Open gl draw turns white after one frame drawn - c++

Question

I am newbie in Opengl. I used the following example from Qt as a start in Opengl as I know Qt.
http://doc.qt.io/qt-5/qtquick-scenegraph-openglunderqml-squircle-cpp.html
I replaced the paint function of the program with following code with an intent to draw chess board pattern. Following is not the paint or render function of my program
paint()
{
if (!m_program) {
initializeOpenGLFunctions();
m_program = new QOpenGLShaderProgram();
m_program->addShaderFromSourceCode(QOpenGLShader::Vertex,
"attribute highp vec4 vertices;"
"varying highp vec2 coords;"
"void main() {"
" gl_Position = vertices;"
" coords = vertices.xy;"
"}");
m_program->addShaderFromSourceCode(QOpenGLShader::Fragment,
"uniform lowp float t;"
"varying highp vec2 coords;"
"void main() {"
" lowp float i = 1. - (pow(abs(coords.x), 4.) + pow(abs(coords.y), 4.));"
" i = smoothstep(t - 0.8, t + 0.8, i);"
" i = floor(i * 20.) / 20.;"
" gl_FragColor = vec4(coords * .5 + .5, i, i);"
"}");
m_program->bindAttributeLocation("vertices", 0);
m_program->link();
}
auto width = static_cast<float>(m_viewportSize.width());
auto height = static_cast<float>(m_viewportSize.height());
auto a = 2.f / width;
auto b = 2.f / height;
std::vector<float> matrix =
{
a , 0, 0, 0,
0, -b, 0, 0,
0, 0, 1, 0,
-1, 1, 0, 1
};
// Set the projection matrix
glMatrixMode(GL_PROJECTION);
glLoadMatrixf(matrix.data());
// Initialize vertices:
std::vector<float> vertices =
{
0, 0,
0, height,
width, height,
width, 0
};
// Initialize colors
std::vector<float> colors =
{
1, 0, 0,
0, 1, 0,
0, 0, 1,
0, 1, 1
};
// Initialize texture virtice
std::vector<float> texCoord =
{
0, 0,
0, 1,
1, 1,
1, 0
};
// Create texture: simple chess board 8x8
auto numRows = 8u;
auto numCols = 8u;
auto character = 172u;
auto remain = 255u - character;
std::vector<unsigned char> texture(numCols * numRows);
for (auto i = 0u; i < texture.size(); ++i)
texture[i] = ((i + (i / numCols)) % 2) * remain + character;
// Upload to GPU texture
unsigned textureID;
glGenTextures(1, &textureID);
glBindTexture(GL_TEXTURE_2D, textureID);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexImage2D(GL_TEXTURE_2D, 0, GL_LUMINANCE, numCols, numRows, 0, GL_LUMINANCE, GL_UNSIGNED_BYTE, texture.data());
// Initialize clear colors
glClearColor(0.f, 0.f, 0.f, 1.f);
// Activate necessary states
glEnable(GL_TEXTURE_2D);
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_COLOR_ARRAY);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
glVertexPointer(2, GL_FLOAT, 0, vertices.data());
glColorPointer(3, GL_FLOAT, 0, colors.data());
glTexCoordPointer(2, GL_FLOAT, 0, texCoord.data());
// render
glClear(GL_COLOR_BUFFER_BIT);
glDrawArrays(GL_QUADS, 0, 4);
m_program->disableAttributeArray(0);
m_program->release();
m_window->resetOpenGLState();
}
The chess board is drawn. But its drawn for a split second & then the screen turns fully white. I want to draw the chess board pattern continiously with each frame draw.
Can someone pls point out what might be going wrong ?

Answer 1

At the very top you have:
if (!m_program) {
then you initialize m_program, at the very bottom you have:
m_program->release();
which as Harish in comments points out is equivalent to calling glUseProgram(0);. So in the next iteration of paint your shader is not bound and not available.
According to docs, the reverse of release(); is bind(); so (I am not expert on this class) the solution might be to call QOpenGLShaderProgram::bind() on the next iteration of your paint.

Answer 2

For future reference, it's a good idea to run opengl programs through a graphics debugger like gDEbugger to give you an insight into what's actually happening inside the API.