Identifies the different resolution units.
Values:
| IG_RESOLUTION_NO_ABS | No absolute units. |
| IG_RESOLUTION_METERS | Pels (Pixels) Per Meter. |
| IG_RESOLUTION_INCHES | Dots (Pixels) Per Inch. |
| IG_RESOLUTION_CENTIMETERS | Pixels Per Centimeter. |
| IG_RESOLUTION_10_INCHES | Dots (Pixels) Per 10 Inches. |
| IG_RESOLUTION_10_CENTIMETERS | Pixels Per 10 Centimeters. |
| IG_RESOLUTION_LAST |