feat(render): load an external colour profile via TILE57_COLORPROFILE

src/chart.zig

@@ -56,7 +56,14 @@ var shared_colors_err: ?anyerror = null;
 fn sharedColors() !*render.resolve.Colors {
     while (colors_state.load(.acquire) != 2) {
         if (colors_state.cmpxchgStrong(@as(u8, 0), @as(u8, 1), .acquire, .monotonic) == null) {
-            if (render.resolve.Colors.init(gpa, embedded_assets.colorprofile[0].bytes)) |c| {
+            // TILE57_COLORPROFILE (via the C shim) overrides the embedded profile —
+            // its bytes are process-lifetime, so the Colors' token keys may slice in.
+            var ov_len: usize = 0;
+            const profile_xml: []const u8 = if (tg_colorprofile_override(&ov_len)) |p|
+                p[0..ov_len]
+            else
+                embedded_assets.colorprofile[0].bytes;
+            if (render.resolve.Colors.init(gpa, profile_xml)) |c| {
                 shared_colors = c;
             } else |e| {
                 shared_colors_err = e;
@@ -74,6 +81,12 @@ fn sharedColors() !*render.resolve.Colors {
 // dir from TILE57_S101_RULES or null. Provided by the portrayal C shim.
 extern fn tg_env_rules() callconv(.c) ?[*:0]const u8;
 
+// TILE57_COLORPROFILE override, also via the C shim: a colorProfile.xml the host
+// points at to recolour the chart (e.g. a monochrome "ink" profile) without a
+// rebuild. Returns the file's bytes (process-lifetime; NULL -> use the embedded
+// profile). See sharedColors().
+extern fn tg_colorprofile_override(len: *usize) callconv(.c) ?[*]const u8;
+
 /// Backend / on-disk format. `auto` sniffs PMTiles first, then S-57.
 pub const Format = enum { auto, pmtiles, s57 };
 

src/portray/lua_shim.c

@@ -80,6 +80,29 @@ const char *tile57_diag_lua_version(void) { return LUA_VERSION; }
  * Used only as a fallback when a caller's rules_dir argument is NULL. */
 const char *tg_env_rules(void) { return getenv("TILE57_S101_RULES"); }
 
+/* TILE57_COLORPROFILE override: if the env var names a readable colorProfile.xml,
+ * slurp it whole and return its bytes with *len set; NULL when unset/unreadable so
+ * the caller falls back to the embedded profile. Mirrors the TILE57_S101_RULES
+ * on-disk rules override, but hands back BYTES (Zig's Colors.init parses a slice).
+ * The buffer is intentionally never freed: it backs a process-lifetime colour table
+ * whose token keys slice INTO it, so it must outlive every render (one leak/process). */
+const char *tg_colorprofile_override(size_t *len) {
+    const char *path = getenv("TILE57_COLORPROFILE");
+    if (!path || !*path) return NULL;
+    FILE *f = fopen(path, "rb");
+    if (!f) return NULL;
+    if (fseek(f, 0, SEEK_END) != 0) { fclose(f); return NULL; }
+    long n = ftell(f);
+    if (n <= 0 || fseek(f, 0, SEEK_SET) != 0) { fclose(f); return NULL; }
+    char *buf = (char *)malloc((size_t)n);
+    if (!buf) { fclose(f); return NULL; }
+    size_t got = fread(buf, 1, (size_t)n, f);
+    fclose(f);
+    if (got != (size_t)n) { free(buf); return NULL; }
+    *len = got;
+    return buf;
+}
+
 /* Suppress the per-cell "[s101] portrayed …" stderr summary. Set once before a
  * parallel bake (many threads would otherwise garble the progress output); read
  * read-only by tg_portray_run thereafter. */

tools/render.zig

@@ -8,6 +8,10 @@ const catalog_embed = @import("catalog"); // embedded portrayal assets (colour p
 const common = @import("common.zig");
 const Flags = common.Flags;
 const usageErr = common.usageErr;
+
+// TILE57_COLORPROFILE override via the portrayal C shim (mirrors the decl in
+// src/chart.zig — env/IO live in C). NULL -> the embedded profile.
+extern fn tg_colorprofile_override(len: *usize) callconv(.c) ?[*]const u8;
 const resolveRulesDir = common.resolveRulesDir;
 
 // tile57 png|pdf <cell.000 | bundle.pmtiles> <z> <x> <y> -o <out> [flags]  (one tile)
@@ -159,7 +163,14 @@ pub fn run(io: std.Io, a: std.mem.Allocator, args: []const [:0]const u8, output:
     }
     defer if (!from_bundle) cell.deinit();
 
-    var colors = try render.resolve.Colors.init(a, catalog_embed.colorprofile[0].bytes);
+    // Honour the TILE57_COLORPROFILE override (same knob the viewer's render path
+    // uses via chart.zig sharedColors) so `tile57 png` previews a custom profile.
+    var ov_len: usize = 0;
+    const profile_xml: []const u8 = if (tg_colorprofile_override(&ov_len)) |p|
+        p[0..ov_len]
+    else
+        catalog_embed.colorprofile[0].bytes;
+    var colors = try render.resolve.Colors.init(a, profile_xml);
     m.data_quality = dq;
     m.size_scale = size_scale;
     const settings = m;