I went making https://github.com/guprobr/WakkaQt instead to learn a bit more Qt before resuming this task
The last ideas we discussed, I never tried... but the last post on projectM made me feel second toughts on the viability of the idea.... anyway, i want to resume this task someday I don't think we should give up but certainly something is missing in the backend inner parts of how Qt works with OpenGL and how projectM works with OpenGL, as it was discussed on the thread on github.

But the first thing i would do is trying with latest Qt LTS, before anything.
Second thing, to try with the same version on Windows!
We have to figure where the problem is. Obviously its a Qt issue, but what made me stop working on it was when SDL window version showed the same issues. That was very disappointing.

PS: sorry to take so long to answer, I just saw this today

#include <QtCore/QtEndian>

void VisualizationSDL2::ConsumeBuffer(GstBuffer *buffer, const int pipeline_id, const QString &format) {
    Q_UNUSED(pipeline_id);

    GstMapInfo map;
    gst_buffer_map(buffer, &map, GST_MAP_READ);

    if (projectm_instance_) {
        unsigned int sample_count = 0;
        const int16_t *int16_data = nullptr;
        const float *float_data = nullptr;

        if (format == QStringLiteral("S16LE") || format == QStringLiteral("S16BE")) {
            sample_count = map.size / sizeof(int16_t) / 2;
            int16_data = reinterpret_cast<const int16_t *>(map.data);
            if (format == QStringLiteral("S16BE")) {
                // Convert big-endian to little-endian if necessary
                std::vector<int16_t> temp_data(sample_count * 2);
                for (unsigned int i = 0; i < sample_count * 2; ++i) {
                    temp_data[i] = qFromBigEndian(int16_data[i]);
                }
                int16_data = temp_data.data();
            }
            projectm_pcm_add_int16(projectm_instance_, int16_data, sample_count, PROJECTM_STEREO);
        } else if (format == QStringLiteral("F32LE") || format == QStringLiteral("F32BE")) {
            sample_count = map.size / sizeof(float) / 2;
            float_data = reinterpret_cast<const float *>(map.data);
            if (format == QStringLiteral("F32BE")) {
                // Convert big-endian to little-endian if necessary
                std::vector<float> temp_data(sample_count * 2);
                for (unsigned int i = 0; i < sample_count * 2; ++i) {
                    temp_data[i] = qFromBigEndian(float_data[i]);
                }
                float_data = temp_data.data();
            }
            projectm_pcm_add_float(projectm_instance_, float_data, sample_count, PROJECTM_STEREO);
        } else if (format == QStringLiteral("S24LE")) {
            // Handle 24-bit audio (3 bytes per sample)
            sample_count = map.size / 3 / 2;
            std::vector<int16_t> int16_samples(sample_count * 2);
            for (unsigned int i = 0; i < sample_count * 2; ++i) {
                int16_samples[i] = static_cast<int16_t>((map.data[i * 3 + 2] << 8) | map.data[i * 3 + 1]);
            }
            projectm_pcm_add_int16(projectm_instance_, int16_samples.data(), sample_count, PROJECTM_STEREO);
        }

        gst_buffer_unmap(buffer, &map);
    }

    gst_buffer_unref(buffer);
}

But now I've tried bypassing the normal way presets are loaded, and only use this, i.e.

projectm_playlist_add_path(projectm_playlist_instance_, "/home/guzpido/presets-cream-of-the-crop", true, true);

it appears to make a bit more presets to work. this could be a clue. Maybe the way we are loading presets has some issue; but I've banging my head to figure it out if this makes even sense.

edit: also i cant determine if ConsumeBuffer is working

#ifndef VISUALIZATIONOPENGLWIDGET_H
#define VISUALIZATIONOPENGLWIDGET_H

#include "config.h"

#include <QOpenGLWindow>
#include <QOpenGLFunctions>

class ProjectMVisualization;

class VisualizationOpenGLWidget : public QOpenGLWindow, protected QOpenGLFunctions {
    Q_OBJECT

public:
    explicit VisualizationOpenGLWidget(ProjectMVisualization* projectm_visualization, QOpenGLContext* sharedContext, QWindow* parent = nullptr);

    void initializeGL() override;

protected:
    void paintGL() override;
    void resizeGL(int width, int height) override;

private:
    void Setup(int width, int height);

    ProjectMVisualization* projectm_visualization_;
    QOpenGLContext* sharedContext_;
};

#endif  // VISUALIZATIONOPENGLWIDGET_H

#include "config.h"

#include <QPainter>

#include "core/logging.h"
#include "visualizationopenglwidget.h"
#include "projectmvisualization.h"

VisualizationOpenGLWidget::VisualizationOpenGLWidget(ProjectMVisualization* projectm_visualization, QOpenGLContext* sharedContext, QWindow* parent)
    : QOpenGLWindow(NoPartialUpdate, parent),
      projectm_visualization_(projectm_visualization),
      sharedContext_(sharedContext) {
    setFormat(sharedContext_->format());
}

void VisualizationOpenGLWidget::initializeGL() {
    QOpenGLWindow::initializeGL();
    sharedContext_->makeCurrent(this);
    initializeOpenGLFunctions();
    projectm_visualization_->Init();
}

void VisualizationOpenGLWidget::paintGL() {
    sharedContext_->makeCurrent(this);

    QPainter p(this);
    p.beginNativePainting();
    
    if (projectm_visualization_) {
        Setup(width(), height());
        projectm_visualization_->RenderFrame(width(), height());
    }
    
    update();

    p.endNativePainting();

    GLenum error = glGetError();
    if (error != GL_NO_ERROR) {
        qWarning() << "OpenGL error in paintGL:" << error;
    }

    qLog(Debug) << __PRETTY_FUNCTION__ << "Completed";
}

void VisualizationOpenGLWidget::resizeGL(int width, int height) {
    sharedContext_->makeCurrent(this);

    Setup(width, height);
    projectm_visualization_->Resize(width, height);

    GLenum error = glGetError();
    if (error != GL_NO_ERROR) {
        qWarning() << "OpenGL error in resizeGL:" << error;
    }
}

void VisualizationOpenGLWidget::Setup(int width, int height) {
    // Ensure the correct OpenGL context is current
    if (!sharedContext_->makeCurrent(this)) {
        qWarning() << "Failed to make OpenGL context current in Setup";
        return;
    }

    // Initialize OpenGL functions
    initializeOpenGLFunctions();

    // Set up OpenGL state
    glShadeModel(GL_SMOOTH);
    glClearColor(0, 0, 0, 0);
    glViewport(0, 0, width, height);
    
    glMatrixMode(GL_TEXTURE);
    glLoadIdentity();
    
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0, width, height, 0, -1, 1);  // Set an orthographic projection matrix
    
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    
    glDrawBuffer(GL_BACK);
    glReadBuffer(GL_BACK);
    
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

    glEnable(GL_LINE_SMOOTH);
    glEnable(GL_POINT_SMOOTH);
    glClearColor(0.0F, 0.0F, 0.0F, 0.0F);
    glLineStipple(2, 0xAAAA);

    GLenum error = glGetError();
    if (error != GL_NO_ERROR) {
        qWarning() << "OpenGL error in Setup:" << error;
    }
}

Its disgusting, but it really makes a HUGE percentage of presets to display instead of black screen.
Ive done this without wraping the openglwindow into a widget;
when you resize the window, it displays, several plugins happens that way.. actually the nicer ones like this one:

Tthe International Open GL NEVER Ugonna SLEEP AGAIN ha ha ha Society.

Decided to go ahead and do the QOpenGLWindow on Strawberry.

Its worse than just the standalone program. Much more bugs. But the few that works are incredible. We are close. Here's what I've been messing around with my IogLnusaha association (I'm on Gnomes)

// Create and setup the QOpenGLWindow
  auto *openGLWindow_ = new VisualizationOpenGLWidget(projectm_visualization_);
  QSurfaceFormat format;
  format.setVersion(3, 3);  // Set OpenGL version to 3.3
  format.setProfile(QSurfaceFormat::CoreProfile);  // Use the core profile
  format.setDepthBufferSize(24);
  format.setStencilBufferSize(8);
  openGLWindow_->setFormat(format);
  openGLWindow_->resize(1280, 720); // Default size for the OpenGL window

  // Wrap the QOpenGLWindow in a QWidget container
  auto *glContainer = QWidget::createWindowContainer(openGLWindow_);
  glContainer->setFocusPolicy(Qt::TabFocus);
  glContainer->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
  glContainer->setParent(this);
  glContainer->setVisible(true);
  ////////////setCentralWidget(glContainer);

  // Add the overlay as a child of the container and set its visibility
  ////////////////overlay_->setParent(glContainer);
  /////////////////overlay_->resize(320, 200); // Resize the overlay 
  //////////////////overlay_->setVisible(true);

  }

#ifndef VISUALIZATIONOPENGLWIDGET_H
#define VISUALIZATIONOPENGLWIDGET_H

#include "config.h"

#include <QOpenGLWindow>
#include <QOpenGLFunctions>

class ProjectMVisualization;

class VisualizationOpenGLWidget : public QOpenGLWindow, protected QOpenGLFunctions {
  Q_OBJECT

 public:
    explicit VisualizationOpenGLWidget(ProjectMVisualization* projectm_visualization, QWindow* parent = nullptr);
  //explicit VisualizationOpenGLWidget(ProjectMVisualization *projectm_visualization, QWidget *parent = nullptr, Qt::WindowFlags f = Qt::WindowFlags());
  //explicit OpenGLWidgetContainer(QOpenGLWindow* openGLWindow, QWidget* parent = nullptr);
  void initializeGL() override;

protected:
  void paintGL() override;
  void resizeGL(const int width, const int height) override;

 private:
  void Setup(const int width, const int height);

 private:
  ProjectMVisualization *projectm_visualization_;
};


#endif  // VISUALIZATIONOPENGLWIDGET_H

.cpp

#include "config.h"

#include <QPainter>

#include "core/logging.h"
#include "visualizationopenglwidget.h"
#include "projectmvisualization.h"

VisualizationOpenGLWidget::VisualizationOpenGLWidget(ProjectMVisualization *projectm_visualization, QWindow *parent)
  : QOpenGLWindow(NoPartialUpdate, parent),
    projectm_visualization_(projectm_visualization) {

    }

void VisualizationOpenGLWidget::initializeGL() {



  QOpenGLWindow::initializeGL();
  QOpenGLFunctions::initializeOpenGLFunctions();

    projectm_visualization_->Init();
  

}

void VisualizationOpenGLWidget::paintGL() {

  QPainter p(this);
  //////////resizeGL(width(), height());
  p.beginNativePainting();
  projectm_visualization_->RenderFrame(width(), height());
  p.endNativePainting();
  update();

  qLog(Debug) << __PRETTY_FUNCTION__ << glGetError();

}

void VisualizationOpenGLWidget::resizeGL(const int width, const int height) {

  
  Setup(width, height);
  projectm_visualization_->Resize(width, height);

}

void VisualizationOpenGLWidget::Setup(const int width, const int height) {

  glShadeModel(GL_SMOOTH);
  glClearColor(0, 0, 0, 0);
  glViewport(0, 0, width, height);
  glMatrixMode(GL_TEXTURE);
  glLoadIdentity();
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();
  glDrawBuffer(GL_BACK);
  glReadBuffer(GL_BACK);
  glEnable(GL_BLEND);

  glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
  glEnable(GL_LINE_SMOOTH);
  glEnable(GL_POINT_SMOOTH);
  glClearColor(0.0F, 0.0F, 0.0F, 0.0F);
  glLineStipple(2, 0xAAAA);

}

ConsumeBuffer:

void ProjectMVisualization::ConsumeBuffer(GstBuffer *buffer, const int pipeline_id, const QString &format) {

  Q_UNUSED(pipeline_id);
  Q_UNUSED(format);

  GstMapInfo map;
  gst_buffer_map(buffer, &map, GST_MAP_READ);

#ifdef HAVE_PROJECTM4
  if (projectm_instance_) {
    const unsigned int samples_per_channel = static_cast<unsigned int> (map.size / sizeof(size_t)) / 2;
    const int16_t *data = reinterpret_cast<int16_t*>(map.data);
    projectm_pcm_add_int16(projectm_instance_, data, samples_per_channel, PROJECTM_STEREO);
  }
#else
  if (projectm_) {
    const short samples_per_channel = static_cast<short>(map.size) / sizeof(short) / 2;
    const short *data = reinterpret_cast<short*>(map.data);
    projectm_->pcm()->addPCM16Data(data, samples_per_channel);
  }
#endif  // HAVE_PROJECTM4

  gst_buffer_unmap(buffer, &map);
  gst_buffer_unref(buffer);

}

InitprojectM

// Create projectM settings
#ifdef HAVE_PROJECTM4
Q_ASSERT(projectm_instance_ == nullptr);
Q_ASSERT(projectm_playlist_instance_ == nullptr);
projectm_instance_ = projectm_create();
projectm_set_preset_duration(projectm_instance_, duration_);
// Set initial window size
  projectm_set_window_size(projectm_instance_, 1280, 720);
  // Additional ProjectM setup
  projectm_set_mesh_size(projectm_instance_, 32, 24);
  projectm_set_fps(projectm_instance_, 60);
  projectm_set_aspect_correction(projectm_instance_, true);
  projectm_set_hard_cut_enabled(projectm_instance_, true);
  projectm_set_hard_cut_duration(projectm_instance_, 10);
  projectm_set_hard_cut_sensitivity(projectm_instance_, 1.0);
  projectm_set_beat_sensitivity(projectm_instance_, 0.5);
  projectm_set_soft_cut_duration(projectm_instance_, 10);
  
