From 7612051f814bf03bd092187a1db8224b9b86d4b5 Mon Sep 17 00:00:00 2001
From: Imanol Fernandez <mortimergoro@gmail.com>
Date: Thu, 24 Jan 2019 17:21:14 +0100
Subject: [PATCH] Add support for rendering geometry range

---
 include/vrb/Geometry.h |  2 ++
 src/Geometry.cpp       | 25 +++++++++++++++++++++++--
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/include/vrb/Geometry.h b/include/vrb/Geometry.h
index 7c2995f..54e19c7 100644
--- a/include/vrb/Geometry.h
+++ b/include/vrb/Geometry.h
@@ -38,6 +38,8 @@ class Geometry : public Node, public Drawable, protected ResourceGL {
   VertexArrayPtr GetVertexArray() const;
   void SetVertexArray(const VertexArrayPtr& aVertexArray);
   void UpdateBuffers();
+  void SetRenderRange(uint32_t aStartIndex, uint32_t aLength);
+  int32_t TriangleCount() const;
 
   void AddFace(
     const std::vector<int> &aVerticies,
diff --git a/src/Geometry.cpp b/src/Geometry.cpp
index 10f7409..b64a263 100644
--- a/src/Geometry.cpp
+++ b/src/Geometry.cpp
@@ -48,8 +48,11 @@ struct Geometry::State : public Node::State, public ResourceGL::State, public Dr
   int triangleCount;
   GLuint vertexObjectId;
   GLuint indexObjectId;
+  uint32_t rangeStart;
+  uint32_t rangeLength;
 
-  State() : vertexCount(0), triangleCount(0), vertexObjectId(0), indexObjectId(0) {}
+  State() : vertexCount(0), triangleCount(0), vertexObjectId(0), indexObjectId(0),
+            rangeStart(0), rangeLength(0) {}
 
 
   GLsizei UVLength() const {
@@ -121,7 +124,14 @@ Geometry::Draw(const Camera& aCamera, const Matrix& aModelTransform) {
     if (kUseTextureCoords) {
       VRB_GL_CHECK(glEnableVertexAttribArray((GLuint)m.renderState->AttributeUV()));
     }
-    VRB_GL_CHECK(glDrawElements(GL_TRIANGLES, m.triangleCount * 3, GL_UNSIGNED_SHORT, 0));
+    const int32_t maxLength = m.triangleCount * 3;
+    if (m.rangeLength == 0) {
+      VRB_GL_CHECK(glDrawElements(GL_TRIANGLES, maxLength, GL_UNSIGNED_SHORT, 0));
+    } else if (m.rangeStart >= maxLength || (m.rangeStart + m.rangeLength) > maxLength) {
+      VRB_WARN("Invalid geometry range (%u-%u). Max geometry length %d", m.rangeStart, m.rangeLength + m.rangeLength, maxLength);
+    } else {
+      VRB_GL_CHECK(glDrawElements(GL_TRIANGLES, m.rangeLength, GL_UNSIGNED_SHORT, (void*)(m.rangeStart * sizeof(GLushort))));
+    }
     VRB_GL_CHECK(glDisableVertexAttribArray((GLuint)m.renderState->AttributePosition()));
     VRB_GL_CHECK(glDisableVertexAttribArray((GLuint)m.renderState->AttributeNormal()));
     if (kUseTextureCoords) {
@@ -225,6 +235,17 @@ Geometry::UpdateBuffers() {
   VRB_GL_CHECK(glBindBuffer(GL_ARRAY_BUFFER, 0));
 }
 
+void
+Geometry::SetRenderRange(uint32_t aStartIndex, uint32_t aLength) {
+  m.rangeStart = aStartIndex;
+  m.rangeLength = aLength;
+}
+
+int32_t
+Geometry::TriangleCount() const {
+  return m.triangleCount;
+}
+
 void
 Geometry::AddFace(
     const std::vector<int>& aVertices,