How could I move image smoothly within QGLWidget window? - image

Question

I am writing a small program to simulate animation. I use QGLWidget to display a dynamic image to mimic picture animation. The basic idea is to use texture and update it through QGLFamebufferOject::blitFramebuffer. Each time, the image array will be changed and then frame buffer will be changed.
The code can run and the image can be moved forward, but image has chopping problem, or flicking, not smooth.
Following is the code for a class:
GLWidget::GLWidget(QWidget *parent) : QGLWidget(QGLFormat(), parent)
{
makeCurrent();
if (QGLFramebufferObject::hasOpenGLFramebufferBlit()) {
QGLFramebufferObjectFormat format;
render_fbo = new QGLFramebufferObject(640, 480, format);
texture_fbo = new QGLFramebufferObject(640, 480);
} else {
render_fbo = new QGLFramebufferObject(640, 480);
texture_fbo = render_fbo;
}
tile_list = glGenLists(1);
glNewList(tile_list, GL_COMPILE);
glBegin(GL_QUADS);
{
glTexCoord2f(0.0f, 0.0f); glVertex3f(-1.0f, -1.0f, 1.0f);
glTexCoord2f(1.0f, 0.0f); glVertex3f( 1.0f, -1.0f, 1.0f);
glTexCoord2f(1.0f, 1.0f); glVertex3f( 1.0f, 1.0f, 1.0f);
glTexCoord2f(0.0f, 1.0f); glVertex3f(-1.0f, 1.0f, 1.0f);
}
glEnd();
glEndList();
backbufferImg = new unsigned char[640 * 480 * sizeof(unsigned char) * 3];
}
GLWidget::~GLWidget()
{
}
void GLWidget::initializeGL()
{
}
void GLWidget::resizeGL(int width, int height)
{
}
void GLWidget::paintGL()
{
}
void GLWidget::saveGLState()
{
glPushAttrib(GL_ALL_ATTRIB_BITS);
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
}
void GLWidget::restoreGLState()
{
glMatrixMode(GL_PROJECTION);
glPopMatrix();
glMatrixMode(GL_MODELVIEW);
glPopMatrix();
glPopAttrib();
}
void GLWidget::changeTextureImage(int nextColumn, int direction)
{
// code about chaning backbufferImg
// ...
// call draw
draw();
}
void GLWidget::draw()
{
makeCurrent();
QPainter p(this); // used for text overlay
// save the GL state set for QPainter
saveGLState();
QPainter fbo_painter(render_fbo);
QImage renderImg(backbufferImg, 640, 480, QImage::Format_RGB888);
fbo_painter.begin(render_fbo);
QRect rect(640, 480);
fbo_painter.drawImage(rect, renderImg, rect);
fbo_painter.end();
if (render_fbo != texture_fbo) {
QGLFramebufferObject::blitFramebuffer(texture_fbo, rect, render_fbo, rect);
} else {
// qDebug() << "render_fbo == texture_fbo";
}
// draw into the GL widget
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glViewport(0, 0, width(), height());
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glBindTexture(GL_TEXTURE_2D, texture_fbo->texture());
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glEnable(GL_TEXTURE_2D);
glPushMatrix();
glScalef(1.0f, 1.0f, 1.0f);
glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
glCallList(tile_list);
glPopMatrix();
// restore the GL state that QPainter expects
restoreGLState();
glFinish();
}

Answer 1

Qt's QGLWidget expects that you perform all drawing operations from its overloaded paintGL function so that it iteracts nicely with Qt's double buffer management.
Also why are you taking that detour over a FBO to update a texture? This is highly inefficient and introduces a lot of synchronization points. Use a Pixel Buffer Object and update the texture with glTexSubImage2D.

Answer 2

Thank you very much! I have changed the code and put the drawing code inside paintGL and didn't use FBO's blit method. MUch better, but still need to improve the code. The following is the code:
GLWidget::GLWidget(QWidget *parent) :
QGLWidget(QGLFormat(), parent)
{
makeCurrent();
fbo = new QGLFramebufferObject(WIDTH, HEIGHT);
tile_list = glGenLists(1);
glNewList(tile_list, GL_COMPILE);
glBegin(GL_QUADS);
{
glTexCoord2f(0.0f, 0.0f); glVertex3f(-1.0f, -1.0f, 1.0f);
glTexCoord2f(1.0f, 0.0f); glVertex3f( 1.0f, -1.0f, 1.0f);
glTexCoord2f(1.0f, 1.0f); glVertex3f( 1.0f, 1.0f, 1.0f);
glTexCoord2f(0.0f, 1.0f); glVertex3f(-1.0f, 1.0f, 1.0f);
}
glEnd();
glEndList();
backbufferImg = new unsigned char[WIDTH * HEIGHT * sizeof(unsigned char) * 3];
}
GLWidget::~GLWidget()
{
glDeleteLists(tile_list, 1);
delete fbo;
delete backbufferImg;
}
void GLWidget::initializeGL()
{
}
void GLWidget::resizeGL(int width, int height)
{
}
void GLWidget::paintGL()
{
glViewport(0, 0, fbo->size().width(), fbo->size().height());
glMatrixMode(GL_MODELVIEW);
// render to the framebuffer object
fbo->bind();
glBindTexture(GL_TEXTURE_2D, refreshTexture);
glCallList(tile_list);
fbo->release();
//////////////////////////////////////////////////////////////////////
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glViewport(0, 0, width(), height());
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glBindTexture(GL_TEXTURE_2D, fbo->texture());
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glEnable(GL_TEXTURE_2D);
glPushMatrix();
glScalef(1.0f, 1.0f, 1.0f);
glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
glCallList(tile_list);
glPopMatrix();
// restore the GL state that QPainter expects
restoreGLState();
glFinish();
}
void GLWidget::saveGLState()
{
glPushAttrib(GL_ALL_ATTRIB_BITS);
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
}
void GLWidget::restoreGLState()
{
glMatrixMode(GL_PROJECTION);
glPopMatrix();
glMatrixMode(GL_MODELVIEW);
glPopMatrix();
glPopAttrib();
}
void GLWidget::refreshTextureImage(int nextColumn, int direction)
{
// upadte backbufferImg
// add code...
QImage renderImg(backbufferImg, WIDTH, HEIGHT, QImage::Format_RGB888);
refreshTexture = bindTexture(renderImg);
updateGL();
}
I will try the method you suggested. But I think framebuffer and pixel buffer in the same level. It means there is no much performance difference. For glTexSubImage2D, I am not quite sure, I need to check it.
Thank you so much.