G13: Parallax, lapis's dots drift with the text at a quarter of its speed

Parallax on|off (off, a pixel shell's; Chrome.fx bit 3). A body region
carries its first line (Region.line, a file's scroll or a terminal's grid
offset; the wire's region record grows a u32). The GUI offsets each
body's page dots by a quarter of its line in rows, and of G6's glide
offset while one runs, so the pattern drifts with the text and settles
with it; the cell shader reads it from the instance's serial slot, whose
per-cell dissolve noise G7 left unused (the dead noise code goes).

Tests: a scrolled body tells its line; the shift handed to the shader is
a quarter of it within the dots' period. Measured in a capture: four lines
move the dots 13 px (140 px / 4 mod 22). Gates as before, goldens
byte-identical. Feel review: .scratch/render/parallax/.

1 files changed, 4 insertions, 14 deletions

diff --git a/shaders/ui.frag.glsl b/shaders/ui.frag.glsl
index cb38fff1..51ec5b8c 100644
--- a/shaders/ui.frag.glsl
+++ b/shaders/ui.frag.glsl
@@ -27,16 +27,6 @@ layout(location = 5) flat in vec4 v_clip;
 
 layout(location = 0) out vec4 o_col;
 
-float cellNoise(uint serial, uint col, uint row) {
-    uint x = serial ^ (col * 0x9e3779b9u) ^ (row * 0x85ebca6bu);
-    x ^= x >> 16;
-    x *= 0x7feb352du;
-    x ^= x >> 15;
-    x *= 0x846ca68bu;
-    x ^= x >> 16;
-    return float(x & 0xffffu) / 65535.0;
-}
-
 void main() {
     if (v_clip.z >= v_clip.x && (gl_FragCoord.x < v_clip.x || gl_FragCoord.x >= v_clip.z ||
         gl_FragCoord.y < v_clip.y || gl_FragCoord.y >= v_clip.w)) discard;
@@ -51,9 +41,6 @@ void main() {
         : clamp(u_ui.background_opacity, 0.0, 1.0);
     uint effect = v_effect.x & 0x07ffffffu;
     float progress = clamp(uintBitsToFloat(v_effect.y), 0.0, 1.0);
-    uint cellCol = v_effect.w & 0xffffu;
-    uint cellRow = v_effect.w >> 16;
-    float noise = cellNoise(v_effect.z, cellCol, cellRow);
 
     // Ink over ground, both premultiplied: v_fg*a + (v_bg*bgA)*(1-a), which is
     // mix() with the ground already premultiplied. At bgA = 1 that is exactly
@@ -63,7 +50,10 @@ void main() {
     // dot every period pixels, fixed to the window, under the text.
     if ((v_effect.x & 0x10000000u) != 0u) {
         float period = u_ui.decor_dot_geom.x;
-        vec2 d = mod(gl_FragCoord.xy, period) - 0.5 * period;
+        // v_effect.z: how far its pane's text has moved the dots (G13's
+        // parallax), in pixels, as a float's bits (0 without).
+        vec2 at = gl_FragCoord.xy - vec2(0.0, uintBitsToFloat(v_effect.z));
+        vec2 d = mod(at, period) - 0.5 * period;
         float dot_a = 1.0 - smoothstep(u_ui.decor_dot_geom.y, u_ui.decor_dot_geom.z, length(d));
         ground = mix(ground, u_ui.decor_dots.rgb, u_ui.decor_dots.a * dot_a);
     }