
• Timing is total time taken to read, downsample (by 2x) and write back 6 EXR files, input resolution 3840x2160, input files are ZIP FP16, ZIP FP32, ZIP w/ mips, ZIP tiled, PIZ and RLE compressed; output is ZIP compressed.

Takeaways

• OpenEXR has been mostly getting smaller over time, nice! Two large size reductions have been where Kimball Thurston has removed very large DWA compression lookup tables (for v3.3, PR #1948, 1.4MB size reduction), and I have just removed some more tables (for v3.4.4, PRs #2126, #2174, 0.5MB size reduction).
• However, addition of the new HTJ2K compression codec in v3.4 has increased the OpenEXR library size by 0.3MB.
• Tinyexr is smaller, but not that much smaller than OpenEXR. And, again, it does not understand some of the compression modes (PXR24, B44, DWA, HTJ2K) and thus can't load some EXR files.
• Tinyexr seems to be about 4x slower than OpenEXR library :(