Specify exponential base in the args and modify README

README.md

 
     light -e -U 5
 
+Increase backlight brightness by 2 percent in exponential mode and specify exponential base K, it must be more then 1.0 and less then 20.0
+
+    light -e2.8 -U 2
+
 Set the minimum cap to 2 in raw value on the sysfs/backlight/acpi_video0 device:
 
     light -Nrs "sysfs/backlight/acpi_video0" 2

src/light.c

         double max_value_d = (double)max_value;
         double percent = 0;
         if(ctx->run_params.mode == LC_MODE_EXPONENTIAL)
-            percent = pow((inraw_d / max_value_d), 1.0 / LIGHT_EXPONENT) * 100.0;
+            percent = pow((inraw_d / max_value_d), 1.0 / ctx->run_params.exponent) * 100.0;
         else
             percent = (inraw_d / max_value_d) * 100.0;
         
     double max_value_d = (double)max_value;
     double target_value_d = 0;
     if(ctx->run_params.mode == LC_MODE_EXPONENTIAL)
-        target_value_d = max_value_d * pow(light_percent_clamp(inpercent) / 100.0, LIGHT_EXPONENT);
+        target_value_d = max_value_d * pow(light_percent_clamp(inpercent) / 100.0, ctx->run_params.exponent);
     else
         target_value_d = max_value_d * (light_percent_clamp(inpercent) / 100.0);
     uint64_t target_value = LIGHT_CLAMP((uint64_t)target_value_d, 0, max_value);
         "\n"
         "Options:\n"
         "  -r          Interpret input and output values in raw mode (ignored for -T)\n"
-        "  -e          Interpret input and output values in exponential mode (ignored for -r and -T)\n"
+        "  -e[K]       Interpret input and output values in exponential mode (ignored for -r)\n"
+        "                 Optional K value specifies base of the exponential mode. Default is 4.0\n"
+        "                 K must be more then 1.0 and less then 20.0 \n"
         "  -s          Specify device target path to use, use -L to list available\n"
         "  -v          Specify the verbosity level (default 0)\n"
         "                 0: Values only\n"
     bool specified_target = false;
     snprintf(ctrl_name, sizeof(ctrl_name), "%s", "sysfs/backlight/auto");
 
-    while((curr_arg = getopt(argc, argv, "HhVGSLMNPAUTOIv:s:re")) != -1)
+    while((curr_arg = getopt(argc, argv, "HhVGSLMNPAUTOIv:s:re?")) != -1)
     {
         switch(curr_arg)
         {
                 ctx->run_params.mode = LC_MODE_RAW;
                 break;
             case 'e':
-                ctx->run_params.mode = LC_MODE_EXPONENTIAL;
+                if(ctx->run_params.mode != LC_MODE_RAW)
+                    ctx->run_params.mode = LC_MODE_EXPONENTIAL;
+                if(optarg)
+                {
+                    if(sscanf(optarg, "%lf", &ctx->run_params.exponent) != 1)
+                    {
+                        fprintf(stderr, "-e argument is not an double.\n\n");
+                        _light_print_usage();
+                        return false;
+                    }
+                    else if(ctx->run_params.exponent <= 1 || ctx->run_params.exponent > 20)
+                    {
+                        fprintf(stderr, "-e argument must be more then 1 and less then 20.\n\n");
+                        _light_print_usage();
+                        return false;
+                    }
+                }
+                else
+                {
+                    ctx->run_params.exponent = LIGHT_EXPONENT;
+                }
                 break;
             
             // Commands
                 break;
             case 'T':
                 _light_set_context_command(ctx, light_cmd_mul_brightness);
-                ctx->run_params.mode = LC_MODE_PERCENTAGE;
+                if(ctx->run_params.mode != LC_MODE_RAW)
+                    ctx->run_params.mode = LC_MODE_PERCENTAGE;
                 need_target = true;
                 need_float_value = true;
                 break;
         break;
     case LC_MODE_PERCENTAGE:
     case LC_MODE_EXPONENTIAL:
+        if(value < 5 && value > 0)
+        { // on small values adequate behaviour
+            value --;
+            break;
+        }   
         if(!_light_raw_to_percent(ctx, value, &percent))
         {
             LIGHT_ERR("failed to convert value from raw to percent for device target");

src/light.h

         // Only one of value and raw_value is populated; which one depends on the command
         uint64_t                value; // The input value, in raw mode
         float                   float_value; // The input value as a float
+        double                  exponent; // Exponent value for the exponential mode
         light_context_mode      mode; // Whether or not we use raw, exponential or percentage mode
         light_device_target_t   *device_target; // The device target to act on
     } run_params;