From b6dbc8fc585acf9f5439f69126e991d2cb1034a7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Vladim=C3=ADr=20Vondru=C5=A1?= <mosra@centrum.cz>
Date: Sun, 9 Oct 2016 22:22:11 +0200
Subject: [PATCH] Check for mesh emptiness even before binding the shader.

Prevents unnecessary state touching.
---
 src/Magnum/Mesh.cpp            |  6 +++---
 src/Magnum/MeshView.cpp        |  6 ++++++
 src/Magnum/Test/MeshGLTest.cpp | 18 +++++++++++-------
 3 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/src/Magnum/Mesh.cpp b/src/Magnum/Mesh.cpp
index 938f1915e..507d58c5b 100644
--- a/src/Magnum/Mesh.cpp
+++ b/src/Magnum/Mesh.cpp
@@ -245,6 +245,9 @@ Mesh& Mesh::setIndexBuffer(Buffer& buffer, GLintptr offset, IndexType type, Unsi
 }
 
 void Mesh::draw(AbstractShaderProgram& shader) {
+    /* Nothing to draw, exit without touching any state */
+    if(!_count || !_instanceCount) return;
+
     shader.use();
 
     #ifndef MAGNUM_TARGET_GLES
@@ -266,9 +269,6 @@ void Mesh::drawInternal(Int count, Int baseVertex, Int instanceCount, GLintptr i
 {
     const Implementation::MeshState& state = *Context::current().state().mesh;
 
-    /* Nothing to draw */
-    if(!count || !instanceCount) return;
-
     (this->*state.bindImplementation)();
 
     /* Non-instanced mesh */
diff --git a/src/Magnum/MeshView.cpp b/src/Magnum/MeshView.cpp
index aa863dca9..e1a55c7a4 100644
--- a/src/Magnum/MeshView.cpp
+++ b/src/Magnum/MeshView.cpp
@@ -130,6 +130,9 @@ void MeshView::multiDrawImplementationDefault(std::initializer_list<std::referen
 #ifdef MAGNUM_TARGET_GLES
 void MeshView::multiDrawImplementationFallback(std::initializer_list<std::reference_wrapper<MeshView>> meshes) {
     for(MeshView& mesh: meshes) {
+        /* Nothing to draw in this mesh */
+        if(!mesh._count) continue;
+
         #ifndef MAGNUM_TARGET_GLES2
         mesh._original.get().drawInternal(mesh._count, mesh._baseVertex, mesh._instanceCount, mesh._indexOffset, mesh._indexStart, mesh._indexEnd);
         #else
@@ -145,6 +148,9 @@ MeshView& MeshView::setIndexRange(Int first) {
 }
 
 void MeshView::draw(AbstractShaderProgram& shader) {
+    /* Nothing to draw, exit without touching any state */
+    if(!_count || !_instanceCount) return;
+
     shader.use();
 
     #ifndef MAGNUM_TARGET_GLES
diff --git a/src/Magnum/Test/MeshGLTest.cpp b/src/Magnum/Test/MeshGLTest.cpp
index 41b4543cc..e798a7423 100644
--- a/src/Magnum/Test/MeshGLTest.cpp
+++ b/src/Magnum/Test/MeshGLTest.cpp
@@ -1853,19 +1853,23 @@ MultiChecker::MultiChecker(AbstractShaderProgram&& shader, Mesh& mesh): framebuf
     mesh.setPrimitive(MeshPrimitive::Points)
         .setCount(2);
 
+    /* Set zero count so we test mesh skipping */
+    MeshView a{mesh};
+    a.setCount(0);
+
     /* Skip first vertex so we test also offsets */
-    MeshView a(mesh);
-    a.setCount(1)
+    MeshView b{mesh};
+    b.setCount(1)
      .setBaseVertex(mesh.baseVertex());
 
-    MeshView b(mesh);
-    b.setCount(1);
+    MeshView c{mesh};
+    c.setCount(1);
     if(mesh.isIndexed()) {
-        b.setBaseVertex(mesh.baseVertex())
+        c.setBaseVertex(mesh.baseVertex())
          .setIndexRange(1);
-    } else b.setBaseVertex(1);
+    } else c.setBaseVertex(1);
 
-    MeshView::draw(shader, {a, b});
+    MeshView::draw(shader, {a, b, c});
 }
 
 template<class T> T MultiChecker::get(PixelFormat format, PixelType type) {