Fix shadow flickering artefacts on platforms that do not support ARB_clip_control.

PiperOrigin-RevId: 743112447
Change-Id: Ibb727685605c0a257547162703f61e5232ff544c

doc/changelog.rst

@@ -24,6 +24,8 @@ Bug fixes
   which the Jacobian is computed, now fixed.
 - Fixed a bug that caused the parent frame of elements in the child worldbody to be incorrectly set when attaching an
   mjSpec to a frame or a site.
+- Fixed a bug that caused shadow rendering to flicker on platforms (e.g., MacOS) that do not support ARB_clip_control.
+  Fixed in collaboration with :github:user:`aftersomemath`.
 
 Python bindings
 ^^^^^^^^^^^^^^^

src/render/render_context.c

@@ -1050,13 +1050,17 @@ static void makeShadow(const mjModel* m, mjrContext* con) {
   }
   glBindFramebuffer(GL_FRAMEBUFFER, con->shadowFBO);
 
-  // create shadow depth texture: in TEXTURE1
+  // Create a shadow depth texture in TEXTURE1 and explicitly select an int24
+  // depth buffer. A depth stencil format is used because that appears to be
+  // more widely supported (MacOS does not support GL_DEPTH_COMPONENT24). Using
+  // a fixed format makes it easier to choose glPolygonOffset parameters that
+  // result in reasonably consistent and artifact free shadows across platforms.
   glGenTextures(1, &con->shadowTex);
   glActiveTexture(GL_TEXTURE1);
   glEnable(GL_TEXTURE_2D);
   glBindTexture(GL_TEXTURE_2D, con->shadowTex);
-  glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT,
+  glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH24_STENCIL8,
-               con->shadowSize, con->shadowSize, 0, GL_DEPTH_COMPONENT, GL_FLOAT, NULL);
+               con->shadowSize, con->shadowSize, 0, GL_DEPTH_STENCIL, GL_UNSIGNED_INT_24_8, NULL);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);

src/render/render_gl3.c

@@ -1211,9 +1211,24 @@ void mjr_render(mjrRect viewport, mjvScene* scn, const mjrContext* con) {
           int cull_face = glIsEnabled(GL_CULL_FACE);
           glDisable(GL_CULL_FACE);  // all faces cast shadows
           glEnable(GL_POLYGON_OFFSET_FILL);
-          float kOffsetFactor = -1.5f;
+
-          float kOffsetUnits = -4.0f;
+          // The limited resolution of the shadow maps means multiple fragments
-          glPolygonOffset(kOffsetFactor, kOffsetUnits);  // prevents "shadow acne"
+          // sample the same texel. When light and camera directions differ on
+          // surfaces that should be lit this causes "shadow acne"  because some
+          // fragments will be lit while adjacent fragments are not. To mitigate
+          // this artifact, an offset is applied to the depth values in the
+          // shadow map. The offset must be large enough to ensure consistent
+          // depth comparison occurs within the limited precision of the depth
+          // buffer. The offset is computed by glPolygonOffset using parameters
+          // that are chosen empirically. We need different values when clip
+          // control is on/off because this setting changes the depth precision.
+          float kOffsetFactor = -16.0f;
+          float kOffsetUnits = -512.0f;
+          if (mjGLAD_GL_ARB_clip_control) {
+            kOffsetFactor = -1.5f;
+            kOffsetUnits = -4.0f;
+          }
+          glPolygonOffset(kOffsetFactor, kOffsetUnits);
 
           // render all geoms to depth texture
           for (int j=0; j < ngeom; j++) {