GLSL smoothstep is NOT lerp, it's 'Hermite linear interpolation' which basically means not a damn thing anyone says it is. mix is what I want.

assets/shaders/ui_shader.frag

@@ -9,13 +9,13 @@ uniform bool is_shape;
 void main() {
   if(is_shape) {
     float tick = (u_time_end - u_time) / u_duration;
-    float blink = smoothstep(1.0, 0.5, tick);
+    float blink = mix(0.5, 1.0, tick);
     vec4 color = vec4(blink, blink, blink, 1.0);
     gl_FragColor = gl_Color * color;
   } else {
     vec4 pixel = texture2D(texture, gl_TexCoord[0].xy);
     float tick = (u_time_end - u_time) / u_duration;
-    float blink = smoothstep(1.0, 0.5, tick);
+    float blink = mix(0.5, 1.0, tick);
     vec4 color = vec4(blink, blink, blink, 1.0);
     gl_FragColor = gl_Color * color * pixel;
   }

combat_ui.cpp

@@ -18,7 +18,7 @@ namespace gui {
     $gui.set<Sprite>(button, {"leather_pouch-128"});
     // $gui.set<Rectangle>(button, {});
     $gui.set<Label>(button, {label});
-    $gui.set<Shader>(button, {.duration=0.2f});
+    $gui.set<Shader>(button, {.duration=0.1f});
     $gui.set<Clickable>(button,
         guecs::make_action(*$level.world, event));
   }