An stb_image wrapper for Nim (including stb_image_write & stb_image_resize). Official Repo here:
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Nim stb_image Wrapper

This is a Nim wrapper of the stb_image single header file libraries (stb_image.h and stb_image_write.h).

Official repo page can be found here:

All other sites should be considered mirrors, though if you log an issue on GitHub, I'll probably address it.

The used header files are included in this repo (they end in the name _header.nim) but you may provide your own. The versions that are used are:

  • stb_image.h: v2.19
  • stb_image_write.h: v1.07

They can be found here:

If something is out of date or incorrect. Please notify me on the GitLab issue tracker so I can fix it. Though if something like a header file is out of date, you should be able to simply exchange it out (unless something was changed in the API).

zlib client

For PNG support, stb_image requires a functioning zlib compressor/decompressor, and one is built in. I'd like to thank RSDuck for exposing the functions with some Nim friend wrappers. Se you want to see how to use it, check tests.nim for some samples.

Callback Functions & stbi_image_resize

Only the callback funtions for stbi_image_write are implemented. I'd like to thank Eduardo Bart (@edubart) for help on this. As for stbi_image_resize, I felt that the scope of this wrapper should be focused on image loading and saving, not processing.

Though if someone would like these included in the wrapper I am open to pull requests.


In the nature of Sean Barret (author of stb) putting his libraries in public domain, so is this code. Specifically it falls under the "Unlicense." Please check the file LICENSE for details.

How To Use

I've tried to write this to be as close as possible to the orignal C API, so if you're already familiar with it, there should be no hassle. If you don't all of the functions are documented in stb_image/read.nim and stb_image/write.nim.

Import what you want to use. I recommend adding the as semantic:

import stb_image/read as stbi
import stb_image/write as stbiw

An original C call would look like this:

#include "stb_image.h"

// Get the image data
int width, height, channels;
unsigned char *data = stbi_load("kevin_bacon.jpeg", &width, &height, &channels, STBI_default);

// Do what you want...

// Cleanup

But becomes this:

import stb_image/read as stbi

  width, height, channels: int
  data: seq[uint8]

data = stbi.load("kevin_bacon.png", width, height, channels, stbi.Default)
# No need to do any GC yourself, as Nim takes care of that for you!

Functions that had names like_this have been turned intoThis. That stbi_ portion has also been dropped.

If you want to write pixels, it's like what you see here, but in reverse:

import stb_image/write as stbiw

# Stuff some pixels
var data: seq[uint8] = @[]

# save it (as monochrome)
stbiw.writeBMP("three.bmp", 3, 1, stbiw.Y, data)

Some of the functions (or variables) that effect the library globally are still there (e.g. setFlipVerticallyOnLoad() in stb_image), but some other has been moved to functions calls as to not act in a global manor (e.g. the useRLE parameter for writeTGA() in stb_image/write).

I also recommend reading through the documentation at the top of the original header files too, as they give you a bit more of a grasp of how things work and the limitations of stb_image.

All of the functions that can do image reading (which are, well, only found in stb_image/read.nim) will throw an STBIException if there was an error retriving the image data. The .message field of the exception will contain some slight information into what went wrong.

Future Plans

  • Return a data structure that descrives an image instead of a sequence of bytes/shorts/floats from the functions. This may be a much more natural way for the programmer to get an image and make a little more sense than returning results in both the "return," statement and "var," parameters. Such a system might have a data structure like this:

      # `T` should only be uint8, uint16, or float32
      STBImage[T]* = ref object of RootObj
        width*: int
        height*: int
        channels*: int # One of the values from `stb_image/components.nim`
        pixelData*: seq[T]

    And the Nim-Friendly functions would change from this:

    # Data
      width, height, channels: int,
      pixels: seq[uint8]
    # Load the image
    pixels = stbi.load("kevin_bacon.jpeg", width, height, channels, stbi.Default)

    Over to this:

    var image = stbi.load("kevin_bacon.jpeg", stbi.Default)

    It may also solve an issue with the pixel data being copied (unecessarly) with the current wrappers. See this thread in the Nim Forum for details:

    The only thing I don't like about this is that it would break the familiarity with the original C API. I don't want to maintain multiple functions that have the same functionality so I would be removing those orignal bindings.

    Trying to figure out how to make the STBImage type play nice with the write functions might be a little more work too (e.g. validating there is enough data and correct parameters).

    I'd like to get some comments on this before moving forward with it.

  • I really would like add unit tests for the functions listed in the Untested Functions section to verify they work, but I'm in need of some very simple test images.

  • Add wrappers/bindings for the stb_image_resize.h library. It's part of the STB toolkit (and falls under it's "image," section), but it wasn't related to image IO so I decided to leave it out. It also looked like quite a bit of work to add in. If someone wants to submit a pull request, I'll review it.

Unbound Functions

Right now stbi_load_gif_from_memory isn't bound, but will be in the future. See: for details on why this hasn't been added in yet.

Untested Functions

All functions should have tests for them, but I wasn't able to find some test images (namely for the HDR) functions. So they do have bindings for completeness, but they haven't been proven to work. If you do happent to have some very simple test images I could use, please provide them. Here is the list of functions that are untested.

From stb_image.h:

  • load16()
  • loadFromFile16()
  • load16FromMemory()
  • loadF()
  • loadFFromMemory()
  • loadFFromFile()
  • HDRToLDRGamma()
  • HDRToLDRScale()
  • LDRToHDRGamma()
  • LDRToHDRScale()
  • isHDRFromMemory()
  • isHDR()
  • isHDRFromFile()
  • setUnpremultiplyOnLoad()
  • stbiConvertIPhoneRGBToPNG()

From stb_image_write.h:

  • writeHDR()


  • Eduardo Bart (@edubart), for help with a segfault issue and write callback functions.