notcurses_render: simplify using new alpha rules

include/notcurses.h

@@ -1000,24 +1000,23 @@ channels_set_bg_default(uint64_t* channels){
   return *channels;
 }
 
-// Returns the result of blending two channels.
+// Returns the result of blending two channels. 'blends' indicates how heavily
+// 'c1' ought be weighed. If 'blends' is 0, 'c1' will be entirely replaced by
+// 'c2'. If 'c1' is otherwise the default color, 'c1' will not be touched,
+// since we can't blend default colors. Likewise, if 'c2' is a default color,
+// it will not be used (unless 'blends' is 0).
 static inline unsigned
 channels_blend(unsigned c1, unsigned c2, unsigned blends){
-  if(channel_default_p(c1)){
-    return c1;
+  if(blends == 0){
+    return c2;
   }
-  if(channel_default_p(c2)){
-    if(blends == 0){
-      channel_set_default(&c1);
-    }
-    return c1;
+  if(!channel_default_p(c2) && !channel_default_p(c1)){
+    int rsum = (channel_get_r(c1) * blends + channel_get_r(c2)) / (blends + 1);
+    int gsum = (channel_get_g(c1) * blends + channel_get_g(c2)) / (blends + 1);
+    int bsum = (channel_get_b(c1) * blends + channel_get_b(c2)) / (blends + 1);
+    channel_set_rgb(&c1, rsum, gsum, bsum);
   }
-  int rsum = (channel_get_r(c1) * blends + channel_get_r(c2)) / (blends + 1);
-  int gsum = (channel_get_g(c1) * blends + channel_get_g(c2)) / (blends + 1);
-  int bsum = (channel_get_b(c1) * blends + channel_get_b(c2)) / (blends + 1);
-  unsigned blend = 0;
-  channel_set_rgb(&blend, rsum, gsum, bsum);
-  return blend;
+  return c1;
 }
 
 // Extract the 32-bit background channel from a cell.

src/lib/render.c

@@ -146,9 +146,9 @@ reshape_shadow_fb(notcurses* nc){
 // tail recursion, though, we instead write first, and then recurse, blending
 // as we descend. α == 0 is opaque. α == 2 is fully transparent.
 static inline ncplane*
-dig_visible_cell(cell* c, int y, int x, ncplane* p, int falpha, int balpha){
-  unsigned fgblends = 1;
-  unsigned bgblends = 1;
+dig_visible_cell(cell* c, int y, int x, ncplane* p){
+  unsigned fgblends = 0;
+  unsigned bgblends = 0;
   // once we decide on our glyph, it cannot be changed by anything below, so
   // lock in this plane for the actual cell return.
   ncplane* glyphplane = NULL;
@@ -178,30 +178,21 @@ dig_visible_cell(cell* c, int y, int x, ncplane* p, int falpha, int balpha){
             c->attrword = vis->attrword;
           }
         }
-        if(falpha > CELL_ALPHA_OPAQUE && cell_get_fg_alpha(vis) < CELL_ALPHA_TRANSPARENT){
-          if(falpha == CELL_ALPHA_BLEND){
-            cell_blend_fchannel(c, cell_get_fchannel(vis), fgblends);
-            ++fgblends;
-          }else{
-            cell_set_fchannel(c, cell_get_fchannel(vis));
-          }
-          falpha = cell_get_fg_alpha(vis);
+        if(cell_get_fg_alpha(c) > CELL_ALPHA_OPAQUE && cell_get_fg_alpha(vis) < CELL_ALPHA_TRANSPARENT){
+          cell_blend_fchannel(c, cell_get_fchannel(vis), fgblends);
+          ++fgblends;
         }
         // Background color takes effect independently of whether we have a
         // glyph. If we've already locked in the background, it has no effect.
         // If it's transparent, it has no effect. Otherwise, update the
         // background channel and balpha.
-        if(balpha > CELL_ALPHA_OPAQUE && cell_get_bg_alpha(vis) < CELL_ALPHA_TRANSPARENT){
-          if(balpha == CELL_ALPHA_BLEND){
-            cell_blend_bchannel(c, cell_get_bchannel(vis), bgblends);
-            ++bgblends;
-          }else{ // balpha == CELL_ALPHA_TRANSPARENT
-            cell_set_bchannel(c, cell_get_bchannel(vis));
-          }
-          balpha = cell_get_bg_alpha(vis);
+        if(cell_get_bg_alpha(c) > CELL_ALPHA_OPAQUE && cell_get_bg_alpha(vis) < CELL_ALPHA_TRANSPARENT){
+          cell_blend_bchannel(c, cell_get_bchannel(vis), bgblends);
+          ++bgblends;
         }
         // if everything's locked in, we're done
-        if((glyphplane && falpha == CELL_ALPHA_OPAQUE && balpha == CELL_ALPHA_OPAQUE)){
+        if((glyphplane && cell_get_fg_alpha(c) == CELL_ALPHA_OPAQUE &&
+              cell_get_bg_alpha(c) == CELL_ALPHA_OPAQUE)){
           return glyphplane;
         }
       }
@@ -219,7 +210,9 @@ dig_visible_cell(cell* c, int y, int x, ncplane* p, int falpha, int balpha){
 static inline ncplane*
 visible_cell(cell* c, int y, int x, ncplane* n){
   cell_init(c);
-  return dig_visible_cell(c, y, x, n, CELL_ALPHA_TRANSPARENT, CELL_ALPHA_TRANSPARENT);
+  cell_set_fg_alpha(c, CELL_ALPHA_TRANSPARENT);
+  cell_set_bg_alpha(c, CELL_ALPHA_TRANSPARENT);
+  return dig_visible_cell(c, y, x, n);
 }
 
