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Collapse per-mesh draws into glMultiDrawElementsIndirect

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Each visible model now issues a single glMultiDrawElementsIndirect
call instead of one glDrawElementsInstancedBaseVertex per mesh.  The
CPU BVH cull populates an array of DrawElementsIndirectCommand
records plus the flat visible-instance list, uploads both, and draws
the whole model in one GL call.

Vertex shaders switch from a uniform u_instance_offset to
gl_BaseInstanceARB (ARB_shader_draw_parameters), so per-draw offset
comes from the indirect command's baseInstance field.

Draw-call counts for BIM scenes with hundreds of unique meshes drop
from hundreds-per-frame to one-per-model, cutting driver overhead.
This also sets up the plumbing for the follow-up compute-shader cull
that will populate the indirect buffer entirely on-GPU.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
This commit is contained in:
Dion Moult
2026-04-12 20:34:47 +10:00
parent 298eca0ab6
commit f0e3056d0a
2 changed files with 86 additions and 57 deletions
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src/ifcviewer/ViewportWindow.h
+24 -9
View File
@@ -38,6 +38,15 @@
#include "InstancedGeometry.h"
#include "SidecarCache.h"
// Matches GL_DRAW_INDIRECT_BUFFER layout for glMultiDrawElementsIndirect.
struct DrawElementsIndirectCommand {
uint32_t count;
uint32_t instanceCount;
uint32_t firstIndex;
uint32_t baseVertex;
uint32_t baseInstance;
};
// Per-model GPU state for the instanced render path.
//
// VBO: local-coord interleaved verts (pos3 + normal3 + color1_packed) — 28 B.
@@ -71,13 +80,15 @@ struct ModelGpuData {
ModelBvh bvh;
// Dynamic visible-instance index buffer (std430, binding = 1).
// Re-uploaded each frame from frame_visible_scratch_.
// Re-uploaded each frame from visible_flat_.
GLuint visible_ssbo = 0;
size_t visible_ssbo_capacity = 0; // bytes
// Per-mesh visible-list offset/count, rebuilt each frame.
std::vector<uint32_t> mesh_vis_first;
std::vector<uint32_t> mesh_vis_count;
// GL_DRAW_INDIRECT_BUFFER of DrawElementsIndirectCommand[], one per
// non-empty mesh. Re-uploaded each frame.
GLuint indirect_buffer = 0;
size_t indirect_capacity = 0; // bytes
uint32_t indirect_command_count = 0; // valid commands this frame
bool finalized = false;
bool hidden = false;
@@ -152,8 +163,9 @@ private:
bool growModelSsbo(ModelGpuData& m, size_t needed_total);
ModelGpuData& getOrCreateModel(uint32_t model_id);
// Populate m.mesh_vis_first / mesh_vis_count and upload visible indices
// to m.visible_ssbo. Uses BVH when available, else linear scan.
// Frustum-cull m's instances (BVH if available, else linear scan),
// build the per-mesh DrawElementsIndirectCommand array + flat visible
// list, and upload both to m.indirect_buffer / m.visible_ssbo.
void cullAndUploadVisible(ModelGpuData& m, const float planes[6][4]);
// Mouse interaction
@@ -194,9 +206,12 @@ private:
// Reused scratch: visible-instance index lists per mesh, flattened into
// `visible_flat_` for upload. Both live in the parent object to avoid
// per-frame allocation.
std::vector<std::vector<uint32_t>> visible_by_mesh_;
std::vector<uint32_t> visible_flat_;
// per-frame allocation. indirect_scratch_ is the matching array of
// DrawElementsIndirectCommand records — forward-declared as bytes so
// the header doesn't need the struct definition.
std::vector<std::vector<uint32_t>> visible_by_mesh_;
std::vector<uint32_t> visible_flat_;
std::vector<DrawElementsIndirectCommand> indirect_scratch_;
// Camera
QVector3D camera_target_{0, 0, 0};