Change the glDepthFunc to GL_GREATER for rendering the outer mesh depth.

[piotrpolatowski]

Hi, I am wondering if it's possible from the client code to change the glDepthFunc from default GL_LESS to GL_GREATER?

Alternatively, do you know where could I modify the source code to achieve what I'm expecting in a hack fast way?

I have modified the OffscreenRendering.cpp example and my need is to get the outer most depth from the perspective of the camera... but I don't know if this is possible to control.

I also have tried changing depth test places that I've found and recompiled the source, but the output does not change.
diff.txt

// ----------------------------------------------------------------------------
// -                        Open3D: www.open3d.org                            -
// ----------------------------------------------------------------------------
// Copyright (c) 2018-2023 www.open3d.org
// SPDX-License-Identifier: MIT
// ----------------------------------------------------------------------------

#include <chrono>
#include "open3d/Open3D.h"
#include "open3d/visualization/rendering/Camera.h"
#include "open3d/visualization/rendering/filament/FilamentEngine.h"
#include "open3d/visualization/rendering/filament/FilamentRenderer.h"
#include <math/mat4.h>
using namespace std::chrono;

using namespace open3d;
using namespace open3d::visualization::gui;
using namespace open3d::visualization::rendering;

using namespace filament::math;

// Headless rendering requires Open3D to be compiled with OSMesa support.
// Add -DENABLE_HEADLESS_RENDERING=ON when you run CMake.
static const bool kUseHeadless = false;

static const std::string kOutputFilename = "offscreen.png";

int main(int argc, const char *argv[]) {
    const int width = 640;
    const int height = 480;

    auto &app = Application::GetInstance();
    app.Initialize(argc, argv);
    if (kUseHeadless) {
        EngineInstance::EnableHeadless();
    }

    auto *renderer =
            new FilamentRenderer(EngineInstance::GetInstance(), width, height,
                                 EngineInstance::GetResourceManager());
    auto *scene = new Open3DScene(*renderer);

    MaterialRecord mat;
    mat.shader = "defaultLit";

    auto mesh_ptr = std::make_shared<geometry::TriangleMesh>();
    if (io::ReadTriangleMesh("/home/polat/nomagic/monomagic/gripper-ros/results/ml_double.stl", *mesh_ptr)) {
        utility::LogInfo("Successfully read {}", argv[2]);
    } else {
        utility::LogWarning("Failed to read {}", argv[2]);
        return 1;
    }

    mesh_ptr->ComputeVertexNormals();
    mesh_ptr->PaintUniformColor({0.0f, 1.0f, 1.0f});
    mesh_ptr->Scale(1e-3, Eigen::Vector3d(0, 0, 0));
    scene->AddGeometry("ml_double", mesh_ptr.get(), mat);
    scene->AddGeometry("ml_double_2", mesh_ptr.get(), mat);

    Eigen::Matrix4d transformation;
    transformation << 1, 0, 0, 0.05, 0, 1, 0, 0, 0, 0, 1, 0.05, 0.0, 0, 0, 1;
    scene->SetGeometryTransform("ml_double_2", transformation);


//    auto torus = open3d::geometry::TriangleMesh::CreateTorus();
//    torus->ComputeVertexNormals();
//    torus->PaintUniformColor({1.0f, 1.0f, 0.0f});
//    scene->AddGeometry("torus", torus.get(), mat);
//    scene->ShowAxes(true);

//    scene->GetCamera()->SetProjection(60.0f, float(width) / float(height), 0.1f,
//                                      1400.0f, Camera::FovType::Vertical);
    auto model_half_size = 0.1f;
    scene->GetCamera()->SetProjection(Camera::Projection::Ortho, -model_half_size, model_half_size, -model_half_size, model_half_size, -0.7, 0.7);
    auto eye_dist = 1.0f;
    auto model_center = 0.075f;
    scene->GetCamera()->LookAt({model_center, 0.0f, model_center}, {model_center, eye_dist, model_center},
                               {1.0f, 0.0f, 0.0f});

    // This example demonstrates rendering to an image without a window.
    // If you want to render to an image from within a window you should use
    // scene->GetScene()->RenderToImage() instead.
    auto start = high_resolution_clock::now();

    auto img = app.RenderToDepthImage(*renderer, scene->GetView(), scene->GetScene(),
                                 width, height);

    auto stop = high_resolution_clock::now();
    auto duration = duration_cast<microseconds>(stop - start);

    std::cout << "Time taken by function: "
         << duration.count() << " microseconds" << std::endl;

    auto output = img->CreateImageFromFloatImage<uint8_t>();
    std::cout << "Writing file to " << kOutputFilename << std::endl;

    io::WriteImage(kOutputFilename, *output);

    // We manually delete these because Filament requires that things get
    // destructed in the right order.
    delete scene;
    delete renderer;

    // Cleanup Filament. Normally this is done by app.Run(), but since we are
    // not using that we need to do it ourselves.
    app.OnTerminate();
}

[piotrpolatowski]

@ssheorey @prewettg since you have contributed to the OffscreenRendering.cpp maybe you will be able to discuss the issue?