Saving an Image

When we save the image, we can use a different format from the original. For instance, JPEG doesn't have transparent colors, but we can write the image back out as a PNG, which does. The stb_image_write library has different functions for each image type. Here's the code to save our image as PNG. 

stbi_write_png("pete.png", width, height, channels, pete,
        width * channels);

Each file type you want to use has its own function, but the first five arguments are the same for each file type:

  1. The file name as a C-style string. Here we've hard-coded pete.png
  2. The width and height returned from stbi_load().
  3. The number of channels (or bytes-per-pixel) used in memory to represent the image.
  4. The pointer to the first byte of the image data in memory.

The last argument, width * channels, is unique to PNG files. It tells the function at what byte the next row begins.

Freeing the Memory

The stbi_load() function returns a pointer, but inside that function it asks the operating system to allocate enough memory to hold the image that it loads from disk. This memory is on the heap, which you met in an earlier lesson. In the C programming language, you have to remember to free that memory before your program ends. We do that by using the function stbi_image_free(pete).