View Full Version : Change zoom from dolly to "real" zoom

Rainer Wahnsinn
02-01-2013, 02:42 AM
I'm trying to change the zoom effect in my application to be based on a change of FoV instead of just a z-translation which is called dolly. User input should be the basis so my current code looks as follows:

void setupMatrices()
// Set ProjectionMatrix
projectionMatrix = glm::perspective(90.0f, (GLfloat)width / (GLfloat) height, 0.1f, 1000.f);
glBindBuffer(GL_UNIFORM_BUFFER, globalMatricesUBO);
glBufferSubData(GL_UNIFORM_BUFFER, 0, sizeof(glm::mat4), glm::value_ptr(projectionMatrix));
glBindBuffer(GL_UNIFORM_BUFFER, 0);

// Set ModelViewMatrix
glm::mat4 identity = glm::mat4(1.0); // Start with the identity as the transformation matrix
glm::mat4 pointTranslateZ = glm::translate(identity, glm::vec3(0.0f, 0.0f, -translate_z)); // Zoom in or out by translating in z-direction based on user input
glm::mat4 viewRotateX = glm::rotate(pointTranslateZ, rotate_x, glm::vec3(1.0f, 0.0f, 0.0f)); // Rotate the whole szene in x-direction based on user input
glm::mat4 viewRotateY = glm::rotate(viewRotateX, rotate_y, glm::vec3(0.0f, 1.0f, 0.0f)); // Rotate the whole szene in y-direction based on user input
glm::mat4 pointRotateX = glm::rotate(viewRotateY, -90.0f, glm::vec3(1.0f, 0.0f, 0.0f)); // Rotate the camera by 90 degrees in negative x-direction to get a frontal look on the szene
glm::mat4 viewTranslate = glm::translate(pointRotateX, glm::vec3(-dimensionX/2.0f, -dimensionY/2.0f, -dimensionZ/2.0f)); // Translate the origin to be the center of the cube
modelViewMatrix = viewTranslate;
glBindBuffer(GL_UNIFORM_BUFFER, globalMatricesUBO);
glBufferSubData(GL_UNIFORM_BUFFER, sizeof(glm::mat4), sizeof(glm::mat4), glm::value_ptr(modelViewMatrix));
glBindBuffer(GL_UNIFORM_BUFFER, 0);

I've tried a lot of things now but couldn't find out which of the code I need to change in order to achieve real zooming. I followed the instructions of a number of tutorials and looked at different example code snippets, but it just didn't worked out. Since I just started learning OpenGL, I hope that someone can tell me which instructions I need to change and how.
rotate_x, rotate_y and translate_z all come from mouse input

02-01-2013, 08:37 AM
Why don't you just change FOV parameter in a projection?

Rainer Wahnsinn
02-01-2013, 03:44 PM
Yeah, that's what I tried, but I couldn't figure out how. Additionally, I probably have to change the modelview matrix as well, since the z translation doesn't make sense anymore. Could you give me a hint or even some sample code on how to change those things exactly? I do understand that I'll need glm::lookat / perspective, but I'm not sure about the parameters if user input is involved.

02-01-2013, 04:48 PM
Have a read of this http://www.terathon.com/gdc07_lengyel.pdf

in particular

e = focal length = 1 / tan(FOV / 2)

As I understand with a telephoto lens what you are changing is the focal length.