Why always produce 4 pictures in my qt opengl program? - c++

Question

Here is my code:
modify from qt example: Examples\Qt-5.14.2\quick\scenegraph\openglunderqml
void SquircleRenderer::init()
{
unsigned char* data = (unsigned char*)malloc(1200*4);
for(int i=0;i<600;i++)
{
data[i*4] = 0;
data[i*4+1] = 255;
data[i*4+2] = 0;
data[i*4+3] = 255;
}
for(int i=600;i<1200;i++)
{
data[i*4] = 0;
data[i*4+1] = 0;
data[i*4+2] = 255;
data[i*4+3] = 255;
}
if (!m_program) {
QSGRendererInterface *rif = m_window->rendererInterface();
Q_ASSERT(rif->graphicsApi() == QSGRendererInterface::OpenGL || rif->graphicsApi() == QSGRendererInterface::OpenGLRhi);
initializeOpenGLFunctions();
if (texs[0])
{
glDeleteTextures(1, texs);
}
glGenTextures(1, texs);
glBindTexture(GL_TEXTURE_2D, texs[0]);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 30, 40, 0, GL_RGBA, GL_UNSIGNED_BYTE, data);
m_program = new QOpenGLShaderProgram();
m_program->addCacheableShaderFromSourceCode(QOpenGLShader::Vertex,
"attribute highp vec4 vertices;"
"varying highp vec2 coords;"
"void main() {"
" gl_Position = vertices;"
" coords = vertices.xy;"
"}");
m_program->addCacheableShaderFromSourceCode(QOpenGLShader::Fragment,
"varying highp vec2 coords;"
"uniform sampler2D inputImageTexture;"
"void main() {"
" gl_FragColor = texture2D(inputImageTexture, coords);"
"}");
m_program->bindAttributeLocation("vertices", 0);
m_program->link();
arrUni[0] = m_program->uniformLocation("inputImageTexture");
}
}
//! [4] //! [5]
void SquircleRenderer::paint()
{
// Play nice with the RHI. Not strictly needed when the scenegraph uses
// OpenGL directly.
m_window->beginExternalCommands();
m_program->bind();
m_program->enableAttributeArray(0);
float values[] = {
-1, 1,
1, 1,
-1, -1,
1, -1
};
// This example relies on (deprecated) client-side pointers for the vertex
// input. Therefore, we have to make sure no vertex buffer is bound.
glBindBuffer(GL_ARRAY_BUFFER, 0);
m_program->setAttributeArray(0, GL_FLOAT, values, 2);//values
//m_program->setUniformValue("t", (float) m_t);
qDebug()<<m_viewportSize.width()<<m_viewportSize.height()<<"\n";
glViewport(0, 0, m_viewportSize.width(), m_viewportSize.height());
glDisable(GL_DEPTH_TEST);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texs[0]);
glUniform1i(arrUni[0], 0);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
m_program->disableAttributeArray(0);
m_program->release();
m_window->endExternalCommands();
}
As the picture you can see,it produces 4 same pictures,could you tell me how to produce 1 picture fill the whole window?:
I tried so many methods, but it didn't work, I guess the problem exists in the values array or the glTexImage2D function.

Answer 1

Textures are mapped accross the [0, 1] range, and values outside of that range are modulo-looped back into it, which creates a repeating pattern. Interpreting the texture over the [-1, 1] range leads to what you are seeing since you are mapping exactly twice the UV range in both axises.
There's a few ways to fix this. But my personal preference for a full-framebuffer pass like this is to have the attribute be normalized, and then have it converted to the expected [-1, 1] range for the clip-space coordinate in the vertex shader:
float values[] = {
0.f, 1.f,
1.f, 1.f,
0.f, 0.f,
1.f, 0.f
};
gl_Position = vertices * 2.0 - vec4(1.0, 1.0,0.0,1.0);
Another common technique is to do away with the attribute buffer altogether, and use gl_VertexID to directly generate both the UVs and coordinates.