aes2501: Detect reversed scans

aes2501 can be mounted 180 degrees rotated (this happens on most part of
laptops), so driver should detect whether sensor is 180degrees rotated
and assemble frames in right order.

THANKS

@@ -6,3 +6,4 @@ Craig Watson (NIST)
 James Vasile (SFLC)
 Toby Howard (University of Manchester)
 Seemant Kulleen
+Vasily Khoruzhick

libfprint/drivers/aes2501.c

@@ -378,11 +378,11 @@ static int sum_histogram_values(unsigned char *data, uint8_t threshold)
 
 /* find overlapping parts of  frames */
 static unsigned int find_overlap(unsigned char *first_frame,
-	unsigned char *second_frame)
+	unsigned char *second_frame, unsigned int *min_error)
 {
 	unsigned int dy;
-	unsigned int min_error = 255 * FRAME_SIZE;
 	unsigned int not_overlapped_height = 0;
+	*min_error = 255 * FRAME_SIZE;
 	for (dy = 0; dy < FRAME_HEIGHT; dy++) {
 		/* Calculating difference (error) between parts of frames */
 		unsigned int i;
@@ -397,8 +397,8 @@ static unsigned int find_overlap(unsigned char *first_frame,
 		/* Normalize error */
 		error *= 15;
 		error /= i;
-		if (error < min_error) {
-			min_error = error;
+		if (error < *min_error) {
+			*min_error = error;
 			not_overlapped_height = dy;
 		}
 		first_frame += FRAME_WIDTH;
@@ -409,11 +409,13 @@ static unsigned int find_overlap(unsigned char *first_frame,
 
 /* assemble a series of frames into a single image */
 static unsigned int assemble(unsigned char *input, unsigned char *output,
-	int num_strips)
+	int num_strips, gboolean reverse, unsigned int *errors_sum)
 {
 	uint8_t *assembled = output;
 	int frame;
 	uint32_t image_height = FRAME_HEIGHT;
+	unsigned int min_error;
+	*errors_sum = 0;
 
 	if (num_strips < 1)
 		return 0;
@@ -422,6 +424,8 @@ static unsigned int assemble(unsigned char *input, unsigned char *output,
 	 * Taken from document describing aes2501 image format
 	 * TODO: move reversing detection here */
 	
+	if (reverse)
+		output += (num_strips - 1) * FRAME_SIZE;
 	for (frame = 0; frame < num_strips; frame++) {
 		int column;
 	    for (column = 0; column < FRAME_WIDTH; column++) {
@@ -433,7 +437,10 @@ static unsigned int assemble(unsigned char *input, unsigned char *output,
 			}
 		}
 
-		output += FRAME_SIZE;
+		if (reverse)
+		    output -= FRAME_SIZE;
+		else
+		    output += FRAME_SIZE;
 	}
 
 	/* Detecting where frames overlaped */
@@ -442,11 +449,12 @@ static unsigned int assemble(unsigned char *input, unsigned char *output,
 		int not_overlapped;
 
 		output += FRAME_SIZE;
-		not_overlapped = find_overlap(assembled, output);
+		not_overlapped = find_overlap(assembled, output, &min_error);
+		*errors_sum += min_error;
 		image_height += not_overlapped;
 		assembled += FRAME_WIDTH * not_overlapped;
 		memcpy(assembled, output, FRAME_SIZE); 
-	} 
+	}
 	return image_height;
 }
 
@@ -512,6 +520,7 @@ static int capture(struct fp_img_dev *dev, gboolean unconditional,
 	int r;
 	struct fp_img *img;
 	unsigned int nstrips;
+	unsigned int errors_sum, r_errors_sum;
 	unsigned char *cooked;
 	unsigned char *imgptr;
 	unsigned char buf[1705];
@@ -574,12 +583,19 @@ static int capture(struct fp_img_dev *dev, gboolean unconditional,
 	if (nstrips == MAX_FRAMES)
 		fp_warn("swiping finger too slow?");
 
-	img->height = assemble(img->data, cooked, nstrips);
+	img->flags = FP_IMG_COLORS_INVERTED;
+	img->height = assemble(img->data, cooked, nstrips, FALSE, &errors_sum);
+	img->height = assemble(img->data, cooked, nstrips, TRUE, &r_errors_sum);
+	
+	if (r_errors_sum > errors_sum) {
+	    img->height = assemble(img->data, cooked, nstrips, FALSE, &errors_sum);
+		img->flags |= FP_IMG_V_FLIPPED | FP_IMG_H_FLIPPED;
+	}
+
 	for (i = 0; i < img->height * FRAME_WIDTH; i++)
 		img->data[i] = (cooked[i] << 4) | 0xf;
 
 	img = fpi_img_resize(img, img->height * FRAME_WIDTH);
-	img->flags = FP_IMG_V_FLIPPED | FP_IMG_H_FLIPPED | FP_IMG_COLORS_INVERTED;
 	*ret = img;
 	return 0;
 err: