Example: opengl_custom_shaders_demo

This example demonstrates how to install a custom OpenGL shader replacing one of MRPT default ones.

In particular, the fragment shader is modified such that depth (raw depth, in opengl internal logarithmic scale) with respect to the eye is visualized as grayscale levels.

C++ example source code:

/*                    _
                     | |    Mobile Robot Programming Toolkit (MRPT)
 _ __ ___  _ __ _ __ | |_
| '_ ` _ \| '__| '_ \| __|          https://www.mrpt.org/
| | | | | | |  | |_) | |_
|_| |_| |_|_|  | .__/ \__|     https://github.com/MRPT/mrpt/
               | |
               |_|

 Copyright (c) 2005-2026, Individual contributors, see AUTHORS file
 See: https://www.mrpt.org/Authors - All rights reserved.
 SPDX-License-Identifier: BSD-3-Clause
*/

#include <mrpt/gui/CDisplayWindow3D.h>
#include <mrpt/opengl/DefaultShaders.h>
#include <mrpt/opengl/Shader.h>
#include <mrpt/opengl/ShaderProgramManager.h>
#include <mrpt/opengl/opengl_api.h>
#include <mrpt/viz/CBox.h>
#include <mrpt/viz/Scene.h>

#include <chrono>
#include <iostream>
#include <thread>

using namespace std::chrono_literals;

// ---------------------------------------------------------------------------
// Custom vertex shader: same inputs as the built-in TRIANGLES_NO_LIGHT shader
// (position, vertexColor, pmv_matrix) plus mv_matrix to compute eye-space z.
// ---------------------------------------------------------------------------
static const char* CUSTOM_VERT = R"XXX(#version 300 es

layout(location = 0) in highp vec3 position;
layout(location = 1) in lowp  vec4 vertexColor;

uniform highp mat4 pmv_matrix;
uniform highp mat4 mv_matrix;

out highp float frag_eyeZ;

void main()
{
    gl_Position = pmv_matrix * vec4(position, 1.0);
    // Eye-space Z (positive = in front of camera)
    frag_eyeZ = -(mv_matrix * vec4(position, 1.0)).z;
}
)XXX";

// ---------------------------------------------------------------------------
// Custom fragment shader: map eye-space depth to a blue→red colour ramp.
// ---------------------------------------------------------------------------
static const char* CUSTOM_FRAG = R"XXX(#version 300 es

in highp float frag_eyeZ;
out lowp vec4 color;

void main()
{
    // Remap depth [0, 30] → [0, 1] and use as a blue→red gradient
    highp float t = clamp(frag_eyeZ / 30.0, 0.0, 1.0);
    color = vec4(t, 0.2, 1.0 - t, 1.0);
}
)XXX";

// ---------------------------------------------------------------------------
static void installCustomShader(mrpt::opengl::ShaderProgramManager& mgr)
{
  std::string errMsg;
  auto prog = mgr.loadCustomProgram("depth_colour", CUSTOM_VERT, CUSTOM_FRAG, "", &errMsg);
  if (!prog)
  {
    std::cerr << "[installCustomShader] Compilation/link error:\n" << errMsg << "\n";
    return;
  }

  std::cout << "[installCustomShader] Custom depth-colour shader compiled OK.\n";

  // Override the no-light triangles slot so all boxes use the new shader:
  mgr.overrideBuiltinProgram(mrpt::opengl::DefaultShaderID::TRIANGLES_NO_LIGHT, prog);
}

// ---------------------------------------------------------------------------
void DemoCustomShaders()
{
  using namespace mrpt;
  using namespace mrpt::gui;
  using namespace mrpt::viz;
  using namespace mrpt::math;

  CDisplayWindow3D win("Demo of MRPT v3 custom shaders", 640, 480);

  Scene::Ptr& theScene = win.get3DSceneAndLock();

  // Add a few boxes that will be rendered with the custom depth-colour shader
  {
    auto obj = viz::CBox::Create(TPoint3D(0, 0, 0), TPoint3D(1, 1, 1), false);
    obj->setLocation(0, 0, 0);
    theScene->insert(obj);

    auto obj2 = viz::CBox::Create(TPoint3D(0, 0, 0), TPoint3D(1, 1, 1), false);
    obj2->setLocation(0, 4, 0);
    theScene->insert(obj2);

    auto obj3 = viz::CBox::Create(TPoint3D(0, 0, 0), TPoint3D(1, 1, 1), false);
    obj3->setLocation(0, 8, 0);
    theScene->insert(obj3);
  }

  win.setCameraZoom(25);
  win.unlockAccess3DScene();

  // Wait for the GL context to be ready before installing the custom shader
  win.wait_for_GL_context();

  // Install the custom shader on the GL thread via ShaderProgramManager
  win.sendFunctionWithShaderManager([](mrpt::opengl::ShaderProgramManager& mgr)
                                    { installCustomShader(mgr); });

  win.repaint();

  std::cout << "Close the window to end.\n";
  while (win.isOpen())
  {
    std::this_thread::sleep_for(50ms);
    win.repaint();
  }
}

// ---------------------------------------------------------------------------
int main()
{
  try
  {
    DemoCustomShaders();
    return 0;
  }
  catch (const std::exception& e)
  {
    std::cerr << mrpt::exception_to_str(e) << "\n";
    return 1;
  }
}