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

[Nuno Santos]

Lazslo,

Thank you very much for your reply.

I was really hopeful that there was some kind of workaround for this limitation. This renders impossible to port an application from Qt 5 to Qt 6 at least as far as my knowledge goes.

Is there any possible workaround for this limitation? Can I do it if I edit the source code in any way? Can you please advise? 

With the current architecture the application relies heavily on this to make blending between layers. 

Thank you in advance!

Best regards,

Nuno

> On 8 May 2024, at 16:42, Laszlo Agocs <laszlo.agocs at qt.io> wrote:
> 
> Hi,
> 
> As you found, QSGMaterialShader::GraphicsPipelineState allows specifying the blend factors, and from Qt 6.5 on it also supports specifying separate RGB and alpha factors.
> 
> However, the blend equation is always ADD. There is no way to change that currently.
> 
> Best regards,
> Laszlo
> 
> From: Interest <interest-bounces at qt-project.org> on behalf of Nuno Santos via Interest <interest at qt-project.org>
> Sent: Monday, May 6, 2024 7:21 PM
> To: interestqt-project. org <interest at qt-project.org>
> Subject: [Interest] Qt 6.7: How to set the blend equation when doing blend on updateGraphicsPipelineState?
>  
> 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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