 // write the cell's UTF-8 grapheme cluster to the provided FILE*. returns the

tests/channel.cpp

@@ -112,19 +112,25 @@ TEST_CASE("ChannelBlend2") {
   CHECK(0x0a == b);
 }
 
-// you can't blend into a default color, at any number of blends
+// you can't blend into a default color at any positive number of blends
 TEST_CASE("ChannelBlendDefaultLeft") {
   uint32_t c1 = 0;
   uint32_t c2 = 0;
   channel_set_rgb(&c2, 0x80, 0x40, 0x20);
-  uint32_t c = channels_blend(c1, c2, 0);
-  CHECK(channel_default_p(c));
+  uint32_t c = channels_blend(c1, c2, 0); // will replace
+  CHECK(!channel_default_p(c));
   unsigned r, g, b;
   channel_get_rgb(c, &r, &g, &b);
+  CHECK(0x80 == r);
+  CHECK(0x40 == g);
+  CHECK(0x20 == b);
+  c = channels_blend(c1, c2, 1); // will not replace
+  CHECK(channel_default_p(c));
+  channel_get_rgb(c, &r, &g, &b);
   CHECK(0 == r);
   CHECK(0 == g);
   CHECK(0 == b);
-  c = channels_blend(c1, c2, 1);
+  c = channels_blend(c1, c2, 2); // will not replace
   CHECK(channel_default_p(c));
   channel_get_rgb(c, &r, &g, &b);
   CHECK(0 == r);
@@ -142,9 +148,9 @@ TEST_CASE("ChannelBlendDefaultRight") {
   CHECK(channel_default_p(c));
   unsigned r, g, b;
   channel_get_rgb(c, &r, &g, &b);
-  CHECK(0x80 == r);
-  CHECK(0x40 == g);
-  CHECK(0x20 == b);
+  CHECK(0 == r);
+  CHECK(0 == g);
+  CHECK(0 == b);
   c = channels_blend(c1, c2, 1);
   CHECK(!channel_default_p(c));
   channel_get_rgb(c, &r, &g, &b);