Fix edge case where color temp is reset to TEMPERATURE_ZERO in delta mode

This quirk is explained in the README. Basically if the brightness that's passed to
get_sct_for_screen() is less than or equal to zero, the temperature is 'set' to 0.
This means that if we were using delta mode to decrease the brightness, the temperature
information will be lost whenever the brightness becomes less than or equal to 0.

Before this commit, this would manifest itself by forcing the temperature to 700K.
For example, doing `xsct 6500 -1; xsct -d 0 1` would result in a red screen, even though
we didn't intend to change the color temperature.

Ideally, the temperature information should stay the same if we are only shifting the brightness
and we reach 0.0, but I couldn't think of a straightforward way to implement this.

Also add a fix to make sure temp and brightness are included when using the delta flag

Author: j-silv

README.md

@@ -120,6 +120,10 @@ Test xsct using the following command:
 xsct 3700 0.9 && xsct
 ~~~
 
+# Quirks
+
+- If the delta mode is used to decrease the brightness to below 0.0 and then increased above 0.0, the temperature will reset to 6500K, regardless of whatever it was before the brightness reached 0. This is because the temperature is reset to 0K when the brightness is set equal to or below 0.0 (to verify this, you can run `xsct 0 0.0; xsct`).
+
 # Resources
 
 The following website by Mitchell Charity provides a table for the conversion between black-body temperatures and color pixel values:

src/xsct.c

@@ -158,11 +158,18 @@ static void sct_for_screen(Display *dpy, int screen, int icrtc, struct temp_stat
 
 static void bound_temp(struct temp_status *const temp)
 {
-    if (temp->temp < TEMPERATURE_ZERO)
+    if (temp->temp <= 0)
+    {
+        // identical behavior when xsct is called in absolute mode with temp == 0
+        fprintf(stderr, "WARNING! Temperatures below %d cannot be displayed.\n", TEMPERATURE_ZERO);
+        temp->temp = TEMPERATURE_NORM;
+    }
+    else if (temp->temp < TEMPERATURE_ZERO)
     {
         fprintf(stderr, "WARNING! Temperatures below %d cannot be displayed.\n", TEMPERATURE_ZERO);
         temp->temp = TEMPERATURE_ZERO;
     }
+
     if (temp->brightness < 0.0)
     {
         fprintf(stderr, "WARNING! Brightness values below 0.0 cannot be displayed.\n");
@@ -297,18 +304,21 @@ int main(int argc, char **argv)
             else
             {
                 // Delta mode: Shift temperature and optionally brightness of each screen by given value
-                if (temp.temp == DELTA_MIN)
+                if (temp.temp == DELTA_MIN || temp.brightness == DELTA_MIN)
                 {
-                    fprintf(stderr, "ERROR! Required temperature value for delta not specified!\n");
+                    fprintf(stderr, "ERROR! Temperature and brightness delta must both be specified!\n");
                 }
                 else
                 {
                     for (screen = screen_first; screen <= screen_last; screen++)
                     {
                         struct temp_status tempd = get_sct_for_screen(dpy, screen, crtc_specified, fdebug);
+
                         tempd.temp += temp.temp;
-                        if (temp.brightness != DELTA_MIN) tempd.brightness += temp.brightness;
+                        tempd.brightness += temp.brightness;
+
                         bound_temp(&tempd);
+
                         sct_for_screen(dpy, screen, crtc_specified, tempd, fdebug);
                     }
                 }