config: add tweak option to allow disabling sixels

Closes #950

CHANGELOG.md

@@ -47,6 +47,8 @@
 * Socket activation for `foot --server` and accompanying systemd unit files
 * Support for [DECNKM](https://vt100.net/docs/vt510-rm/DECNKM.html), which
   allows setting/saving/restoring/querying the keypad mode.
+* Sixel support can be disabled by setting `[tweak].sixel=no`
+  (https://codeberg.org/dnkl/foot/issues/950).
 
 
 ### Changed

config.c

@@ -2433,6 +2433,9 @@ parse_section_tweak(struct context *ctx)
     else if (strcmp(key, "font-monospace-warn") == 0)
         return value_to_bool(ctx, &conf->tweak.font_monospace_warn);
 
+    else if (strcmp(key, "sixel") == 0)
+        return value_to_bool(ctx, &conf->tweak.sixel);
+
     else {
         LOG_CONTEXTUAL_ERR("not a valid option: %s", key);
         return false;
@@ -2974,6 +2977,7 @@ config_load(struct config *conf, const char *conf_path,
             .box_drawing_base_thickness = 0.04,
             .box_drawing_solid_shades = true,
             .font_monospace_warn = true,
+            .sixel = true,
         },
 
         .notifications = tll_init(),

config.h

@@ -316,6 +316,7 @@ struct config {
         float box_drawing_base_thickness;
         bool box_drawing_solid_shades;
         bool font_monospace_warn;
+        bool sixel;
     } tweak;
 
     user_notifications_t notifications;

csi.c

@@ -795,8 +795,13 @@ csi_dispatch(struct terminal *term, uint8_t final)
              *
              * Note: tertiary DA responds with "FOOT".
              */
-            static const char reply[] = "\033[?62;4;22c";
-            term_to_slave(term, reply, sizeof(reply) - 1);
+            if (term->conf->tweak.sixel) {
+                static const char reply[] = "\033[?62;4;22c";
+                term_to_slave(term, reply, sizeof(reply) - 1);
+            } else {
+                static const char reply[] = "\033[?62;22c";
+                term_to_slave(term, reply, sizeof(reply) - 1);
+            }
             break;
         }
 
@@ -1418,6 +1423,11 @@ csi_dispatch(struct terminal *term, uint8_t final)
             break;
 
         case 'S': {
+            if (!term->conf->tweak.sixel) {
+                UNHANDLED();
+                break;
+            }
+
             unsigned target = vt_param_get(term, 0, 0);
             unsigned operation = vt_param_get(term, 1, 0);
 

dcs.c

@@ -420,6 +420,9 @@ dcs_hook(struct terminal *term, uint8_t final)
     case 0:
         switch (final) {
         case 'q': {
+            if (!term->conf->tweak.sixel) {
+                break;
+            }
             int p1 = vt_param_get(term, 0, 0);
             int p2 = vt_param_get(term, 1,0);
             int p3 = vt_param_get(term, 2, 0);

doc/foot.ini.5.scd

@@ -1207,6 +1207,10 @@ any of these options.
 	
 	Default: _512_. Maximum allowed: _2048_ (2GB).
 
+*sixel*
+	Boolean. When enabled, foot will process sixel images. Default: _yes_
+
+
 # SEE ALSO
 
 *foot*(1), *footclient*(1)