I’ve finally added image coloring to Kolori! Kolori can now take an image as input, by either selecting a image file using a native file dialog or dragging and dropping a image onto the window! The image below was created using the feature–using one of the gallery images as the input image.

The hard part about this was, not actually loading the image file, reading the pixel data, uploading that into the GPU, and rendering it, but instead managing the file dialog by which a user can upload an image into the application.

We use SDL3 for windowing and make use of the SDL_ShowOpenFileDialog function to, well, show an open file dialog. I initially put all of the image loading and OpenGL calls into the dialog callback, but something strange happened then. All of my OpenGL calls were triggering an GL_INVALID_OPERATION error–and specifically on Windows–but why? All of my OpenGL calls were fine, no invalid arguments passed in (or in the wrong order), so everything should’ve been fine.

The answer lies in the SDL_ShowOpenFileDialog and how OpenGL works with multiple threads. SDL_ShowOpenFileDialog is an asynchronous function–it will return immediately, and the result of the dialog will be passed on to the callback. However, the callback function may be called on a different thread depending on the operating system, which explains the difference in effect on Windows, and the GL_INVALID_OPERATION errors were caused by the absence of an active OpenGL context on the new thread.

SDL3 does not provide an API for setting the current thread’s OpenGL context, so I had to cope. I moved most of the business logic out of the callback function, only keeping the image loading logic, and instead performed the texture uploading into the UI rendering logic, where we check if there is any new image data that happened to be uploaded, upload the data onto the GPU as a texture and show it on screen.