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Sep 25, 2026
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NHW Image Codec is Raphael Canut's free, open-source experimental wavelet compressor, a pair of command-line tools that squeeze a 512 by 512 bitmap into a .nhw file faster than WebP or AVIF can manage.
NHW Image Codec is a one-man research project into a different way of compressing photographs, and the download here is the working proof of it.
It comes from Raphael Canut, who has been publishing it as open source since 2012 and still ships new builds - the current one landed on 7 September 2026.
It is free, royalty-free and patent-free, and it is also strictly experimental, which is the part the rest of this page is about.
READ THIS FIRST
NHW compresses one thing and one thing only: a 512 by 512 pixel, 24-bit colour BMP file. Not JPEGs, not photos from your camera, not any other size. This is a limitation the developer states plainly, and it has not changed in fourteen years of releases.
What It Actually Does
The download is a zip holding two small command-line programs - an encoder and a decoder. There is no installer, no window, no right-click menu and nothing that plugs into Windows or an image viewer.
You feed the encoder a bitmap and it writes a .nhw file. You feed the decoder that .nhw file and it writes a bitmap back. Nothing else on your computer can open a .nhw file, and nothing else ever will unless the format catches on.
Quality is set with a switch from -q1 to -q23, defaulting to 20, and the developer says the range he has actually tuned is -q8 to -q23. Higher numbers mean better quality and bigger files, the same way a JPEG quality slider works.
What Makes It Different
Most image compression chops a picture into small blocks and compresses each one. JPEG uses 8 by 8 pixel blocks, which is why a heavily compressed JPEG goes blocky around edges and lettering.
NHW runs a wavelet transform across the whole image instead, so the errors it makes are spread out rather than squared off. The developer's stated goal is what he calls "neatness" - keeping edges crisp - and he accepts losing some fine detail and colour precision to get it.
The other design goal is raw speed. NHW encodes and decodes considerably faster than AVIF, WebP or a still frame pushed through x265, which is what makes it interesting as research even though it is not yet a tool you can use.
Answering the One-Star Review
The review on this page says NHW only works on 512 by 512 images and cannot really be called a codec. The first half is correct and it is why the warning above sits where it does.
The second half is harsher than it needs to be. NHW is a genuine wavelet codec with an encoder, a decoder, a bitstream and a quality scale - it is simply an experiment that has never been extended to arbitrary image sizes, and the developer has never claimed otherwise on his own site.
Who Should Look Elsewhere
Almost everybody, honestly. If any row below describes you, the right column is the tool you actually want.
| If this is you | Use this instead |
|---|---|
| You want smaller photos that still open everywhere | WebP, which every browser reads, or libavif for AVIF |
| You want smaller files with no quality loss at all | HALIC for lossless, or JPEG XL, which can shrink existing JPEGs losslessly |
| You have a folder of images to resize or convert | Converseen or FastStone Photo Resizer |
| Windows will not show a format you have | The matching store component, such as the HEIC Image Extension or JPEG XL Image Extension |
| You are editing photographs | GIMP, or darktable if you shoot RAW |
| You are compressing images for a website | WebP or AVIF, both of which browsers already decode - a .nhw file would show nothing |
Who It Is Actually For
Codec developers and researchers, mainly. The source is on GitHub under a permissive licence with no patents attached, the encoder is a few thousand lines of plain C, and it compiles with gcc or CMake on Windows, Linux and macOS alike.
It is also worth a look if you are simply curious what a wavelet codec does to an image compared with a block-based one. Compress one of the developer's own test images at -q10 and the difference in how the two approaches fail is obvious at a glance.
If you came here looking for something you can actually use on your pictures, our graphics tools and image viewers categories are where the finished ones live.
What Changed in 0.3.4+2
Releases come often and each one is small. The version numbering runs 0.3.4, then 0.3.4+1, then 0.3.4+2, and the recent ones all refine the same internal weighting that decides how much of each wavelet band survives compression.
- 0.3.4+2, 7 September 2026 - continued improvement to the weight sequence.
- 0.3.4+1, 28 August 2026 - fine-tuning of the nhw_kernel weight computation.
- 0.3.4, 19 August 2026 - further work on the weight sequence.
The developer's own summary of the current build is "better neatness, better precision, still very fast". There is no changelog beyond that, and no list of fixed bugs, because the project has never had a formal one.
What You Need To Run It
The zip on the download page holds prebuilt Windows executables, and the whole codec is small enough to be startling next to any other download here. You run it from a Command Prompt, so there is nothing to install and nothing to uninstall.
Your input file has to be a 512 by 512 24-bit colour BMP. Any image editor will crop and export one - GIMP or even Paint will do it - and the developer publishes a small set of test images if you would rather not prepare your own.
If you want to check what you actually produced, ExifTool reads back the dimensions and colour depth of the bitmap you fed in and the one that came out.
Quick questions
Can I compress my photos with it?
Not usefully. It only accepts a 512 by 512 24-bit BMP, and nothing but its own decoder can read the .nhw file it produces. For real photos use WebP or AVIF.
Is NHW lossless?
No, it is lossy - it throws information away to make the file smaller, like JPEG does. For lossless compression, HALIC is the one on this site that keeps every pixel.
Does it install into Windows or my image viewer?
No. There is no installer and no shell integration - the download is two command-line executables in a zip, and .nhw files stay invisible to Explorer, Photos and every other program.
Is it really faster than WebP and HEVC?
On encoding and decoding speed, yes, and that is the project's main claim. It is a fair comparison only on 512 by 512 images, which is the only size it handles.
Is it safe and is it free?
It is free, open source, royalty-free and carries no patents, and the source is on GitHub for anyone to read. There is nothing bundled with it.
Is the project still going?
Yes. Three releases landed in August and September 2026 alone, though each is a small refinement rather than a new capability.
Download NHW Image Codec 0.3.4+2 if you want to see what a wavelet codec does, or tell us how you got on if you have already tried it.
JO
NHW is just a mediocre project that can only work with 512*512 image sizes. It has no advantages whatsoever. It can't even be considered a real codec. It's a great achievement that something like this was seen and showcased as soon as it was released!
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