Backends: OpenGL3: Fix building with pre-3.2 GL bindings which do not expose glDrawElementsBaseVertex(), using runtime GL version to decide if we set ImGuiBackendFlags_RendererHasVtxOffset. (#2866, #2852) [@dpilawa]

docs/CHANGELOG.txt

@@ -73,6 +73,8 @@ Other Changes:
 - Demo: Added simple item reordering demo in Widgets -> Drag and Drop section. (#2823, #143) [@rokups]
 - Examples: DX12: Using IDXGIDebug1::ReportLiveObjects() when DX12_ENABLE_DEBUG_LAYER is enabled.
 - Examples: Emscripten: Removed NO_FILESYSTEM from Makefile, seems to fail on some setup. (#2734) [@Funto]
+- Backends: OpenGL3: Fix building with pre-3.2 GL loaders which do not expose glDrawElementsBaseVertex(),
+  using runtime GL version to decide if we set ImGuiBackendFlags_RendererHasVtxOffset. (#2866, #2852) [@dpilawa]
 - Backends: OSX: Fix using Backspace key. (#2578, #2817, #2818) [@DiligentGraphics]
 - Backends: GLFW: Previously installed user callbacks are now restored on shutdown. (#2836) [@malte-v]
 - CI: Set up a bunch of continuous-integration tests using GitHub Actions. We now compile many of the example

examples/imgui_impl_opengl3.cpp

@@ -13,6 +13,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2019-10-25: OpenGL: Using a combination of GL define and runtime GL version to decide whether to use glDrawElementsBaseVertex(). Fix building with pre-3.2 GL loaders.
 //  2019-09-22: OpenGL: Detect default GL loader using __has_include compiler facility.
 //  2019-09-16: OpenGL: Tweak initialization code to allow application calling ImGui_ImplOpenGL3_CreateFontsTexture() before the first NewFrame() call.
 //  2019-05-29: OpenGL: Desktop GL only: Added support for large mesh (64K+ vertices), enable ImGuiBackendFlags_RendererHasVtxOffset flag.
@@ -77,7 +78,7 @@
 #include "TargetConditionals.h"
 #endif
 
-// Auto-detect GL version
+// Auto-enable GLES on matching platforms
 #if !defined(IMGUI_IMPL_OPENGL_ES2) && !defined(IMGUI_IMPL_OPENGL_ES3)
 #if (defined(__APPLE__) && (TARGET_OS_IOS || TARGET_OS_TV)) || (defined(__ANDROID__))
 #define IMGUI_IMPL_OPENGL_ES3           // iOS, Android  -> GL ES 3, "#version 300 es"
@@ -119,16 +120,16 @@
 #endif
 #endif
 
-// Desktop GL has glDrawElementsBaseVertex() which GL ES and WebGL don't have.
+// Desktop GL 3.2+ has glDrawElementsBaseVertex() which GL ES and WebGL don't have.
 #if defined(IMGUI_IMPL_OPENGL_ES2) || defined(IMGUI_IMPL_OPENGL_ES3) || !defined(GL_VERSION_3_2)
-#define IMGUI_IMPL_OPENGL_MAY_HAVE_DRAW_WITH_BASE_VERTEX     0
+#define IMGUI_IMPL_OPENGL_MAY_HAVE_VTX_OFFSET   0
 #else
-#define IMGUI_IMPL_OPENGL_MAY_HAVE_DRAW_WITH_BASE_VERTEX     1
+#define IMGUI_IMPL_OPENGL_MAY_HAVE_VTX_OFFSET   1
 #endif
 
