diff --git a/src/xrt/compositor/shaders/layer.comp b/src/xrt/compositor/shaders/layer.comp
index 586e710af..acdb9c8ff 100644
--- a/src/xrt/compositor/shaders/layer.comp
+++ b/src/xrt/compositor/shaders/layer.comp
@@ -1,4 +1,5 @@
 // Copyright 2021-2023, Collabora Ltd.
+// Copyright 2025, NVIDIA CORPORATION.
 // Author: Jakob Bornecrantz <jakob@collabora.com>
 // Author: Christoph Haag <christoph.haag@collabora.com>
 // SPDX-License-Identifier: BSL-1.0
@@ -7,15 +8,8 @@
 #extension GL_GOOGLE_include_directive : require
 
 #include "srgb.inc.glsl"
+#include "layer_defines.inc.glsl"
 
-//! @todo should this be a spcialization const?
-#define XRT_LAYER_PROJECTION 0
-#define XRT_LAYER_PROJECTION_DEPTH 1
-#define XRT_LAYER_QUAD 2
-#define XRT_LAYER_CUBE 3
-#define XRT_LAYER_CYLINDER 4
-#define XRT_LAYER_EQUIRECT1 5
-#define XRT_LAYER_EQUIRECT2 6
 
 const float PI = acos(-1);
 
@@ -460,19 +454,18 @@ vec4 do_layers(vec2 view_uv)
 		vec4 rgba = vec4(0, 0, 0, 0);
 
 		switch (ubo.layer_type_and_unpremultiplied[layer].x) {
-		case XRT_LAYER_CYLINDER:
+		case LAYER_COMP_TYPE_QUAD:
+			rgba = do_quad(view_uv, layer);
+			break;
+		case LAYER_COMP_TYPE_CYLINDER:
 			rgba = do_cylinder(view_uv, layer);
 			break;
-		case XRT_LAYER_EQUIRECT2:
+		case LAYER_COMP_TYPE_EQUIRECT2:
 			rgba = do_equirect2(view_uv, layer);
 			break;
-		case XRT_LAYER_PROJECTION:
-		case XRT_LAYER_PROJECTION_DEPTH:
+		case LAYER_COMP_TYPE_PROJECTION:
 			rgba = do_projection(view_uv, layer);
 			break;
-		case XRT_LAYER_QUAD:
-			rgba = do_quad(view_uv, layer);
-			break;
 		default: break;
 		}
 
diff --git a/src/xrt/compositor/shaders/layer_defines.inc.glsl b/src/xrt/compositor/shaders/layer_defines.inc.glsl
new file mode 100644
index 000000000..9f601c7e3
--- /dev/null
+++ b/src/xrt/compositor/shaders/layer_defines.inc.glsl
@@ -0,0 +1,22 @@
+// Copyright 2025, NVIDIA CORPORATION.
+// SPDX-License-Identifier: BSL-1.0
+
+/*
+ * This file is included by both C code, like comp_render_cs.c, and GLSL code,
+ * like layer.comp, so it uses a very limited set of features of both.
+ */
+#ifndef LAYER_DEFINES_INC_GLSL
+#define LAYER_DEFINES_INC_GLSL
+
+//! To handle invalid/unsupported layer types.
+#define LAYER_COMP_TYPE_NOOP 0
+//! Maps to XRT_LAYER_QUAD (not numerically)
+#define LAYER_COMP_TYPE_QUAD 1
+//! Maps to XRT_LAYER_CYLINDER (not numerically)
+#define LAYER_COMP_TYPE_CYLINDER 2
+//! Maps to XRT_LAYER_EQUIRECT2 (not numerically)
+#define LAYER_COMP_TYPE_EQUIRECT2 3
+//! Maps to XRT_LAYER_PROJECTION[_DEPTH] (not numerically)
+#define LAYER_COMP_TYPE_PROJECTION 4
+
+#endif // LAYER_DEFINES_INC_GLSL
diff --git a/src/xrt/compositor/util/comp_render_cs.c b/src/xrt/compositor/util/comp_render_cs.c
index f7da44e7f..6b8ed8b30 100644
--- a/src/xrt/compositor/util/comp_render_cs.c
+++ b/src/xrt/compositor/util/comp_render_cs.c
@@ -26,6 +26,8 @@
 
 #include "render/render_interface.h"
 
+#include "shaders/layer_defines.inc.glsl"
+
 #include "util/comp_render.h"
 #include "util/comp_render_helpers.h"
 #include "util/comp_base.h"
@@ -33,7 +35,7 @@
 
 /*
  *
- * Compute layer data builders.
+ * Helpers
  *
  */
 
@@ -54,6 +56,27 @@ get_layer_depth_image(const struct comp_layer *layer, uint32_t swapchain_index,
 	return &sc->images[image_index];
 }
 
+
+static inline uint32_t
+xrt_layer_to_cs_layer_type(const struct xrt_layer_data *data)
+{
+	switch (data->type) {
+	case XRT_LAYER_QUAD: return LAYER_COMP_TYPE_QUAD;
+	case XRT_LAYER_CYLINDER: return LAYER_COMP_TYPE_CYLINDER;
+	case XRT_LAYER_EQUIRECT2: return LAYER_COMP_TYPE_EQUIRECT2;
+	case XRT_LAYER_PROJECTION:
+	case XRT_LAYER_PROJECTION_DEPTH: return LAYER_COMP_TYPE_PROJECTION;
+	default: U_LOG_E("Invalid layer type! %u", data->type); return LAYER_COMP_TYPE_NOOP;
+	}
+}
+
+
+/*
+ *
+ * Compute layer data builders.
+ *
+ */
+
 /// Data setup for a cylinder layer
 static inline void
 do_cs_cylinder_layer(const struct comp_layer *layer,
@@ -660,7 +683,7 @@ comp_render_cs_layer(struct render_compute *render,
 			continue;
 		}
 
-		ubo_data->layer_type[cur_layer].val = data->type;
+		ubo_data->layer_type[cur_layer].val = xrt_layer_to_cs_layer_type(data);
 		ubo_data->layer_type[cur_layer].unpremultiplied = is_layer_unpremultiplied(data);
 
 		// Finally okay to increment the current layer.
@@ -671,7 +694,7 @@ comp_render_cs_layer(struct render_compute *render,
 	ubo_data->layer_count.value = cur_layer;
 
 	for (uint32_t i = cur_layer; i < RENDER_MAX_LAYERS; i++) {
-		ubo_data->layer_type[i].val = UINT32_MAX;
+		ubo_data->layer_type[i].val = LAYER_COMP_TYPE_NOOP; // Explicit no-op.
 	}
 
 	//! @todo: If Vulkan 1.2, use VK_DESCRIPTOR_BINDING_PARTIALLY_BOUND_BIT and skip this