ARToolkit using a still image - c

Question

I'm trying to use ARtoolkit, but with a static image instead of a video stream. I need to be able to load an image, identify markers, and locate them. I'm using SDL for loading the image. I'm able to obtain the RGB values for each pixel from the loaded image, but I'm unsure how to format the data for ARToolkit to work with it.
ARToolkit stores its images as type ARUint8* (an unsigned char*). I'm confused as to how this format works. Right now I have this code inside the main loop that runs continuously as the program is executing. This code (should) print out the RGB values for each pixel in the frame.
ARUint8* dataPtr;
dataPtr = arVideoGetImage(); // Get a new frame from the webcam
int width, height;
if (arVideoInqSize(&width, &height) == 0) // if width and height could be obtained
{
for (int y = 0; y < height; y++)
{
for (int x = 0; x < width; x++)
{
printf("pixel %i, %i: %i, %i, %i\n", x, y, dataPtr[(y * 320) + x], dataPtr[(y * 320) + x + 1], dataPtr[(y * 320) + x + 2]);
}
}
}
Typical output:
pixel 5, 100: 0, 0, 0
pixel 6, 100: 178, 3, 0
pixel 7, 100: 0, 0, 177
etc...
It seems to be accessing the RGB values correctly, but I'm unsure how to copy over the image data (from SDL's format) into this new format.

Answer 1

Figured it out. Posting answer in case anyone else ever needs it.
On Windows, ARToolkit defaults to BGRA for the dataPtr array. The following function will load an image (using SDL) and return a pointer to a ARUint8 (that contains the image data).
ARUint8* loadImage(char* filename, int* w, int* h)
{
SDL_Surface* img = IMG_Load(filename);
if (!img)
{
printf("Image '%s' failed to load. Error: %s\n", filename, IMG_GetError());
return NULL;
}
*w = img->w; // Assign width and height to the given pointers
*h = img->h;
ARUint8* dataPtr = (ARUint8*)calloc(img->w * img->h * 4, sizeof(ARUint8)); // Allocate space for image data
// Write image data to the dataPtr variable
for (int y = 0; y < img->h; y++)
{
for (int x = 0; x < img->w; x++)
{
Uint8 r, g, b;
SDL_GetRGB(getpixel(img, x, y), img->format, &r, &g, &b); // Get the RGB values
int i = ((y * img->w) + x) * 4; // Figure out index in array
dataPtr[i] = b;
dataPtr[i + 1] = g;
dataPtr[i + 2] = r;
dataPtr[i + 3] = 0; // Alpha
}
}
SDL_FreeSurface(img);
return dataPtr;
}
The getpixel function is borrowed from here: http://sdl.beuc.net/sdl.wiki/Pixel_Access
This function allowed me to use a photograph instead of a video feed from a webcam.