elan: Set max frame height to 50, add note about performance

Frame height is a loosely experience-backed guesstimation. 50 seems to be
a good middle ground between swipe speed and quality.

libfprint/drivers/elan.c

@@ -19,6 +19,23 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+/*
+ * The algorithm which libfprint uses to match fingerprints doesn't like small
+ * images like the ones these drivers produce. There's just not enough minutiae
+ * (recognizable print-specific points) on them for a reliable match. This means
+ * that unless another matching algo is found/implemented, these readers will
+ * not work as good with libfprint as they do with vendor drivers.
+ *
+ * To get bigger images the driver expects you to swipe the finger over the
+ * reader. This works quite well for readers with a rectangular 144x64 sensor.
+ * Worse than real swipe readers but good enough for day-to-day use. It needs
+ * a steady and relatively slow swipe. There are also square 96x96 sensors and
+ * I don't know whether they are in fact usable or not because I don't have one.
+ * I imagine they'd be less reliable because the resulting image is even
+ * smaller. If they can't be made usable with libfprint, I might end up dropping
+ * them because it's better than saying they work when they don't.
+ */
+
 #define FP_COMPONENT "elan"
 
 #include "drivers_api.h"

libfprint/drivers/elan.h

@@ -48,7 +48,7 @@
 #define ELAN_MAX_FRAMES 30
 
 /* crop frames to this height to improve stitching */
-#define ELAN_MAX_FRAME_HEIGHT 30
+#define ELAN_MAX_FRAME_HEIGHT 50
 
 /* number of frames to drop at the end of capture because frames captured
  * while the finger is being lifted can be bad */