//projectm_set_window_size(projectm_instance_, 512, 512);
const char *texture_search_paths[] = { "/usr/local/share/projectM/textures" };
projectm_set_texture_search_paths(projectm_instance_, texture_search_paths, 1);
projectm_playlist_instance_ = projectm_playlist_create(projectm_instance_);
projectm_playlist_set_shuffle(projectm_playlist_instance_, false);

"patched" SetImmediatePreset (FOR TESTING, but works on v3)
void ProjectMVisualization::SetImmediatePreset(const int index) {

#ifdef HAVE_PROJECTM4
  if (projectm_playlist_instance_) {
    projectm_playlist_set_position(projectm_playlist_instance_, index, true);
 /* projectm_playlist_play_previous(projectm_playlist_instance_, true);
    projectm_set_preset_duration(projectm_instance_, 1);
    projectm_set_preset_locked(projectm_instance_, false);

     QTimer::singleShot(1500, this, [index,this]() {
      projectm_set_preset_locked(projectm_instance_, true);
      projectm_set_preset_duration(projectm_instance_, duration_);
    }); */
  }
#else
  if (projectm_) {
    projectm_->selectPreset(index, true);
  }
#endif  // HAVE_PROJECTM4

}

It appears that when it wraps to a QWidget, the shader codes don't like and start to stop doing its things. ITs not a matter of context, but its like a corruption happens when you wrap.
I could manage to bind key_S to open the visualizations selector, when setting as central, because I could not figure out why the overlay and interface don't work

the FPS on the video is very low due: * frameskipping at the desktop recorder

• fps was set t o 10 fps for testing purposes on projectm instance

LoooooooooooooooooooooooooooL!

PS: still, several GL error 1282 in the majority of presets tested.

while some actually works

A few days ago I installed those newest cream-of-cream set of plugins. Awesome by the way.
They seem all working fine. I spent days testing on random mode. I still want to compare one by one with the SDL app.

Now, problem is, when I go to the select visualization window, all the previews gets bugged if The first ones of the list (mostly called "Fast transition..") are selected. They don't seem to be normal plugins. Anyway, Thing is, if you select for preview any of these first ones, it bugs all other subsequent plugins. glClear color is not black anymore and they render all dumb. Only restarting Strawberry for normal behaviour again.
Gonna look at the milkdrop code, later, to compare the "normal" ones from these "Fast transition..." ones.

glShadeModel(GL_SMOOTH);
glClearColor(0, 0, 0, 0);
glViewport(0, 0, width(), height());
glMatrixMode(GL_TEXTURE);
glLoadIdentity();
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glDrawBuffer(GL_BACK);
glReadBuffer(GL_BACK);
glEnable(GL_BLEND);

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_LINE_SMOOTH);
glEnable(GL_POINT_SMOOTH);
glClearColor(0.0F, 0.0F, 0.0F, 0.0F);
glLineStipple(2, 0xAAAA);

ALSO, you need to run the commands always before renderFrame, at drawBackground method.

https://lwn.net/Articles/750152/

if (projectm_) {
  projectm_->setShuffleEnabled(false);
  projectm_->selectPreset(index, true);
  projectm_->changePresetDuration(1);
  projectm_->setPresetLock(false);
  projectm_->selectPrevious(index);

  QTimer::singleShot(1250, this, [index,this]() {
    projectm_->setPresetLock(true);
    projectm_->changePresetDuration(duration_);
    projectm_->setShuffleEnabled(true);
  });
}

Here's an improved version 0.1b to prevent altering shuffle globally in the settings. But don't forget to set smoothPresetDuration = 0 in the settings, this is required.

B I N G O
MilkDrop was heavily Windows-based, implemented with DirectX, Win32 APIs, and assembler. ProjectM did a good job of replicating the functionality in a cross-platform manner but one DirectX-specific piece remains: the shader code in the preset files. Some presets can contain GPU shader programs as mentioned previously. Because they were written for MilkDrop, they are in HLSL, a shader language for DirectX. Support for HLSL was provided in projectM by NVIDIA's Cg toolkit, but that has long been deprecated and is unsupported. Either manual or automatic conversion (possibly using something along the lines of HLSL2GLSL for Unity) needs to be added along with code to compile and upload the shaders. This would greatly increase performance and capabilities, enable the most advanced presets, and drop the dependency on an out-of-date and unsupported proprietary framework.

