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  • callbacks: (WIP) Same as RAII + callbacks support, allowing progress monitoring and cancelation (partial).
  • RAII: (WIP) Same as master + RAII patches for memory management.
  • master: Same as master-sourceforge-cmake + ASan patches.
  • master-sourceforge-cmake: Same as master-sourceforge + CMake support.
  • master-sourceforge: Clone of (trunk Jan. 2021)


What is changed

  • Added new FreeImageCB struct, containing callback functions for start, progress, finish and message output for an operation.
  • Added new FreeImageLoadArgs struct, serving as a more extensible way of passing arguments to the loading functions.

How to use it

For now only the following loading functions have callback support: JPEG, PNG, BPM, TIFF, TARGA, PSD, RAW-Preview, WebP, EXR, HDR

The new functionality is exposed via new public functions that take an FreeImageLoadArgs instead of the old int flags argument:

FIBITMAP * FreeImage_LoadAdv(FREE_IMAGE_FORMAT fif, const char *filename, const FreeImageLoadArgs* args);

Versions for loading from handle and memory, as well as supporting wchar filenames are also available.

FreeImageLoadArgs contains all loading options, along side the new FreeImageCB struct:

struct FreeImageLoadArgs {
  unsigned flags;      //< lower 16 bits: same as old flags argument
  unsigned option;     //< lower 16 bits: FIF_JPEG: desired downscale size
  unsigned cbOption;   //< lower 8 bits: number of times onProgress should be called while loading
  const FreeImageCB* cb;
  void* more;
  • flags is equivalent to old flags argument with the notable exception of passing integer values (like downscale size). These are handled by a separate option member.
  • option is an integer value, the loading function might accept. Currently only JPEG loading uses this for passing downscale size.
  • cbOption controls how the callback is used. Currently it is interpreted as the desired number of times FreeImageCB::onProgress is invoked.
  • cb holds the actual callbacks.

FreeImageCB contains the callbacks as function pointers, as well as a pointer to user-data:

struct FreeImageCB {
  void* user;
  FI_OnStartedProc onStarted;
  FI_OnProgressProc onProgress;
  FI_OnFinishedProc onFinished;
  FI_OnMessageProc onMessage;

  void* more;

These are all (optionally) supplied by the user in order to have feedback about the state of operation.
For example, here is how to create a simple setup to monitor start and finish of image loading:

// prepare the callbacks

DLL_CALLCONV BOOL OnStartedProc(void* user, FREE_IMAGE_OPERATION operation, unsigned which, void*) {
  // signal "started" 

  return true; //< do continue with the operation
DLL_CALLCONV void OnFinishedProc(void* user, const BOOL* success, void*) {
  // signal "finished"

int main() {
  // init FreeImage if needed

  // setup callbacks

  FreeImageCB cb{};
  cb.onStarted = &OnStartedProc;
  cb.onFinished = &OnFinishedProc;

  // setup loading argument

  FreeImageLoadArgs args{};
  args.cb = &cb;

  // load the image

  auto* dib = FreeImage_LoadAdv(FIF_JPEG, "some-path/image.jpg", &args);

  //  use the dib

  // de-init FreeImage if needed

  return 0;

In the example above, the user-supplied OnStartedProc will be called just before actual load of image data starts. The user has the option to cancel loading at that point by returning false. On the other hand OnFinishedProc will be called just after the all data from the image is either loaded, an error occurred or the operation is canceled.
All currently available callbacks are as follows:

BOOL (*FI_OnStartedProc) (void* user, FREE_IMAGE_OPERATION operation, unsigned which, void* more);

This one is called when an operation has started - the input is successfully opened and data is about to be read.

  • user has the value of the FreeImageCB::user member, supplied when the struct is created.
  • operation is a new enum denoting different operations, executed by FreeImage, like loading, saving, rotation, tone-mapping etc. Currently callbacks are implemented only in loading and FI_OP_LOAD will be the only value operation will have.
  • which depends on the operation and denotes the type of operation. For FI_OP_LOAD (the only implemented operation) this is the FREE_IMAGE_FORMAT of the started loading.
  • return TRUE to continue with the operation; FALSE to cancel. If canceled, loading functions return null.
BOOL (*FI_OnProgressProc)(void* user, double progress, void* more);

This is called while the operation is in progress. In the case of image loading, when it is called is controlled by the FreeImageLoadArgs::cbOption member. At the moment only the first 8 bits ot this member are used and are interpreted as an simple count of how many times this callback is to be invoked (up 255 times). In future more options could be envisioned like number of lines skipped or number of milliseconds passed.

