Fix issue 203 - Text fade isn't smooth

- Change calculation of "dt" to ensure proper numeric range of text alpha
- Ensure text alpha actually reaches zero to prevent still-visible ghosted text
- Reorder some code to ensure slow image processing doesn't impact text fade time
- Remove some commented-out code

picframe/viewer_display.py

@@ -452,15 +452,14 @@ def slideshow_is_running(self, pics=None, time_delay = 200.0, fade_time = 10.0,
         loop_running = self.__display.loop_running()
         tm = time.time()
         if pics is not None:
-            #self.__sbg = self.__sfg # if the first tex_load fails then __sfg might be Null TODO should fn return if None?
+            new_sfg = self.__tex_load(pics, (self.__display.width, self.__display.height))
+            tm = time.time()
             self.__next_tm = tm + time_delay
             self.__name_tm = tm + fade_time + self.__show_text_tm # text starts after slide transition
-            new_sfg = self.__tex_load(pics, (self.__display.width, self.__display.height))
             if new_sfg is not None: # this is a possible return value which needs to be caught
                 self.__sbg = self.__sfg
                 self.__sfg = new_sfg
             else:
-                #return (True, False) # return early
                 (self.__sbg, self.__sfg) = (self.__sfg, self.__sbg) # swap existing images over
             self.__alpha = 0.0
             if fade_time > 0.5:
@@ -494,13 +493,9 @@ def slideshow_is_running(self, pics=None, time_delay = 200.0, fade_time = 10.0,
                 self.__xstep, self.__ystep = (self.__slide.unif[i] * 2.0 / (time_delay - fade_time) for i in (48, 49))
                 self.__slide.unif[48] = 0.0
                 self.__slide.unif[49] = 0.0
-                #self.__kb_up = not self.__kb_up # just go in one direction
 
         if self.__kenburns and self.__alpha >= 1.0:
             t_factor = time_delay - fade_time - self.__next_tm + tm
-            #t_factor = self.__next_tm - tm
-            #if self.__kb_up:
-            #    t_factor = time_delay - t_factor
             # add exponentially smoothed tweening in case of timing delays etc. to avoid 'jumps'
             self.__slide.unif[48] = self.__slide.unif[48] * 0.95 + self.__xstep * t_factor * 0.05
             self.__slide.unif[49] = self.__slide.unif[49] * 0.95 + self.__ystep * t_factor * 0.05
@@ -520,18 +515,22 @@ def slideshow_is_running(self, pics=None, time_delay = 200.0, fade_time = 10.0,
 
         if self.__alpha >= 1.0 and tm < self.__name_tm:
             # this sets alpha for the TextBlock from 0 to 1 then back to 0
-            dt = 1.1 - (self.__name_tm - tm) / self.__show_text_tm # i.e. dt from 0.1 to 1.1
+            dt = 1.0 - (self.__name_tm - tm) / self.__show_text_tm
+            if dt > 0.995: dt = 1 # ensure that calculated alpha value fully reaches 0 (TODO: Improve!)
             ramp_pt = max(4.0, self.__show_text_tm / 4.0) # always > 4 so text fade will always < 4s
+
             # create single saw tooth over 0 to __show_text_tm
             alpha = max(0.0, min(1.0, ramp_pt * (1.0 - abs(1.0 - 2.0 * dt)))) # function only run if image alpha is 1.0 so can use 1.0 - abs...
+
+            # if we have text, set it's current alpha value to fade in/out
             for block in self.__textblocks:
                 if block is not None:
                     block.sprite.set_alpha(alpha)
 
-            if self.__text_bkg_hgt:
+            # if we have a text background to render (and we currently have text), set its alpha and draw it
+            if self.__text_bkg_hgt and any(block is not None for block in self.__textblocks): #txt_len > 0: #only draw background if text there
                 self.__text_bkg.set_alpha(alpha)
-                if any(block is not None for block in self.__textblocks): #txt_len > 0: #only draw background if text there
-                    self.__text_bkg.draw()
+                self.__text_bkg.draw()
 
         for block in self.__textblocks:
             if block is not None:
@@ -540,4 +539,4 @@ def slideshow_is_running(self, pics=None, time_delay = 200.0, fade_time = 10.0,
         return (loop_running, False) # now returns tuple with skip image flag added
 
     def slideshow_stop(self):
-        self.__display.destroy()
+        self.__display.destroy()