I saw on some presets some kind of shader language. The ones that bug!

The channels I tought it wouldnt be an issue, but thinking about it, it is.

###GstMapInfo

A structure containing the result of a map operation such as Memory.map. It contains the data and size.

struct GstMapInfo {
GstMemory* memory;
GstMapFlags flags;
ubyte* data;
size_t size;
size_t maxsize;
void*[4] userData;
void*[4] GstReserved;
}

its not the buffer, its the SIZE of the buffer, thats why its not 16 bit like the buffer itself!
Maybe thats why

• we got ConsumeBuffer consuming
• we got drawBackground drawing
• we got playlist preview selector selecting and previewing
• we aint got no segfault, mon!

YES WE HAVE PROJECTM v3 (and v2 maybe) working

void ProjectMVisualization::SetImmediatePreset(const int index) {

#ifdef HAVE_PROJECTM4
  if (projectm_playlist_instance_) {
    projectm_playlist_set_position(projectm_playlist_instance_, index, true);
  }
#else
  if (projectm_) {
    projectm_->selectPreset(index, true);
    projectm_->changePresetDuration(1);
    projectm_->setPresetLock(false);
    projectm_->selectPrevious(index);
  
    QTimer::singleShot(1500, this, [index,this]() {
      projectm_->setPresetLock(true);
      projectm_->changePresetDuration(duration_);
    }); 
     
  }
#endif  // HAVE_PROJECTM4

}

So I managed to do this ugly hack, that sets the previous preset, unlock, lets projectM do the transition, then "quickly" locks again after the timer

Read my previous posts, it was quite a journey. Thats the only thing missing I believe, to work with v3.

v2 is used on ubuntu and has some issues, i posted previously about that too.

The -1 would segfault, so I force the LAST preset to begin the first, when index = 0 is selected.

if ( index <= 0 )
  projectm_->selectPresetPosition(projectm_->getPlaylistSize());
else
  projectm_->selectPresetPosition(index-1);

This looks wrong, since the index starts with zero, getPlaylistSize will be too high, needs to do - 1. Also, why are you using index-1 when the index is set? Doesn't IndexOfPreset return the correct index?

  s.smoothPresetDuration = 0;
  s.presetDuration = duration_;
  s.shuffleEnabled = false;

and my brand new ugly disgusting hack that now makes the preview work for the first element:

void ProjectMVisualization::SetImmediatePreset(const int index) {

#ifdef HAVE_PROJECTM4
  if (projectm_playlist_instance_) {
    projectm_playlist_set_position(projectm_playlist_instance_, index, true);
  }
#else
  if (projectm_) {
    Lock(false);
    projectm_->changePresetDuration(1);
    if ( index <= 0 )
      projectm_->selectPresetPosition(projectm_->getPlaylistSize());
    else
      projectm_->selectPresetPosition(index-1);
    // Create a QTimer to call Lock(true) after a delay
        QTimer::singleShot(500, this, [this]() {
            Lock(true);
            projectm_->changePresetDuration(duration_);
        });
  }
#endif  // HAVE_PROJECTM4

}

I believe we have a working v3 integration with projectM! I executed the program for more than ten hours without segfaults or bugs.

void ProjectMVisualization::SetImmediatePreset(const int index) {

#ifdef HAVE_PROJECTM4
  if (projectm_playlist_instance_) {
    projectm_playlist_set_position(projectm_playlist_instance_, index, true);
  }
#else
  if (projectm_) {
    if ( index <= 0 )
      return;
    Lock(false);
    projectm_->changePresetDuration(1);
    projectm_->selectPresetPosition(index-1);
    // Create a QTimer to call Lock(true) after a delay
        QTimer::singleShot(500, this, [this]() {
            Lock(true);
            projectm_->changePresetDuration(duration_);
        });
  }
#endif  // HAVE_PROJECTM4

}

pretty ugly, but its the proof of concept about what I've posted before. It works. Every single plugin works in the preview now, except the first one (yuck)

PS: the idea here is, instead of setting the preset directly, making projectM make a transition as if it was playing as a playlist queue of the main window. This way, it does not brake the preview, since they all work when projectM manages the transition.