Merge pull request #89 from claudiofelber/master

Support horizontally flipped QR-codes according to ISO 18004:2015

.gitignore

@@ -9,3 +9,4 @@ libquirc.so*
 .*.swp
 *~
 .DS_Store
+.idea

README.md

@@ -174,6 +174,23 @@ for (i = 0; i < num_codes; i++) {
 `quirc_code` and `quirc_data` are flat structures which don't need to be
 initialized or freed after use.
 
+In case you also need to support horizontally flipped QR-codes (mirrored
+images according to ISO 18004:2015, pages 6 and 62), you can make a second
+decode attempt with the flipped image data whenever you get an ECC failure:
+
+```C
+    err = quirc_decode(&code, &data);
+    if (err == QUIRC_ERROR_DATA_ECC) {
+        quirc_flip(&code);
+        err = quirc_decode(&code, &data);
+    }
+
+    if (err)
+        printf("DECODE FAILED: %s\n", quirc_strerror(err));
+    else
+        printf("Data: %s\n", data.payload);
+```
+
 Copyright
 ---------
 Copyright (C) 2010-2012 Daniel Beer <<dlbeer@gmail.com>>

lib/decode.c

@@ -916,3 +916,18 @@ quirc_decode_error_t quirc_decode(const struct quirc_code *code,
 
 	return QUIRC_SUCCESS;
 }
+
+void quirc_flip(struct quirc_code *code)
+{
+	struct quirc_code flipped = {0};
+	unsigned int offset = 0;
+	for (int y = 0; y < code->size; y++) {
+		for (int x = 0; x < code->size; x++) {
+			if (grid_bit(code, y, x)) {
+				flipped.cell_bitmap[offset >> 3u] |= (1u << (offset & 7u));
+			}
+			offset++;
+		}
+	}
+	memcpy(&code->cell_bitmap, &flipped.cell_bitmap, sizeof(flipped.cell_bitmap));
+}

lib/quirc.h

@@ -168,6 +168,9 @@ void quirc_extract(const struct quirc *q, int index,
 quirc_decode_error_t quirc_decode(const struct quirc_code *code,
 				  struct quirc_data *data);
 
+/* Flip a QR-code according to optional mirror feature of ISO 18004:2015 */
+void quirc_flip(struct quirc_code *code);
+
 #ifdef __cplusplus
 }
 #endif

tests/inspect.c

@@ -34,6 +34,10 @@ static void dump_info(struct quirc *q)
 
 		quirc_extract(q, i, &code);
 		err = quirc_decode(&code, &data);
+		if (err == QUIRC_ERROR_DATA_ECC) {
+			quirc_flip(&code);
+			err = quirc_decode(&code, &data);
+		}
 
 		dump_cells(&code);
 		printf("\n");

tests/qrtest.c

@@ -125,9 +125,16 @@ static int scan_file(const char *path, const char *filename,
 
 		quirc_extract(decoder, i, &code);
 
-		if (!quirc_decode(&code, &data))
+		quirc_decode_error_t err = quirc_decode(&code, &data);
+		if (err == QUIRC_ERROR_DATA_ECC) {
+			quirc_flip(&code);
+			err = quirc_decode(&code, &data);
+		}
+
+		if (!err) {
 			info->decode_count++;
 		}
+	}
 
 	(void)clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &tp);
 	info->total_time += MS(tp) - total_start;
@@ -150,12 +157,14 @@ static int scan_file(const char *path, const char *filename,
 
 			if (want_verbose) {
 				struct quirc_data data;
-				quirc_decode_error_t err =
+				quirc_decode_error_t err = quirc_decode(&code, &data);
-					quirc_decode(&code, &data);
+				if (err == QUIRC_ERROR_DATA_ECC) {
+					quirc_flip(&code);
+					err = quirc_decode(&code, &data);
+				}
 
 				if (err) {
-					printf("  ERROR: %s\n\n",
+					printf("  ERROR: %s\n\n", quirc_strerror(err));
-					       quirc_strerror(err));
 				} else {
 					printf("  Decode successful:\n");
 					dump_data(&data);