Sprite sheets, atlases, game UI, time of day, weather presets, headless captures

- BillboardInstance: per-instance uv sub-rect, feet anchor, turn, dithered alpha, flash;
  BillboardBatch::sharedTexture; Texture createEmpty/update/cellUV/cellAnchor/nearest
- SpriteAtlas: fixed-cell runtime atlas with gutters
- hd2d::ui: Octopath-style panels, fonts (Cinzel, EB Garamond), dialogue box with
  typewriter + portraits + choices, menus, nameplates, toasts, ImGui theme
- DayCycle (dawn/day/dusk/night keyframes, sun + moon arcs), Weather overcast/mist/
  lightning, night-aware presets, stars and moon in the sky
- Hidden-window offscreen captures (Window visible=false, Renderer output FBO,
  OffscreenTarget); --screenshot never opens a window
- Camera::worldToScreen; CMake usable via add_subdirectory (HD2D_BUILD_DEMO,
  HD2D_ENGINE_ASSET_DIR)

assets/shaders/billboard.vert

@@ -1,8 +1,11 @@
 #version 450 core
 #include "common.glsl"
 
+// Per-instance attributes; must match BillboardInstance (Billboard.h).
 layout(location = 0) in vec4 iPosSway;      // pivot xyz, sway
 layout(location = 1) in vec4 iSizeFlipTint; // size xy, flip, tint
+layout(location = 2) in vec4 iRect;         // uv sub-rect (minU, minV, maxU, maxV)
+layout(location = 3) in vec4 iAnchorFx;     // anchor v (< 0 = uOpaque.y), turn, alpha, flash
 
 uniform vec4 uOpaque;  // opaque pixel bounds in uv (minX, minY, maxX, maxY)
 uniform int uPass;     // 0 = color, 1 = shadow
@@ -13,22 +16,34 @@ out VS {
     vec3 pivot;
     vec2 local;  // (horizontal offset, height above pivot) in world units
     float tint;
+    flat vec2 fx;  // alpha, flash
 } o;
 
 void main() {
     vec2 corner = vec2(float(gl_VertexID & 1), float(gl_VertexID >> 1));
     vec2 size = iSizeFlipTint.xy;
     vec3 pivot = iPosSway.xyz;
+    vec4 rect = iRect;
+    float rectH = max(rect.w - rect.y, 1e-6);
+    float anchor = iAnchorFx.x < 0.0 ? uOpaque.y : iAnchorFx.x;
+    float anchorLocal = (anchor - rect.y) / rectH;
     float lx = (corner.x - 0.5) * size.x;
-    float ly = (corner.y - uOpaque.y) * size.y;  // lowest opaque row sits on the pivot
+    float ly = (corner.y - anchorLocal) * size.y;  // the feet row sits on the pivot
 
-    // Shadow cards face the sun so silhouettes are never edge-on to the light.
-    vec3 right = uPass == 1 ? uShadowRight.xyz : uBillboardRight.xyz;
+    // Shadow cards face the sun so silhouettes are never edge-on to the light;
+    // `turn` yaws color cards away from camera-facing (paper turn).
+    vec3 right = uBillboardRight.xyz;
+    if (iAnchorFx.y != 0.0) {
+        float ct = cos(iAnchorFx.y), st = sin(iAnchorFx.y);
+        right = vec3(right.x * ct - right.z * st, 0.0, right.x * st + right.z * ct);
+    }
+    if (uPass == 1) right = uShadowRight.xyz;
     vec3 wp = pivot + right * lx + vec3(0.0, ly, 0.0);
 
     float sway = iPosSway.w;
     if (sway > 0.0) {
-        float visibleHeight = max(size.y * (uOpaque.w - uOpaque.y), 1e-3);
+        // Feet to the top of the rect (or of the opaque pixels, if lower).
+        float visibleHeight = max(size.y * (min(uOpaque.w, rect.w) - anchor) / rectH, 1e-3);
         float k = clamp(ly / visibleHeight, 0.0, 1.0);
         k *= k;
         float ph = dot(pivot.xz, vec2(0.37, 0.61));
@@ -46,13 +61,14 @@ void main() {
         wp.y -= push * k * sway * 0.3 * visibleHeight;
     }
 
-    vec2 uv = corner;
-    if (iSizeFlipTint.z > 0.5) uv.x = 1.0 - uv.x;
-    o.uv = uv;
+    vec2 c = corner;
+    if (iSizeFlipTint.z > 0.5) c.x = 1.0 - c.x;  // mirror inside the sub-rect
+    o.uv = mix(rect.xy, rect.zw, c);
     o.worldPos = wp;
     o.pivot = pivot;
     o.local = vec2(lx, ly);
     o.tint = iSizeFlipTint.w;
+    o.fx = iAnchorFx.zw;
     gl_Position = uViewProj * vec4(wp, 1.0);
     gl_ClipDistance[0] = dot(vec4(wp, 1.0), uClipPlane);
 }