Wire ssbo rendering into sandbox

build.zig

@@ -8,6 +8,7 @@ pub fn build(b: *std.Build) void {
         .target = target,
         .optimize = optimize,
         .linux_display_backend = .X11,
+        .opengl_version = .gl_4_3,
     });
 
     // sandbox executable

src/sandbox.zig

@@ -286,3 +286,35 @@ test "update respawns entity at edge when reaching center" {
 
     try std.testing.expect(on_left or on_right or on_top or on_bottom);
 }
+
+// GPU entity for SSBO rendering (position + color only, no velocity)
+pub const GpuEntity = extern struct {
+    x: f32,
+    y: f32,
+    color: u32,
+};
+
+test "GpuEntity struct has correct size for SSBO" {
+    // SSBO layout: x(4) + y(4) + color(4) = 12 bytes
+    try std.testing.expectEqual(@as(usize, 12), @sizeOf(GpuEntity));
+}
+
+test "GpuEntity can be created from Entity" {
+    const entity = Entity{
+        .x = 100.0,
+        .y = 200.0,
+        .vx = 1.5, // ignored for GPU
+        .vy = -0.5, // ignored for GPU
+        .color = 0x00FFFF,
+    };
+
+    const gpu_entity = GpuEntity{
+        .x = entity.x,
+        .y = entity.y,
+        .color = entity.color,
+    };
+
+    try std.testing.expectEqual(@as(f32, 100.0), gpu_entity.x);
+    try std.testing.expectEqual(@as(f32, 200.0), gpu_entity.y);
+    try std.testing.expectEqual(@as(u32, 0x00FFFF), gpu_entity.color);
+}

src/sandbox_main.zig

@@ -5,6 +5,7 @@ const std = @import("std");
 const rl = @import("raylib");
 const sandbox = @import("sandbox.zig");
 const ui = @import("ui.zig");
+const SsboRenderer = @import("ssbo_renderer.zig").SsboRenderer;
 
 const SCREEN_WIDTH = sandbox.SCREEN_WIDTH;
 const SCREEN_HEIGHT = sandbox.SCREEN_HEIGHT;
@@ -26,7 +27,7 @@ const HEARTBEAT_INTERVAL: f32 = 10.0; // seconds between periodic logs
 
 // auto-benchmark settings
 const BENCH_RAMP_INTERVAL: f32 = 2.0; // seconds between entity ramps
-const BENCH_RAMP_AMOUNT: usize = 10_000; // entities added per ramp
+const BENCH_RAMP_AMOUNT: usize = 50_000; // entities added per ramp
 const BENCH_EXIT_THRESHOLD_MS: f32 = 25.0; // exit when frame time exceeds this
 const BENCH_EXIT_SUSTAIN: f32 = 1.0; // must stay above threshold for this long
 
@@ -154,14 +155,19 @@ pub fn main() !void {
     // parse args
     var bench_mode = false;
     var use_instancing = false;
+    var use_ssbo = false;
     var args = try std.process.argsWithAllocator(std.heap.page_allocator);
     defer args.deinit();
     _ = args.skip(); // skip program name
     while (args.next()) |arg| {
         if (std.mem.eql(u8, arg, "--bench")) {
             bench_mode = true;
+            use_ssbo = true; // bench mode uses SSBO by default
         } else if (std.mem.eql(u8, arg, "--gpu")) {
             use_instancing = true;
+            use_ssbo = false; // explicit --gpu overrides SSBO
+        } else if (std.mem.eql(u8, arg, "--ssbo")) {
+            use_ssbo = true;
         }
     }
 
@@ -221,6 +227,21 @@ pub fn main() !void {
         if (transforms) |t| std.heap.page_allocator.free(t);
     }
 
+    // SSBO rendering setup (only if --ssbo flag)
+    var ssbo_renderer: ?SsboRenderer = null;
+
+    if (use_ssbo) {
+        ssbo_renderer = SsboRenderer.init(circle_texture) orelse {
+            std.debug.print("failed to initialize SSBO renderer\n", .{});
+            return;
+        };
+        std.debug.print("SSBO instancing mode enabled\n", .{});
+    }
+
+    defer {
+        if (ssbo_renderer) |*r| r.deinit();
+    }
+
     // load UI font (embedded)
     const font_data = @embedFile("verdanab.ttf");
     const ui_font = rl.loadFontFromMemory(".ttf", font_data, 32, null) catch {
@@ -301,8 +322,11 @@ pub fn main() !void {
         rl.beginDrawing();
         rl.clearBackground(BG_COLOR);
 
-        if (use_instancing) {
+        if (use_ssbo) {
-            // GPU instancing path
+            // SSBO instanced rendering path (12 bytes per entity)
+            ssbo_renderer.?.render(&entities);
+        } else if (use_instancing) {
+            // GPU instancing path (64 bytes per entity)
             const xforms = transforms.?;
             // fill transforms array with entity positions
             for (entities.items[0..entities.count], 0..) |entity, i| {