 // OpenGL Data
-static char         g_GlslVersionString[32] = "";
+static GLuint       g_GlVersion = 0;                // Extracted at runtime using GL_MAJOR_VERSION, GL_MINOR_VERSION queries.
-static GLuint       g_GlVersion = 0;
+static char         g_GlslVersionString[32] = "";   // Specified by user or detected based on compile time GL settings.
 static GLuint       g_FontTexture = 0;
 static GLuint       g_ShaderHandle = 0, g_VertHandle = 0, g_FragHandle = 0;
 static int          g_AttribLocationTex = 0, g_AttribLocationProjMtx = 0;                                // Uniforms location
@@ -138,7 +139,7 @@ static unsigned int g_VboHandle = 0, g_ElementsHandle = 0;
 // Functions
 bool    ImGui_ImplOpenGL3_Init(const char* glsl_version)
 {
-    // query for GL version
+    // Query for GL version
 #if !defined(IMGUI_IMPL_OPENGL_ES2)
     GLint major, minor;
     glGetIntegerv(GL_MAJOR_VERSION, &major);
@@ -151,12 +152,13 @@ bool    ImGui_ImplOpenGL3_Init(const char* glsl_version)
     // Setup back-end capabilities flags
     ImGuiIO& io = ImGui::GetIO();
     io.BackendRendererName = "imgui_impl_opengl3";
-#if IMGUI_IMPL_OPENGL_MAY_HAVE_DRAW_WITH_BASE_VERTEX
+#if IMGUI_IMPL_OPENGL_MAY_HAVE_VTX_OFFSET
     if (g_GlVersion >= 3200)
         io.BackendFlags |= ImGuiBackendFlags_RendererHasVtxOffset;  // We can honor the ImDrawCmd::VtxOffset field, allowing for large meshes.
 #endif
 
-    // Store GLSL version string so we can refer to it later in case we recreate shaders. Note: GLSL version is NOT the same as GL version. Leave this to NULL if unsure.
+    // Store GLSL version string so we can refer to it later in case we recreate shaders. 
+    // Note: GLSL version is NOT the same as GL version. Leave this to NULL if unsure.
 #if defined(IMGUI_IMPL_OPENGL_ES2)
     if (glsl_version == NULL)
         glsl_version = "#version 100";
@@ -356,14 +358,12 @@ void    ImGui_ImplOpenGL3_RenderDrawData(ImDrawData* draw_data)
 
                     // Bind texture, Draw
                     glBindTexture(GL_TEXTURE_2D, (GLuint)(intptr_t)pcmd->TextureId);
-#if IMGUI_IMPL_OPENGL_MAY_HAVE_DRAW_WITH_BASE_VERTEX
+#if IMGUI_IMPL_OPENGL_MAY_HAVE_VTX_OFFSET
                     if (g_GlVersion >= 3200)
                         glDrawElementsBaseVertex(GL_TRIANGLES, (GLsizei)pcmd->ElemCount, sizeof(ImDrawIdx) == 2 ? GL_UNSIGNED_SHORT : GL_UNSIGNED_INT, (void*)(intptr_t)(pcmd->IdxOffset * sizeof(ImDrawIdx)), (GLint)pcmd->VtxOffset);
                     else
-                        glDrawElements(GL_TRIANGLES, (GLsizei)pcmd->ElemCount, sizeof(ImDrawIdx) == 2 ? GL_UNSIGNED_SHORT : GL_UNSIGNED_INT, (void*)(intptr_t)(pcmd->IdxOffset * sizeof(ImDrawIdx)));                    
-#else
-                    glDrawElements(GL_TRIANGLES, (GLsizei)pcmd->ElemCount, sizeof(ImDrawIdx) == 2 ? GL_UNSIGNED_SHORT : GL_UNSIGNED_INT, (void*)(intptr_t)(pcmd->IdxOffset * sizeof(ImDrawIdx)));
 #endif
+                    glDrawElements(GL_TRIANGLES, (GLsizei)pcmd->ElemCount, sizeof(ImDrawIdx) == 2 ? GL_UNSIGNED_SHORT : GL_UNSIGNED_INT, (void*)(intptr_t)(pcmd->IdxOffset * sizeof(ImDrawIdx)));
                 }
             }
         }