Convert an OpenCV 2 Image to an Allegro 5 Image In C/C++

Convert an OpenCV 2 Image to an Allegro 5 Image In C/C++

Just a quick sample for converting an OpenCV 2 image (Mat) to an Allegro 5 image (ALLEGRO_BITMAP).

First we need to setup some things and have places to store some stuff:

#include <allegro5/allegro.h>
#include <allegro5/allegro_image.h>
#include <cv.h>
#include <highgui.h>

cv::VideoCapture video([device number/filename]);
cv::Mat frame;
ALLEGRO_BITMAP *image = al_create_bitmap([width], [height]);

Next the guts:

video >> frame;
if ( !frame.empty() ) {
	al_set_target_bitmap(image);
	al_lock_bitmap(image, ALLEGRO_PIXEL_FORMAT_ANY, ALLEGRO_LOCK_WRITEONLY);
	for ( int y = 0; y < [height]; y++ ) {
		for ( int x = 0; x < [width]; x++ ) {
			cv::Vec3b &pixel = frame.at(y, x);
			al_put_pixel(x, y, al_map_rgb(pixel[2], pixel[1], pixel[0]));
		}
	}
	al_unlock_bitmap(image);
}

A few notes:

  • OpenCV 2 does not often work in RGB unless you make it. It is typically the reverse, BGR. Unless you have a specific need I see no reason not to do the conversion on-the-fly as above.
  • This sample assumes everything is the same width, height, color depth, ect, so watch out for that. Allegro, in particular, may slow to a crawl if you do not watch your conversions.
  • I am very not happy with the performance of this so it does need some work in that respect. It does, however, work very well otherwise. My goal is to get my Atom-based netbook running this smoothly. The Raspberry Pi may be a pipe dream but I am going to try.
  • This was tested in Linux with hardware I know what to expect out of. If there is any chance your webcam/video/whatever may return something other than a 24-bit (uint8, uint8, uint8) BGR color space you will need to account for that. Both OpenCV and Allegro have a number of functions/macros for that kind of thing.

This is mostly for my own notes but I figured someone else might also be interested. None of this is meant to be complete but, if you are struggling like I was, this should be all you need to pass that hurdle. Give a man a fish… alright, back to my cold, week-old “chinese” food and root beer.

Update 2012.11.28
After some more experimentation (and a nudge in the right direction from Peter Wang) I have tweaked the guts and it now runs much, much faster:

video >> frame;
if ( !frame.empty() ) {
	ALLEGRO_LOCKED_REGION *region = al_lock_bitmap(image, ALLEGRO_PIXEL_FORMAT_ANY, ALLEGRO_LOCK_WRITEONLY);
	for ( int y = 0; y < [height]; y++ ) {
		for ( int x = 0; x < [width]; x++ ) {
			uint32_t *ptr32 = (uint32_t *)region->data + x + y * (region->pitch / 4);
			*ptr32 = (frame.data[y * webcam_width * 3 + x * 3 + 0] << 16) | (frame.data[y * webcam_width * 3 + x * 3 + 1] << 8) | (frame.data[y * webcam_width * 3 + x * 3 + 2] << 0);
		}
	}
	al_unlock_bitmap(image);
}

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