[Interest] Qt 6.7: How to set the blend equation when doing blend on updateGraphicsPipelineState?

[Nuno Santos]

Hi,

I’m porting an app from Qt 5.15 to Qt 6.7 but I’m stumbling on the fact that there is no equivalent to blend equation in OpenGL.

Note: At the time being, I’m still using OpenGL as graphics backend to minimise the already huge ongoing port.


To achieve blending mode between nodes, I was using a mix between glBlendFunc and glBlendEquation:

    case Normal:
        QOpenGLContext::currentContext()->functions()->glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
        QOpenGLContext::currentContext()->functions()->glBlendEquation(GL_FUNC_ADD);
        break;
    case Screen:
        QOpenGLContext::currentContext()->functions()->glBlendFunc(GL_SRC_COLOR, GL_ONE);
        QOpenGLContext::currentContext()->functions()->glBlendEquation(GL_FUNC_ADD);
        break;
    case Overlay:
        QOpenGLContext::currentContext()->functions()->glBlendFunc(GL_SRC_COLOR, GL_ONE);
        QOpenGLContext::currentContext()->functions()->glBlendEquation(GL_MAX);
        break;
    case Difference:
        QOpenGLContext::currentContext()->functions()->glBlendFunc(GL_SRC_COLOR, GL_ONE);
        QOpenGLContext::currentContext()->functions()->glBlendEquation(GL_FUNC_REVERSE_SUBTRACT);
        break;

But in the newest version of QSGMaterialShader class documentation it says:

"The shader pipeline state changes are less often used. One use case is materials that wish to use a specific blend mode. The relevant function is updateGraphicsPipelineState(). This function is not called unless the QSGMaterialShader has opted in by setting the flag UpdatesGraphicsPipelineState. The task of the function is to update the GraphicsPipelineState struct instance that is passed to it with the desired changes. Currently only blending and culling-related features are available, other states cannot be controlled by materials.”

So I’m now trying to use 

bool QSGMaterialShader::updateGraphicsPipelineState(QSGMaterialShader::RenderState &state, QSGMaterialShader::GraphicsPipelineState *ps, QSGMaterial *newMaterial, QSGMaterial *oldMaterial)

case Normal:
    ps->srcColor = QSGMaterialShader::GraphicsPipelineState::One;
    ps->dstColor = QSGMaterialShader::GraphicsPipelineState::OneMinusSrcAlpha;
    break;
case Screen:
    ps->srcColor = QSGMaterialShader::GraphicsPipelineState::SrcColor;
    ps->dstColor = QSGMaterialShader::GraphicsPipelineState::One;
    break;
case Overlay:
    ps->srcColor = QSGMaterialShader::GraphicsPipelineState::SrcColor;
    ps->dstColor = QSGMaterialShader::GraphicsPipelineState::One;
    break;
case Difference:
    ps->srcColor = QSGMaterialShader::GraphicsPipelineState::SrcColor;
    ps->dstColor = QSGMaterialShader::GraphicsPipelineState::One;
    break;


When the blendEquation GL_FUNC_ADD was used, it seems to be working.

But when I get to modes like Overlay and Difference, where the SRC and DST are the same, without a blend equation, I don’t know how to do it.

Is there support to this use case?

Thank you in advance!

Best regards,

Nuno
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