render: skip textures the classic renderer can't upload

KTX textures crash the classic OpenGL renderer and the Python viewer with
"Number of channels not supported: 1": mjCTexture::LoadKTX stores the file as
an opaque single-channel blob for the Filament renderer, and mjr_uploadTexture
aborts via mju_error because the classic path only handles 3- or 4-channel
textures.

Skip textures the classic renderer can't upload (nchannel not 3 or 4) with a
warning instead of aborting, so the model still loads and the other textures
render. LoadKTX and the Filament path are unchanged. The guard also covers the
cube/skybox path, which previously over-read a single-channel blob as RGB.

Add a render_test.py regression test that builds a model with a KTX texture and
constructs an MjrContext; it aborts before this change and passes after.

Addresses #3343.

python/mujoco/render_test.py

@@ -14,6 +14,8 @@
 # ==============================================================================
 """Tests for MuJoCo Python rendering."""
 
+import os
+
 from absl.testing import absltest
 import mujoco
 import numpy as np
@@ -104,6 +106,34 @@ class MuJoCoRenderTest(absltest.TestCase):
     context.free()
     context.free()
 
+  def test_ktx_texture_does_not_crash_classic_renderer(self):
+    # KTX textures are stored as opaque single-channel blobs for the Filament
+    # renderer; mjCTexture::LoadKTX does not decode them. The classic renderer
+    # must skip such textures with a warning rather than aborting in
+    # mjr_uploadTexture (https://github.com/google-deepmind/mujoco/issues/3343).
+    ktx_path = os.path.join(self.create_tempdir().full_path, 'texture.ktx')
+    with open(ktx_path, 'wb') as f:
+      # LoadKTX stores the raw payload as a 1-channel blob without decoding, so
+      # the contents do not matter; start with the KTX 2.0 identifier for realism.
+      f.write(b'\xabKTX 20\xbb\r\n\x1a\n' + bytes(64))
+
+    model = mujoco.MjModel.from_xml_string(f"""
+      <mujoco>
+        <asset>
+          <texture name="ktx" type="2d" file="{ktx_path}"/>
+          <material name="mat" texture="ktx"/>
+        </asset>
+        <worldbody>
+          <geom type="plane" size="1 1 .1" material="mat"/>
+        </worldbody>
+      </mujoco>""")
+    self.assertEqual(model.tex_nchannel[0], 1)
+
+    # Building the render context uploads all textures; before the fix this
+    # aborted with "Number of channels not supported: 1". It must not raise now.
+    context = mujoco.MjrContext(model, mujoco.mjtFontScale.mjFONTSCALE_150)
+    context.free()
+
   def test_mjrrect_repr(self):
     rect = mujoco.MjrRect(1, 2, 3, 4)
     rect_repr = repr(rect)

src/render/classic/render_context.c

@@ -1365,6 +1365,16 @@ void mjr_uploadTexture(const mjModel* m, const mjrContext* con, int texid) {
   int w = m->tex_width[texid];
   float plane[4];
 
+  // the classic renderer only uploads 3- (RGB) or 4-channel (RGBA) textures. KTX
+  // and other encoded textures are single-channel blobs for the Filament renderer
+  // (see LoadKTX); skip them with a warning instead of aborting (issue #3343).
+  if (m->tex_nchannel[texid] != 3 && m->tex_nchannel[texid] != 4) {
+    mju_warning("texture %d: classic renderer can't upload %d-channel data, "
+                "skipping (KTX/encoded textures need the Filament renderer)",
+                texid, m->tex_nchannel[texid]);
+    return;
+  }
+
   // 2D texture
   if (m->tex_type[texid] == mjTEXTURE_2D) {
     // OpenGL settings
@@ -1390,11 +1400,9 @@ void mjr_uploadTexture(const mjModel* m, const mjrContext* con, int texid) {
     if (m->tex_nchannel[texid] == 3) {
       type = GL_RGB;
       internaltype = (m->tex_colorspace[texid] == mjCOLORSPACE_SRGB) ? GL_SRGB8_EXT : GL_RGB;
-    } else if (m->tex_nchannel[texid] == 4) {
+    } else {  // 4 channels, guaranteed by the channel check at the top of the function
       type = GL_RGBA;
       internaltype = (m->tex_colorspace[texid] == mjCOLORSPACE_SRGB) ? GL_SRGB8_ALPHA8_EXT : GL_RGBA;
-    } else {
-      mju_error("Number of channels not supported: %d", m->tex_nchannel[texid]);
     }
 
     glTexImage2D(GL_TEXTURE_2D, 0, internaltype, m->tex_width[texid],