OpenGL image viewer, need some help on implementation

[minimoog]

First thing.

ResizeGL method looks suspicouis, viewport depends of the image width and height, and doesn't make sense. Just use:

void GLWidget::resizeGL(int width, int height) {
          glMatrixMode(GL_PROJECTION);
          glLoadIdentity();
          gluOrtho2D(0, width, 0, height);
          glViewport(0, 0, width, height);
 };

void GLWidget::resizeGL(int width, int height) {
          glMatrixMode(GL_PROJECTION);
          glLoadIdentity();
          gluOrtho2D(0, width, 0, height);
          glViewport(0, 0, width, height);
 };

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This way you get texture to pixel exact mapping. So if vertex coordinate is x, y it's (x, y) coordinate on the window (be careful y axis is inverted).

For zooming you can scale, or change vertex coordinates (it's the same).

But I will think about the part scaling staying the same and in the middle.

What do you mean with "get the image coordinates from the mouseMove function so that I can display the raw color values of the image"?

BTW, this is mostly OpenGL related.

[minimoog]

Try this

void GLWidget::resizeGL(int width, int height)
{
	glViewport(0, 0, width, height);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluOrtho2D(0.0, width, 0.0, height);
}

void GLWidget::resizeGL(int width, int height)
{
	glViewport(0, 0, width, height);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluOrtho2D(0.0, width, 0.0, height);
}

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void GLWidget::paintGL()
{
  ................
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
	glTranslatef(width() / 2.0f, height() / 2.0f,  0.0f);
	glScalef(scaleFactor, scaleFactor, 0.0f);
	glTranslatef(-imageWidth / 2.0f, -imageHeight / 2.0f, 0.0f);
 
	glBegin(GL_QUADS);
			glTexCoord2f(0.0f, 0.0f);  glVertex2f(0.0f, 0.0f);
			glTexCoord2f(1.0f, 0.0f);  glVertex2f(imageWidth, 0.0f);
			glTexCoord2f(1.0f, 1.0f);  glVertex2f(imageWidth, imageHeight);
			glTexCoord2f(0.0f, 1.0f);  glVertex2f(0.0f, imageHeight);
	glEnd();
}

void GLWidget::paintGL()
{
  ................
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
	glTranslatef(width() / 2.0f, height() / 2.0f,  0.0f);
	glScalef(scaleFactor, scaleFactor, 0.0f);
	glTranslatef(-imageWidth / 2.0f, -imageHeight / 2.0f, 0.0f);
		
	glBegin(GL_QUADS);
			glTexCoord2f(0.0f, 0.0f);  glVertex2f(0.0f, 0.0f);
			glTexCoord2f(1.0f, 0.0f);  glVertex2f(imageWidth, 0.0f);
			glTexCoord2f(1.0f, 1.0f);  glVertex2f(imageWidth, imageHeight);
			glTexCoord2f(0.0f, 1.0f);  glVertex2f(0.0f, imageHeight);
	glEnd();
}

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You must know the basics of matrix and coordinate transformations if you want to understand above code.

[Randulf]

Ah, yes!!
This is working perfectly. Thanks a lot!!!

What do you mean with "get the image coordinates from the mouseMove function so that I can display the raw color values of the image"?

Im going to have a widget with information about wich pixel the mouse pointer is over and what color values it has. Since I only get the window coordinates I have to translate them to image space. What I ment with raw valus is that I have two arrays with pixel valus, one with 32 bit HDR data and another with clamped values. Anyway that getting the valus from the array wont be a problem it just the translation that i'm unshure of.

Thanks again for the help.

[Randulf]

Hi!
Now I got everything working except from the translation from window coordinates to image.
This is how i do it now:

glMatrixMode( GL_MODELVIEW );
glLoadIdentity();
glTranslatef(this->size().width() / 2.0f, this->size().height() / 2.0f, 0.0f);
glTranslatef(imageOffset.x(), -imageOffset.y(), 0.0f);
glScalef(zoomFactor, zoomFactor, 0.0f);
glTranslatef(-imageWidth / 2.0f, -imageHeight / 2.0f, 0.0f);

Then in the mouse move method:

void GLWidget::mouseMoveEvent(QMouseEvent *event)
{
QPoint pos = event->pos();
QMatrix q,t1,t2,s,t3;
t1.translate(this->size().width() / 2, this->size().height() / 2);
t2.translate(imageOffset.x(), imageOffset.y());
s.scale(zoomFactor, zoomFactor);
t3.translate(- imageWidth / 2, - imageHeight / 2);
q = t1*t2*s*t3;
QMatrix qinv = q.inverted();
QPoint im = qinv.map(pos);
qDebug() << im;
}

This works fine when as long as I dont scale the image. I can move around the image in the window and it translates the coordinates.
This is is supposed to be simple.

[Randulf]

Seems like I multiplied the matrices in the wrong order.
Doing it the opposite way solved the problem.
q = t3*s*t2*t1;

[minimoog]

Well you have something like this transformations:

M = T1 * T2 * S * T3

You transform vertex with above transformations. Because projection is simple, with given window coordinate you want to know that position in object (untransformed) space, you need to untransform window coordinate to object space...

Vwindow = M * V

V = inverse(M) * Vwindow

inverse(T1 * T2 * S * T3) = inverse(T3) * inverse(T2) * inverse(S) * inverse(T1)

Because inverse of translation is just changing the sign, and inverse of scale is reciprocal

inverse(M) = (-T3) * (-T2) * (1 / S) * (-T1)

With this matrix you can transform window coordinate to object space (the image).

It's easy to build this matrix by yourself.