Crafter.Graphics/shaders/ui-text.comp.glsl

#version 460
#extension GL_GOOGLE_include_directive : enable
#extension GL_KHR_shader_subgroup_basic : enable
#extension GL_KHR_shader_subgroup_arithmetic : enable
#include "ui-shared.glsl"

// One workgroup per 8×8 screen tile. The workgroup cooperatively streams the
// GlyphItem list in chunks of 64 (see ui-shared.glsl), culling each chunk
// against the tile and compacting survivors — in buffer order — into shared
// memory; every thread then accumulates over only those survivors.
layout(push_constant) uniform PC {
    UIDispatchHeader hdr;
    uint  fontTextureSlot;
    uint  fontSamplerSlot;
    uint  _p0;
    uint  _p1;
} pc;

layout(local_size_x = 8, local_size_y = 8, local_size_z = 1) in;

// SDF tuning — must match Crafter::FontAtlas::kOnEdgeValue / kPixelDistScale.
const float ON_EDGE    = 128.0 / 255.0;
const float DIST_SCALE = 32.0;

shared vec4 s_rect[UI_CHUNK];
shared vec4 s_uv[UI_CHUNK];
shared vec4 s_color[UI_CHUNK];
shared uint s_keep[UI_CHUNK];
shared uint s_order[UI_CHUNK];
shared uint s_count;
shared uint s_subTotals[UI_CHUNK];  // per-subgroup survivor totals (carry step)

// Stable in-order compaction of one chunk's survivors via a per-subgroup
// exclusive prefix-sum plus a carry across subgroups, so a survivor's slot in
// s_order[] equals the number of survivors with a smaller local index — buffer
// (draw) order preserved. Replaces the old lane-0 serial scan. See
// ui-fused.comp.glsl for the full rationale; the body is identical.
void uiCompactChunk() {
    uint lid       = gl_LocalInvocationIndex;
    uint keep      = s_keep[lid];
    uint subPrefix = subgroupExclusiveAdd(keep);
    uint subTotal  = subgroupAdd(keep);
    if (subgroupElect())
        s_subTotals[gl_SubgroupID] = subTotal;
    barrier();

    uint base  = 0u;
    uint total = 0u;
    for (uint s = 0u; s < gl_NumSubgroups; ++s) {
        uint t = s_subTotals[s];
        if (s < gl_SubgroupID) base += t;
        total += t;
    }
    if (lid == 0u) s_count = total;

    if (keep != 0u)
        s_order[base + subPrefix] = lid;
}

void main() {
    ivec2 screenPx;
    bool  valid = uiResolveScreenPixel(pc.hdr, screenPx);

    vec4 dst = vec4(0.0);
    vec2 sp  = vec2(0.0);
    if (valid) {
        dst = imageLoad(uiImages[pc.hdr.outImage], screenPx);
        sp  = vec2(screenPx) + 0.5;
    }

    vec2 tileMin, tileMax;
    uiTileBounds(tileMin, tileMax);

    uint lid = gl_LocalInvocationIndex;
    for (uint base = 0u; base < pc.hdr.itemCount; base += UI_CHUNK) {
        uint idx  = base + lid;
        bool keep = false;
        if (idx < pc.hdr.itemCount) {
            s_rect[lid]  = uiGlyphHeap[pc.hdr.itemBuffer].items[idx].rect;
            s_uv[lid]    = uiGlyphHeap[pc.hdr.itemBuffer].items[idx].uv;
            s_color[lid] = uiGlyphHeap[pc.hdr.itemBuffer].items[idx].color;
            keep = uiAabbOverlapsTile(s_rect[lid].xy, s_rect[lid].xy + s_rect[lid].zw,
                                      tileMin, tileMax);
        }
        s_keep[lid] = keep ? 1u : 0u;
        barrier();

        uiCompactChunk();
        barrier();

        if (valid) {
            for (uint j = 0u; j < s_count; ++j) {
                uint c = s_order[j];

                vec2 lo = s_rect[c].xy;
                vec2 hi = s_rect[c].xy + s_rect[c].zw;
                if (sp.x < lo.x || sp.y < lo.y) continue;
                if (sp.x >= hi.x || sp.y >= hi.y) continue;

                vec2 t  = (sp - s_rect[c].xy) / s_rect[c].zw;
                vec2 uv = mix(s_uv[c].xy, s_uv[c].zw, t);

                // Font slots are push constants — provably dynamically uniform, so no
                // nonuniformEXT: lets the compiler hoist the descriptor load out of the
                // per-pixel glyph loop and avoid the divergent-descriptor path.
                float sdf = texture(
                    sampler2D(uiTextures[pc.fontTextureSlot],
                              uiSamplers[pc.fontSamplerSlot]),
                    uv
                ).r;

                // Distance in atlas-pixels (negative inside the glyph).
                float dAtlas = (ON_EDGE - sdf) * DIST_SCALE;

                // Atlas-px per screen-px along this glyph's transform — keeps AA crisp
                // at any rendering size. uvSpan * atlasSize / screenSpan.
                vec2  uvSpan         = s_uv[c].zw - s_uv[c].xy;
                // FontAtlas::kAtlasSize = 1024.
                vec2  atlasPerScreen = (uvSpan * 1024.0) / s_rect[c].zw;
                float scalePx        = max(atlasPerScreen.x, atlasPerScreen.y);
                // 1-screen-px AA band, expressed in atlas-pixel units of dAtlas.
                float band           = max(scalePx, 0.0001);

                float a = clamp(0.5 - dAtlas / band, 0.0, 1.0);
                if (a <= 0.0) continue;

                vec4 col = s_color[c];
                vec4 src = vec4(col.rgb, col.a * a);
                dst = uiBlendOver(dst, src);
            }
        }
        barrier();
    }

    if (valid) imageStore(uiImages[pc.hdr.outImage], screenPx, dst);
}