Phong-Alpha material transparency

Question

I'm using a pre-built material of Qt3D:

Qt3DRender::QMaterial *MyClass::createMaterial()
{
    Qt3DExtras::QPhongAlphaMaterial *mat = new Qt3DExtras::QPhongAlphaMaterial();
    mat->setAmbient(QColor("#576675"));
    mat->setDiffuse(QColor("#5F6E7D"));
    mat->setSpecular(QColor("#61707F"));
    mat->setShininess(0.0f);
    mat->setAlpha(0.5f);
    return mat;
}

I set alpha to 0.5f, so I expect the material be semi-transparent. But the model looks mostly white except some regions:

When I check the source code I see this settings for alpha-blending:

m_blendState->setSourceRgb(QBlendEquationArguments::SourceAlpha);
m_blendState->setDestinationRgb(QBlendEquationArguments::OneMinusSourceAlpha);
m_blendEquation->setBlendFunction(QBlendEquation::Add);

I wonder why the model looks white?

---

As suggested by @Macke, the object on black background looks fine!

When I set alpha to 1.0, I observe this:

---

UPDATE

As pointed out by @Macke, one issue was related to depth test. On the source code, depth mask is disabled by default:

// ...
, m_noDepthMask(new QNoDepthMask())
// ...

m_phongAlphaGL3RenderPass->addRenderState(m_noDepthMask);

m_phongAlphaGL2RenderPass->addRenderState(m_noDepthMask);

m_phongAlphaES2RenderPass->addRenderState(m_noDepthMask);

I enabled depth mask by removing QNoDepthMask stuff, and now with alpha = 1.0 the rendering result is fine:

---

UPDATE

Suggested by @EddyAlleman, I added such lines of code:

blendState->setSourceAlpha(Qt3DRender::QBlendEquationArguments::Zero);
blendState->setDestinationAlpha(Qt3DRender::QBlendEquationArguments::One);

Then, the transparency (alpha = 0.4) is fine even on gray background:

Answer 1

try this to set the blendequation state

set sourceAlphaArg to Qt3DRender::QBlendEquationArguments::Zero
set destinationAlphaArg to Qt3DRender::QBlendEquationArguments::One

info from enum QBlendEquationArguments::Blending

Constant Value OpenGL Qt3DRender::QBlendEquationArguments::Zero 0 GL_ZERO Qt3DRender::QBlendEquationArguments::One 1 GL_ONE

EDIT: a good explanation can be found here: learnopengl.com/Advanced-OpenGL/Blending

Answer 2

Try not using blendequation, seems like the color computed by blendstate is added to the gray background. (maybe good for got particle sparks, less so for objects)

How does it look with black background?