[Feature] OpenGL Backend

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Feature description
This feature proposes the introduction of a Backend System for ImPlot3D. Currently, depth sorting is performed on the CPU at a per-triangle level (Z-sorting), which is computationally expensive for large datasets and fails to resolve visual artifacts when triangles intersect or overlap complexly. By supporting a dedicated OpenGL Backend, ImPlot3D can leverage the GPU's hardware depth buffer (Z-buffer) for per-pixel depth testing, ensuring perfect intersections and significantly higher performance. When no backend is initialized, ImPlot3D should fall back to CPU Z-sorting.

This issue will also resolve #171 #126 #121 #111 #100

Tasks

• Study ImGui OpenGL backend
• Define how this could be implemented
• Create plot RGBA texture with hardcoded colors
• Create plot depth texture
• Render plot rect as plot RGBA texture
• Create 3D rendering OpenGL shader
• Render plot RGBA texture from ImDrawList3D using depth texture
• Alpha Blending with sorted triangles
• Per-pixel plot clipping
• Instanced line rendering
• Instanced marker rendering
• Render images

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My current idea is to create two textures per visible 3D plot. One texture will be an RGBA, and the other will be a depth texture. A GPU shader will be responsible for transforming the vertices from plot coordinates to pixel coordinates. This GPU shader will use the depth texture during depth testing.

Below the PlotRect is being rendered from a hardcoded RGBA texture generated from the OpenGL3 backend.

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First output from the opengl3 backend shader

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Rendering looking great

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ImGui rendering triangles (per-triangle Z buffer)

ImPlot3D backend rendering (per-pixel Z buffer)

Exciting!!

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Working great when triangles have alpha=1, alpha blending still needs some work

Kooha-2026-02-15-11-10-15.mp4

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For now the user will need to add 4 more function calls in their rendering loop

    // Setup backend
    ImGui_ImplGlfw_InitForOpenGL(window, true);
    ImGui_ImplOpenGL3_Init(glsl_version);
+   ImPlot3D_ImplOpenGL3_Init();

    // Main loop
    while (!glfwWindowShouldClose(window)) {
        glfwPollEvents();

        // Start frame
        ImGui_ImplOpenGL3_NewFrame();
        ImGui_ImplGlfw_NewFrame();
        ImGui::NewFrame();

        // Demo windows
        ImGui::ShowDemoWindow();
        ImPlot::ShowDemoWindow();
        ImPlot3D::ShowDemoWindow();

        // Prepare draw data for rendering
        ImGui::Render();
+       ImPlot3D::Render();

        // Render draw data
        int display_w, display_h;
        glfwGetFramebufferSize(window, &display_w, &display_h);
        glViewport(0, 0, display_w, display_h);
        glClearColor(0.1f, 0.1f, 0.1f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);
+       ImPlot3D_ImplOpenGL3_RenderDrawData(ImPlot3D::GetDrawData());
        ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());

        // Swap buffers
        glfwSwapBuffers(window);
    }

    // Cleanup
+   ImPlot3D_ImplOpenGL3_Shutdown();
    ImGui_ImplOpenGL3_Shutdown();
    ImGui_ImplGlfw_Shutdown();
    ImPlot3D::DestroyContext();
    ImPlot::DestroyContext();
    ImGui::DestroyContext();
    glfwDestroyWindow(window);
    glfwTerminate();