Fix glClipControl(GL_UPPER_LEFT) handling in opengl3.

docs/CHANGELOG.txt

@@ -44,6 +44,8 @@ Other Changes:
 - TreeNode: Fixed bug where dragging a payload over a TreeNode() with either _OpenOnDoubleClick
   or _OpenOnArrow would open the node. (#143)
 - Backends: Win32: Support for #define NOGDI, won't try to call GetDeviceCaps(). (#3137, #2327)
+- Backends: OpenGL: Fixed handling of GL 4.5+ glClipControl(GL_UPPER_LEFT) by inverting the
+  projection matrix top and bottom values. (#3143, #3146) [@u3shit]
 
 
 -----------------------------------------------------------------------

examples/imgui_impl_opengl3.cpp

@@ -13,6 +13,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2020-04-21: OpenGL: Fixed handling of glClipControl(GL_UPPER_LEFT) by inverting projection matrix.
 //  2020-04-12: OpenGL: Fixed context version check mistakenly testing for 4.0+ instead of 3.2+ to enable ImGuiBackendFlags_RendererHasVtxOffset.
 //  2020-03-24: OpenGL: Added support for glbinding 2.x OpenGL loader.
 //  2020-01-07: OpenGL: Added support for glbinding 3.x OpenGL loader.
@@ -243,6 +244,14 @@ static void ImGui_ImplOpenGL3_SetupRenderState(ImDrawData* draw_data, int fb_wid
     glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
 #endif
 
+    // Support for GL 4.5 rarely used glClipControl(GL_UPPER_LEFT)
+    bool clip_origin_lower_left = true;
+#if defined(GL_CLIP_ORIGIN) && !defined(__APPLE__)
+    GLenum current_clip_origin = 0; glGetIntegerv(GL_CLIP_ORIGIN, (GLint*)&current_clip_origin);
+    if (current_clip_origin == GL_UPPER_LEFT)
+        clip_origin_lower_left = false;
+#endif
+
     // Setup viewport, orthographic projection matrix
     // Our visible imgui space lies from draw_data->DisplayPos (top left) to draw_data->DisplayPos+data_data->DisplaySize (bottom right). DisplayPos is (0,0) for single viewport apps.
     glViewport(0, 0, (GLsizei)fb_width, (GLsizei)fb_height);
@@ -250,6 +259,7 @@ static void ImGui_ImplOpenGL3_SetupRenderState(ImDrawData* draw_data, int fb_wid
     float R = draw_data->DisplayPos.x + draw_data->DisplaySize.x;
     float T = draw_data->DisplayPos.y;
     float B = draw_data->DisplayPos.y + draw_data->DisplaySize.y;
+    if (!clip_origin_lower_left) { float tmp = T; T = B; B = tmp; } // Swap top and bottom if origin is upper left
     const float ortho_projection[4][4] =
     {
         { 2.0f/(R-L),   0.0f,         0.0f,   0.0f },
@@ -318,12 +328,6 @@ void    ImGui_ImplOpenGL3_RenderDrawData(ImDrawData* draw_data)
     GLboolean last_enable_cull_face = glIsEnabled(GL_CULL_FACE);
     GLboolean last_enable_depth_test = glIsEnabled(GL_DEPTH_TEST);
     GLboolean last_enable_scissor_test = glIsEnabled(GL_SCISSOR_TEST);
-    bool clip_origin_lower_left = true;
-#if defined(GL_CLIP_ORIGIN) && !defined(__APPLE__)
-    GLenum last_clip_origin = 0; glGetIntegerv(GL_CLIP_ORIGIN, (GLint*)&last_clip_origin); // Support for GL 4.5's glClipControl(GL_UPPER_LEFT)
-    if (last_clip_origin == GL_UPPER_LEFT)
-        clip_origin_lower_left = false;
-#endif
 
     // Setup desired GL state
     // Recreate the VAO every time (this is to easily allow multiple GL contexts to be rendered to. VAO are not shared among GL contexts)
@@ -371,10 +375,7 @@ void    ImGui_ImplOpenGL3_RenderDrawData(ImDrawData* draw_data)
                 if (clip_rect.x < fb_width && clip_rect.y < fb_height && clip_rect.z >= 0.0f && clip_rect.w >= 0.0f)
                 {
                     // Apply scissor/clipping rectangle
-                    if (clip_origin_lower_left)
-                        glScissor((int)clip_rect.x, (int)(fb_height - clip_rect.w), (int)(clip_rect.z - clip_rect.x), (int)(clip_rect.w - clip_rect.y));
-                    else
-                        glScissor((int)clip_rect.x, (int)clip_rect.y, (int)clip_rect.z, (int)clip_rect.w); // Support for GL 4.5 rarely used glClipControl(GL_UPPER_LEFT)
+                    glScissor((int)clip_rect.x, (int)(fb_height - clip_rect.w), (int)(clip_rect.z - clip_rect.x), (int)(clip_rect.w - clip_rect.y));
 
                     // Bind texture, Draw
                     glBindTexture(GL_TEXTURE_2D, (GLuint)(intptr_t)pcmd->TextureId);