Use glBlendFunc in QOpenGLWidget

Question

I'm trying to use glBlendFunc in QOpenGLWidget (in paintGL), but objects do not mix (alpha is works).

My code:

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glEnable(GL_DEPTH_TEST);
glEnable(GL_CULL_FACE);
glEnable(GL_BLEND);

glBlendFunc(blenFunc, GL_ONE);

m_world.setToIdentity();
m_world.rotate((m_xRot / 16.0f), 1, 0, 0);
m_world.rotate(m_yRot / 16.0f, 0, 1, 0);
m_world.rotate(m_zRot / 16.0f, 0, 0, 1);

QOpenGLVertexArrayObject::Binder vaoBinder(&m_vao);
m_program->bind();
m_tex->bind();
fillYoffsetLightning();

const GLfloat scaleFactor = 0.05f;
m_world.scale(scaleFactor, scaleFactor, 0.0f);
m_world.translate(0.f, 0.0f, 0.0f);

const GLfloat fact = 1 / scaleFactor;
const uint8_t X = 0, Y = 1;

for(int i = 0; i < maxElem; ++i) {
    const GLfloat offX = m_ELECT[i][X] * fact;
    const GLfloat offY = m_ELECT[i][Y] * fact;
    m_world.translate(offX, offY);

    m_program->setUniformValue(m_projMatrixLoc, m_proj);
    m_program->setUniformValue(m_mvMatrixLoc, m_camera * m_world);
    QMatrix3x3 normalMatrix = m_world.normalMatrix();
    m_program->setUniformValue(m_normalMatrixLoc, normalMatrix);

glDrawArrays(GL_TRIANGLE_FAN, 0, m_logo.vertexCount());

    update();
    m_world.translate(-offX, -offY);
}

m_program->release();

shaders are simple:

// vertex
"attribute highp vec4 color;\n"
"varying highp vec4 colorVertex;\n"
//......... main:
"colorVertex = color;\n"

// fragment
"varying highp vec4 colorVertex;\n"
//......... main:
"gl_FragColor = colorVertex;\n"

Color is: a pentagon with a gradient from white from center to blue to the edges is drawn (center color is 1,1,1, edges is 0,0,0.5)

screenshoot

Why is this happening?

You have to disable the depth test: `glDisable(GL_DEPTH_TEST);` Use a blend function like this: `glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)` — Rabbid76, Aug 28 '18 at 16:03

score 0 · Accepted Answer · answered Aug 29 '18 at 06:32

If you want to achieve a blending effect, the you have to disable the depth test:

glDisable(GL_DEPTH_TEST);

Note, the default depth test function is GL_LESS. If a fragment is draw on a place of a previous fragment, then it is discarded by the depth test, because this condition is not full filled.
If the depth test is disabled, then the fragments are "blended" by the blending function (glBlendFunc) and equation (glBlendEquation).

I recommend to use the following blending function:

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

iforce2d · Answer 2 · 2022-02-05T16:50:33.203

In my case (Qt 5.15.2) I found that using a color call with no alpha component (eg. glColor3f(1,0,0) ) causes the blending to be disabled for any subsequent rendering. To my surprise I could not even recover it by re-issuing these commands:

glEnable(GL_BLEND); // wtf has no effect
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

Blending simply remained disabled until the next paint begins. This did not happen with the original QGLWidget class. It only happens with QOpenGLWidget and only on Windows (Mac + Linux are fine).

The good-enough solution for me was to replace any non-alpha color calls with alpha equivalents, at least for cases where you need to use blending later in the render. Eg.

glColor3f(1,0,0);    // before
glColor4f(1,0,0,1);  // after

Another issue that might come up is if you use QPainter along with direct rendering, because the QPainter will trash your OpenGL state. See the mention of 'beginNativePainting' in the docs:

https://doc.qt.io/qt-5/qopenglwidget.html#painting-techniques

EDIT: I'll add this here because my comment on Rabbid's answer was deleted for some reason - the depth test does NOT need to be disabled to use blending. Rabbid might be thinking of disabling depth buffer writes which is sometimes done to allow drawing all translucent objects without having to sort them in order of furthest to nearest:
Why we disable Z-write in blending

Use glBlendFunc in QOpenGLWidget

2 Answers2