genesys: Implement a way to override DPISET on GL124 and GL843

backend/genesys_gl124.cc

@@ -1005,8 +1005,13 @@ static void gl124_init_optical_regs_scan(Genesys_Device* dev, const Genesys_Sens
         r->value |= REG05_GMMENB;
     }
 
-    reg->set16(REG_DPISET, dpiset * ccd_size_divisor);
-    DBG (DBG_io2, "%s: dpiset used=%d\n", __func__, dpiset * ccd_size_divisor);
+    unsigned dpiset_reg = dpiset * ccd_size_divisor;
+    if (sensor.dpiset_override != 0) {
+        dpiset_reg = sensor.dpiset_override;
+    }
+
+    reg->set16(REG_DPISET, dpiset_reg);
+    DBG (DBG_io2, "%s: dpiset used=%d\n", __func__, dpiset_reg);
 
   r = sanei_genesys_get_address (reg, REG06);
   r->value |= REG06_GAIN4;

backend/genesys_gl843.cc

@@ -1145,6 +1145,10 @@ static void gl843_init_optical_regs_scan(Genesys_Device* dev, const Genesys_Sens
 
     unsigned dpiset = session.output_resolution * session.ccd_size_divisor *
             ccd_pixels_per_system_pixel;
+
+    if (sensor.dpiset_override != 0) {
+        dpiset = sensor.dpiset_override;
+    }
     reg->set16(REG_DPISET, dpiset);
     DBG(DBG_io2, "%s: dpiset used=%d\n", __func__, dpiset);
 

backend/genesys_sensor.h

@@ -162,9 +162,14 @@ struct Genesys_Sensor {
     // the scan method used with the sensor
     ScanMethod method = ScanMethod::FLATBED;
 
-    // The scanner may be setup to use a constant dpihw. If non-zero, the overridden dpihw is set
+    // The scanner may be setup to use a custom dpihw that does not correspond to any actual
+    // resolution. The value zero does not set the override.
     unsigned dpihw_override = 0;
 
+    // The scanner may be setup to use a custom dpiset value that does not correspond to any actual
+    // resolution. The value zero does not set the override.
+    unsigned dpiset_override = 0;
+
     // CCD may present itself as half or quarter-size CCD on certain resolutions
     int ccd_size_divisor = 1;