[Qt3D] Using a SSBO in Qt3D

FredFred

Hello,

I can't get a SSBO working using Qt3D. I'm also unable to find a single example displaying how it is supposed to be done.
Here are the main parts of my code:

Buffer init:

    QByteArray ssboData;
    ssboData.resize(1000);
    ssboData.fill(0);

    mySSBOBuffer = new Qt3DRender::QBuffer(this);
    mySSBOBuffer->setUsage(Qt3DRender::QBuffer::DynamicRead);
    mySSBOBuffer->setAccessType(Qt3DRender::QBuffer::AccessType::ReadWrite);
    mySSBOBuffer->setData(ssboData);

    QByteArray atomicCounterData;
    atomicCounterData.resize(4);
    atomicCounterData.fill(0);

    myAtomicCounterBuffer = new Qt3DRender::QBuffer(this);
    myAtomicCounterBuffer->setUsage(Qt3DRender::QBuffer::DynamicRead);
    myAtomicCounterBuffer->setAccessType(Qt3DRender::QBuffer::AccessType::ReadWrite);
    myAtomicCounterBuffer->setData(atomicCounterData);

Passing the buffers as QParameters to the shader.

    myMaterial->addParameter(new Qt3DRender::QParameter("acCountFrags", QVariant::fromValue(myAtomicCounterBuffer->id()), myMaterial));
    myMaterial->addParameter(new Qt3DRender::QParameter("ssboBuffer", QVariant::fromValue(mySSBOBuffer->id()), myMaterial));

I also tried

    myMaterial->addParameter(new Qt3DRender::QParameter("acCountFrags", QVariant::fromValue(myAtomicCounterBuffer), myMaterial));
    myMaterial->addParameter(new Qt3DRender::QParameter("ssboBuffer", QVariant::fromValue(mySSBOBuffer), myMaterial));

Fragment Shader (color has no use, just to check shader is working):

#version 430 core

layout(binding = 0, offset = 0) uniform atomic_uint acCountFrags;

layout (std430) buffer ssboBuffer
{
    uint fragIds[];
};

out vec4 color;

void main()
{
    uint index = atomicCounterIncrement(acCountFrags);
    fragIds[index] = 5;

    color = vec4(0.2, 0.2, 0.2, 1.0);
}

In all of my tries, nothing is written to the buffers after rendering. They are still full of 0 like after init.
Does anybody know if i'm doing something wrong ? Or somewhere I could find a working example ?

Thank you.

Oeystein

Hi,

I just got SSBO working with Qt3D. This might be too late for you, but it might help somebody else. Change you shader to something like:

#version 430 core

layout(binding = 0, offset = 0) uniform atomic_uint acCountFrags;

layout (std430) buffer ssboBuffer
{
    uint fragIds[];
} myssbo;

out vec4 color;

void main()
{
    uint index = atomicCounterIncrement(acCountFrags);
    myssbo.fragIds[index] = 5;

    color = vec4(0.2, 0.2, 0.2, 1.0);
}

I have had success passing data from compute shaders into vertex and fragment shaders.

Best regards,
Øystein