5 files changed, 117 insertions, 0 deletions
diff --git a/shaders/CMakeLists.txt b/shaders/CMakeLists.txt
index c78d42b..a0227d5 100644
--- a/shaders/CMakeLists.txt
+++ b/shaders/CMakeLists.txt
@@ -10,6 +10,7 @@ add_executable(shaders
 
   aero.cc
   polygon.cc
+  subpixel.cc
   vfog.cc
 )
 target_link_libraries(shaders PRIVATE common)
diff --git a/shaders/shaders.cc b/shaders/shaders.cc
index 214d598..86dcfdb 100644
--- a/shaders/shaders.cc
+++ b/shaders/shaders.cc
@@ -598,6 +598,9 @@ SDL_AppResult SDLCALL SDL_AppEvent(void*, SDL_Event* event)
     if (event->key.key == SDLK_TAB) {
       G.ui_hidden = !G.ui_hidden;
     }
+    if (event->key.key == SDLK_SPACE) {
+      G.pause = !G.pause;
+    }
   } break;
   case SDL_EVENT_MOUSE_MOTION: {
     if (is_3d && (SDL_GetMouseState(NULL, NULL) & SDL_BUTTON_MASK(SDL_BUTTON_LEFT))) {
diff --git a/shaders/shaders.hh b/shaders/shaders.hh
index 00365fb..d23a13d 100644
--- a/shaders/shaders.hh
+++ b/shaders/shaders.hh
@@ -7,6 +7,7 @@
 #include "imgui.h"
 
 #include <algorithm>
+#include <cstring>
 #include <vector>
 
 #include <SDL3/SDL.h>
diff --git a/shaders/subpixel.cc b/shaders/subpixel.cc
new file mode 100644
index 0000000..73a5d51
--- /dev/null
+++ b/shaders/subpixel.cc
@@ -0,0 +1,22 @@
+#include "shaders.hh"
+
+static float u_quad[2] = { 256.0f, 2.0f };
+
+static void ui()
+{
+  ImGui::SliderFloat("quad width", &u_quad[0], 1.0f, 512.0f);
+  ImGui::SliderFloat("quad height", &u_quad[1], 1.0f, 512.0f);
+}
+
+static void uniforms(Shader* shader)
+{
+  GL(glUniform2fv(shader->get_required_uniform("u_quad"), 1, u_quad));
+}
+
+static Shader subpixel = {
+  .path   = "subpixel.glsl",
+  .model  = MODEL_QUAD,
+  .ui_fn  = ui,
+  .uf_fn  = uniforms,
+};
+ENABLE_SHADER(subpixel);
diff --git a/shaders/subpixel.glsl b/shaders/subpixel.glsl
new file mode 100644
index 0000000..df65b91
--- /dev/null
+++ b/shaders/subpixel.glsl
@@ -0,0 +1,90 @@
+#version 330 core
+
+// --------------------------------------------------------------------------------
+// Basic subpixel rendering with LCD filtering.
+// Assumes R-G-B LCD with no compositor scaling
+//
+// ref: https://www.shadertoy.com/view/NtVXWc (spinning quad)
+//
+// SPDX-License-Identifier: 0BSD
+// --------------------------------------------------------------------------------
+
+// --------------------------------------------------------------------------------
+// Uniforms
+// --------------------------------------------------------------------------------
+uniform float u_time;
+uniform vec2  u_res;
+uniform vec2  u_quad;
+
+// --------------------------------------------------------------------------------
+// Vertex outputs
+// --------------------------------------------------------------------------------
+in vec2 v_p;
+in vec2 v_t;
+
+// --------------------------------------------------------------------------------
+// Fragment outputs
+// --------------------------------------------------------------------------------
+out vec4 f_c;
+
+// --------------------------------------------------------------------------------
+// Entry point
+// --------------------------------------------------------------------------------
+
+vec2 rotate(vec2 v, float theta)
+{
+  // ref: https://en.wikipedia.org/wiki/Rotation_matrix
+  return mat2(cos(theta), -sin(theta), sin(theta), cos(theta)) * v;
+}
+
+float box_d(vec2 v)
+{
+  return max(v.x, v.y);
+}
+
+float spinning_quad(vec2 pos)
+{
+  pos = rotate(pos, u_time * -0.25f);
+  vec2 halfsize = u_quad / 2.0f;
+  vec2 q = abs(pos) - halfsize;
+  return 1.0f - step(0.0f, box_d(q));
+}
+
+vec3 spinning_quad_subpixel(vec2 pos)
+{
+  vec2 one_third_x = vec2(1.0f / 3.0f, 0.0f);
+
+  float sL = spinning_quad(pos - one_third_x);
+  float sC = spinning_quad(pos);
+  float sR = spinning_quad(pos + one_third_x);
+
+#if 1
+  // 3-tap low pass filter
+  float weights[3] = float[3](0.25f, 0.50f, 0.25f);
+  float r = weights[1] * sL + weights[0] * sC;
+  float g = weights[0] * sL + weights[1] * sC + weights[2] * sR;
+  float b = weights[0] * sC + weights[1] * sR;
+
+  r *= (1.0f / 0.75f);
+  b *= (1.0f / 0.75f);
+#else
+  float r = sL;
+  float g = sC;
+  float b = sR;
+#endif
+
+  return vec3(r, g, b);
+}
+
+void main()
+{
+  vec2 cursor = gl_FragCoord.xy - (u_res / 2.0f);
+
+  // top quad
+  vec3 c1 = vec3(spinning_quad(cursor - vec2(0, u_res.y / 4.0f)));
+
+  // bottom quad
+  vec3 c2 = spinning_quad_subpixel(cursor + vec2(0, u_res.y / 4.0f));
+
+  f_c = vec4(mix(c2, c1, step(u_res.y / 2.0f, gl_FragCoord.y)), 1.0f);
+}