Fix IdxBuffer related comments. (#4845, #4863)

imgui.cpp

@@ -295,6 +295,7 @@ CODE
        // TODO: Setup viewport covering draw_data->DisplayPos to draw_data->DisplayPos + draw_data->DisplaySize
        // TODO: Setup orthographic projection matrix cover draw_data->DisplayPos to draw_data->DisplayPos + draw_data->DisplaySize
        // TODO: Setup shader: vertex { float2 pos, float2 uv, u32 color }, fragment shader sample color from 1 texture, multiply by vertex color.
+       ImVec2 clip_off = draw_data->DisplayPos;
        for (int n = 0; n < draw_data->CmdListsCount; n++)
        {
           const ImDrawList* cmd_list = draw_data->CmdLists[n];
@@ -309,9 +310,11 @@ CODE
              }
              else
              {
-                 // The texture for the draw call is specified by pcmd->GetTexID().
-                 // The vast majority of draw calls will use the Dear ImGui texture atlas, which value you have set yourself during initialization.
-                 MyEngineBindTexture((MyTexture*)pcmd->GetTexID());
+                 // Project scissor/clipping rectangles into framebuffer space
+                 ImVec2 clip_min(pcmd->ClipRect.x - clip_off.x, pcmd->ClipRect.y - clip_off.y);
+                 ImVec2 clip_max(pcmd->ClipRect.z - clip_off.x, pcmd->ClipRect.w - clip_off.y);
+                 if (clip_max.x <= clip_min.x || clip_max.y <= clip_min.y)
+                     continue;
 
                  // We are using scissoring to clip some objects. All low-level graphics API should support it.
                  // - If your engine doesn't support scissoring yet, you may ignore this at first. You will get some small glitches
@@ -322,14 +325,16 @@ CODE
                  //   - In the interest of supporting multi-viewport applications (see 'docking' branch on github),
                  //     always subtract draw_data->DisplayPos from clipping bounds to convert them to your viewport space.
                  // - Note that pcmd->ClipRect contains Min+Max bounds. Some graphics API may use Min+Max, other may use Min+Size (size being Max-Min)
-                 ImVec2 pos = draw_data->DisplayPos;
-                 MyEngineScissor((int)(pcmd->ClipRect.x - pos.x), (int)(pcmd->ClipRect.y - pos.y), (int)(pcmd->ClipRect.z - pos.x), (int)(pcmd->ClipRect.w - pos.y));
+                 MyEngineSetScissor(clip_min.x, clip_min.y, clip_max.x, clip_max.y);
+
+                 // The texture for the draw call is specified by pcmd->GetTexID().
+                 // The vast majority of draw calls will use the Dear ImGui texture atlas, which value you have set yourself during initialization.
+                 MyEngineBindTexture((MyTexture*)pcmd->GetTexID());
 
                  // Render 'pcmd->ElemCount/3' indexed triangles.
                  // By default the indices ImDrawIdx are 16-bit, you can change them to 32-bit in imconfig.h if your engine doesn't support 16-bit indices.
-                 MyEngineDrawIndexedTriangles(pcmd->ElemCount, sizeof(ImDrawIdx) == 2 ? GL_UNSIGNED_SHORT : GL_UNSIGNED_INT, idx_buffer, vtx_buffer);
+                 MyEngineDrawIndexedTriangles(pcmd->ElemCount, sizeof(ImDrawIdx) == 2 ? GL_UNSIGNED_SHORT : GL_UNSIGNED_INT, idx_buffer + pcmd->IdxOffset, vtx_buffer, pcmd->VtxOffset);
              }
-             idx_buffer += pcmd->ElemCount;
           }
        }
     }

imgui.h

@@ -2278,16 +2278,16 @@ typedef void (*ImDrawCallback)(const ImDrawList* parent_list, const ImDrawCmd* c
 #define ImDrawCallback_ResetRenderState     (ImDrawCallback)(-1)
 
 // Typically, 1 command = 1 GPU draw call (unless command is a callback)
-// - VtxOffset/IdxOffset: When 'io.BackendFlags & ImGuiBackendFlags_RendererHasVtxOffset' is enabled,
-//   those fields allow us to render meshes larger than 64K vertices while keeping 16-bit indices.
-//   Pre-1.71 backends will typically ignore the VtxOffset/IdxOffset fields.
+// - VtxOffset: When 'io.BackendFlags & ImGuiBackendFlags_RendererHasVtxOffset' is enabled,
+//   this fields allow us to render meshes larger than 64K vertices while keeping 16-bit indices.
+//   Backends made for <1.71. will typically ignore the VtxOffset fields.
 // - The ClipRect/TextureId/VtxOffset fields must be contiguous as we memcmp() them together (this is asserted for).
 struct ImDrawCmd
 {
     ImVec4          ClipRect;           // 4*4  // Clipping rectangle (x1, y1, x2, y2). Subtract ImDrawData->DisplayPos to get clipping rectangle in "viewport" coordinates
     ImTextureID     TextureId;          // 4-8  // User-provided texture ID. Set by user in ImfontAtlas::SetTexID() for fonts or passed to Image*() functions. Ignore if never using images or multiple fonts atlas.
     unsigned int    VtxOffset;          // 4    // Start offset in vertex buffer. ImGuiBackendFlags_RendererHasVtxOffset: always 0, otherwise may be >0 to support meshes larger than 64K vertices with 16-bit indices.
-    unsigned int    IdxOffset;          // 4    // Start offset in index buffer. Always equal to sum of ElemCount drawn so far.
+    unsigned int    IdxOffset;          // 4    // Start offset in index buffer.
     unsigned int    ElemCount;          // 4    // Number of indices (multiple of 3) to be rendered as triangles. Vertices are stored in the callee ImDrawList's vtx_buffer[] array, indices in idx_buffer[].
     ImDrawCallback  UserCallback;       // 4-8  // If != NULL, call the function instead of rendering the vertices. clip_rect and texture_id will be set normally.
     void*           UserCallbackData;   // 4-8  // The draw callback code can access this.