blend: conditionally blend in alpha

include/notcurses.h

@@ -1117,13 +1117,19 @@ channels_blend(unsigned c1, unsigned c2, unsigned blends){
   unsigned rsum, gsum, bsum;
   if(blends == 0){
     // don't just return c2, or you set wide status and all kinds of crap
-    channel_rgb(c2, &rsum, &gsum, &bsum);
-    channel_set_rgb(&c1, rsum, gsum, bsum);
+    if(channel_default_p(c2)){
+      channel_set_default(&c1);
+    }else{
+      channel_rgb(c2, &rsum, &gsum, &bsum);
+      channel_set_rgb(&c1, rsum, gsum, bsum);
+    }
+    channel_set_alpha(&c1, channel_alpha(c2));
   }else if(!channel_default_p(c2) && !channel_default_p(c1)){
     rsum = (channel_r(c1) * blends + channel_r(c2)) / (blends + 1);
     gsum = (channel_g(c1) * blends + channel_g(c2)) / (blends + 1);
     bsum = (channel_b(c1) * blends + channel_b(c2)) / (blends + 1);
     channel_set_rgb(&c1, rsum, gsum, bsum);
+    channel_set_alpha(&c1, channel_alpha(c2));
   }
   return c1;
 }

src/demo/witherworm.c

@@ -17,6 +17,12 @@ mathplane(struct notcurses* nc){
   const int HEIGHT = 9;
   const int WIDTH = dimx;
   struct ncplane* n = ncplane_new(nc, HEIGHT, WIDTH, dimy - HEIGHT - 1, dimx - WIDTH, NULL);
+  cell b = CELL_TRIVIAL_INITIALIZER;
+  cell_set_fg_rgb(&b, 0xff, 0xff, 0xff);
+  cell_set_fg_alpha(&b, CELL_ALPHA_BLEND);
+  cell_set_bg_alpha(&b, CELL_ALPHA_TRANSPARENT);
+  ncplane_set_base(n, &b);
+  cell_release(n, &b);
   ncplane_set_fg_rgb(n, 0xff, 0xff, 0xff);
   if(n){
     ncplane_printf_aligned(n, 0, NCALIGN_RIGHT, "∮E⋅da=Q,n→∞,∑f(i)=∏g(i)⎧⎡⎛┌─────┐⎞⎤⎫");
@@ -67,6 +73,7 @@ surrounding_cells(struct ncplane* n, cell* cells, int y, int x){
 static int
 lightup_surrounding_cells(struct ncplane* n, const cell* cells, int y, int x){
   cell c = CELL_TRIVIAL_INITIALIZER;
+  /*
   cell_duplicate(n, &c, &cells[0]);
   lighten(n, &c, 2, y - 1, x - 1);
   cell_duplicate(n, &c, &cells[1]);
@@ -91,6 +98,7 @@ lightup_surrounding_cells(struct ncplane* n, const cell* cells, int y, int x){
   lighten(n, &c, 2, y, x - 2);
   cell_duplicate(n, &c, &cells[11]);
   lighten(n, &c, 2, y, x + 2);
+  */
   cell_duplicate(n, &c, &cells[12]);
   lighten(n, &c, 0, y, x);
   cell_release(n, &c);

tests/channel.cpp

@@ -143,16 +143,13 @@ TEST_CASE("ChannelBlendDefaultRight") {
   uint32_t c1 = 0;
   uint32_t c2 = 0;
   channel_set_rgb(&c1, 0x80, 0x40, 0x20);
+  CHECK(!channel_default_p(c1));
   CHECK(channel_default_p(c2));
   uint32_t c = channels_blend(c1, c2, 0);
   CHECK(channel_default_p(c));
-  unsigned r, g, b;
-  channel_rgb(c, &r, &g, &b);
-  CHECK(0 == r);
-  CHECK(0 == g);
-  CHECK(0 == b);
   c = channels_blend(c1, c2, 1);
   CHECK(!channel_default_p(c));
+  unsigned r, g, b;
   channel_rgb(c, &r, &g, &b);
   CHECK(0x80 == r);
   CHECK(0x40 == g);