Note that currently this count is just a hint! The end result may vary greatly.

  • user has the value of the FreeImageCB::user member, supplied when the struct is created.
  • progress argument ranges from (0, 1] indicating the progress made.

Note that the final value(s) of progress might not be reached and that there will be uneven jumps b/w received values. This argument is only to have some overall visual feedback, not to give detailed information. In general, the primary purpose of the progress callback is to have cancellation support.

  • return TRUE to continue with the operation; FALSE to cancel. If canceled, loading functions return null.
void (*FI_OnFinishedProc)(void* user, const BOOL* success, void* more);

When the operation completes, no matter if successfully or not, this callback is invoked.

  • user has the value of the FreeImageCB::user member, supplied when the struct is created.
  • success argument indicates in what state the operation completes. If the argument is null, the operation has been canceled. Otherwise its value denotes success (TRUE) or failure (FALSE) in the case of error.
void (*FI_OnMessageProc) (void* user, const char* msg, void* more);

The last callback is an improved version of the existing FreeImage_OutputMessageFunction. It will receive the same messages as the old procedure, however it allows dedicated messages (in contrast of having a global function), as well as passing user-data.

Note, all more arguments (and members) are reserved for future expansion. Reserved are also all unused bits in the FreeImageLoadArgs members.

CMake support

What is changed ?

  • Added CMake support for the base library and its dependencies.
  • Fixed build issues, discovered using mingw-w64 under Windows.

How to use it ?

Please note that for now, only the core library is supported (the Source directory).

The project can be configured, using the standard CMake procedures.

From Source, one can simply execute:

cmake -B <build-directory>

Where <build-directory> is the desired build destination directory. This will create a build configuration for a dynamic library in Release mode, using the default CMake generator.

To specify a generatior, add -G <generator>, where <generator> is one of the supported generators. To see all generators run cmake --help.

Currently the CMake support does not introduce any configuration options. It does, however, respect the common CMake ones. In particular:

  • To build as static library add -DBUILD_SHARED_LIBS=OFF.
  • To build in Debug add -DCMAKE_BUILD_TYPE=Debug

Extended Example

The following line, ran from Source, configures a static library in Debug, using the Ninja generator in a ./build subdirectory:


To build, You can run the build command for the chosen generator.

ninja -C build

Or You can use CMake to call the build command for you.

cmake --build build

In case You are using the Visual Studio generator, You can open the generated solution and not use the command line for the build step.
Please consult the CMake manual for additional information.

Implementation Notes

Whenever possible, the CMakeLists.txt files for the depend libraries are the ones from the original project repositories, for the currently used version.
Some modifications are made, though. These are minor fixups, as well as adding ways to skip configuring install, tests and examples, but also skipping extensive code gen.

NOTE: After CMake runs the following original source files will be renamed:
ZLib/zconf.h => ZLib/zconf.h.included LibPNG/pnglibconf.h => LibPNG/pnglibconf.h.old
If you want to compile the original Visual Studio solution after that, remember to restore them back.


What is FreeImage ?

FreeImage is an Open Source library project for developers who would like to support popular graphics image formats like PNG, BMP, JPEG, TIFF and others as needed by today's multimedia applications. FreeImage is easy to use, fast, multithreading safe, and cross-platform (works with Windows, Linux and Mac OS X).

Thanks to it's ANSI C interface, FreeImage is usable in many languages including C, C++, VB, C#, Delphi, Java and also in common scripting languages such as Perl, Python, PHP, TCL, Lua or Ruby.

The library comes in two versions: a binary DLL distribution that can be linked against any WIN32/WIN64 C/C++ compiler and a source distribution. Workspace files for Microsoft Visual Studio provided, as well as makefiles for Linux, Mac OS X and other systems.


The FreeImage library with some additional features. These include: CMake support, progress and cancelation support (partial).