• Advertisement
Sign in to follow this  

OpenGL The zooming in blues

This topic is 2730 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

Hi there,

I'm building a 2d application (of a graph if anyone's interested!) using OpenGl in C++. In it, users can drag and also zoom. There is a main function that is called everytime there is any action, the relevant parts are:


//Only scale Y with timeline (not X):
glScaled(1, currentScale, 1);

//Draw timeline (NB, this will always stay in the same place):
//cout << "scale is: " << currentScale << endl;
glTranslated(-totalMovedX, 0, 0);
graph->timeline->draw();
glTranslated(totalMovedX, 0, 0);

//Now, scale the rest:
glScaled(currentScale, 1, 1);
graph->draw();

..(not important)...

glScaled(1/currentScale, 1/currentScale, 1);
//Now, move it a bit if dragging:
if (parent->leftButtonDown){
//cout << "dragging by " << -(mousex-curx) << " " << -(mousey-cury) << endl;
glTranslated(-(mousex-curx), -(mousey-cury), 0);
totalMovedX += (-(mousex-curx));
totalMovedY += (-(mousey-cury));


}



Now, what this does is draws a timeline on the left (along the Y axis), only scales that (if there's a zoom, currentScale starts off as 1), and allows the user to drag the canvas to view different parts of the graph.

What I've been stuck on for an embarrassing number of hours is the zoom. I've got the scale as you've seen above, and I have a zoomIn function:


void RefMapGLScene::zoomIn(bool toMouse){
if (currentScale > 3){
return;
}
pthread_mutex_lock(&parent->gtkLock);
currentScale = currentScale+zoomFactor;

//now zoom in the centre:
double xAdjust = (get_width())*zoomFactor/2;
double yAdjust = get_height()*zoomFactor/2;
glTranslated(-xAdjust, -yAdjust, 0);
totalMovedX += -xAdjust;
totalMovedY += -yAdjust;


Zoomfactor is 0.1, there is an accompanying zoomOut function, but it's just the same, except we subtract zoomFactor.

Now, the problem comes with zooming into the center. Normally scale will appear to zoom towards 0,0 (ie, the bottom left), so I need to do a translate to move the zoom towards the centre. This translate works fine so long as the user does not drag inside the application to see a different part of the graph. If the user does drag to see another part of the graph, it doesn't zoom into the area that the user is looking, but rather, if they dragged to see what's on the right (ie, dragged mouse left), then the app zooms into the left, and vice versa for if they dragged to see what's on the left, the zoom zooms into the right.

To represent this problem visually, imagine you start looking in the centre (where 0 is the origin):
----0----
Then zooming in and out works fine.

But if you look to the right
-0-------
Then zooming in will make everything bigger, but the translate will not leave you zooming into the correct area infront of you, ie, you will head towards the origin:
--0------
---0-----

I hope that kind of makes sense to somebody... :)

Thank you for reading all that. If it doesn't leave you a little bit confused, then you're a bright person! I'd greatly appreciate any help. I've tried using glOrtho to do the zoom in and that's just not working. I just don't see why this translation isn't doing it right... I've played around with it lots, and it still doesn't work... Thank you for any replies in advance!

Share this post


Link to post
Share on other sites
Advertisement
Here is some code from a C# application which I think does what you need:

ret.Translate(Display.ClientSize.Width / 2, Display.ClientSize.Height / 2);
ret.Scale(m_zoom, m_zoom);
ret.Translate(-m_page_bounds.Width / 2 - Hscroll.Value, -m_page_bounds.Height / 2 - Vscroll.Value);

The first line places the origin in the center of the drawing area, the second zooms, and the third moves to the center of the page bounds (your graph maybe)
HScroll and VScroll are scrollbars.

In my application, this is a function which returns a matrix which is needed to properly retrieve the right mouse coordinates (when you zoom out your drawing, you will zoom in your mouse coordinates) in which case you need the inverse o that matrix to retrieve the correct mouse coordinates.

Share this post


Link to post
Share on other sites
Hey,

Thanks very much for that reply.

I've reworked my algorithm a little, to try to make it simpler and closer to your one, but it's still not working unfortunantly.

With every zoom:


double xAdjust = (get_width())*zoomFactor/2;
double yAdjust = get_height()*zoomFactor/2;

zoomTranslateX -= xAdjust;
zoomTranslateY -= yAdjust;


And then in the draw loop:


//So far, we have no translates
glTranslated(0, -zoomTranslateY, 0);

//Only scale Y with timeline (not X):
glScaled(1, currentScale, 1);
glTranslated(0, totalMovedY/currentScale, 0);
//Draw timeline (NB, this will always stay in the same place):
graph->timeline->draw();
glTranslated(0, -totalMovedY/currentScale, 0);

glScaled(1, 1/currentScale, 1);

//Now we are back without any translates or scales
//So move to where you think you need to be:
glTranslated(-zoomTranslateX, 0, 0);


glScaled(currentScale, currentScale, 1);
glTranslated(totalMovedX/currentScale, totalMovedY/currentScale, 0);

do other drawing here...


//now, end the drawing:
glTranslated(-totalMovedX/currentScale, -totalMovedY/currentScale, 0);
glScaled(1/currentScale, 1/currentScale, 1);

//Now we're back without any translates or scales:
glTranslated(zoomTranslateX, zoomTranslateY, 0);



And I'm having exactly the same problem as before I'm afraid! The code is perhaps a bit simpler now, but the same thing's happening, ie, the zoom in always go towards a centre point where we start, whether or not I've moved to the side.. So, just to restate, if I've dragged my canvas so I'm seeing the area to the right, the zoom in zooms to the left (and vice versa with the other side)!

Thanks again for your reply though, it made it simpler in my head...

Share this post


Link to post
Share on other sites
Sorry to double post, I've been looking at: http://www.opengl.org/discussion_boards/ubbthreads.php?ubb=showflat&Number=108344

I shouldn't have to call GlOrtho again after I change the scaling factor, or after I translate, should I? I've tried it, but didn't have any luck... but perhaps I was missing something if I was meant to call that :P

Share this post


Link to post
Share on other sites
I would have to look through your code carefully to be sure (and even then there might not be enough context to be certain), but I have a feeling you might be going about this the wrong way.

First, minor detail, but this:
glTranslated(-totalMovedX, 0, 0);
graph->timeline->draw();
glTranslated(totalMovedX, 0, 0);
Should probably be implemented using glPushMatrix() and glPopMatrix() (since you appear to be using the fixed-function matrix stack).

Let me ask this: are you resetting the modelview matrix to identity at the beginning of each frame, or are you just letting transforms accumulate from frame to frame, and using (implicit) inverse transforms in order to 'cancel out' previous transforms?

If the latter, I don't think you want to be doing it that way (unless there's some good reason for doing it that way that I'm not aware of). It's more typical to track whatever information you need (e.g. zoom factor, camera position, etc.) in your own code, and then build transforms for rendering 'from scratch' (that is, starting with identity) each frame.

Also, I'd think it would be more sensible to implement zoom in terms of the parameters of the orthographic projection rather than via scaling. You can zoom in and out by adjusting the projection, and move the camera by adjusting the camera position (which is then used to set up the 'view' part of the modelview matrix, typically by inversion/negation).

Maybe I'm wrong and the way you're doing it is fine, but I get the sense from glancing over your code that something may be amiss.

Share this post


Link to post
Share on other sites
Hey,

Thanks for that jyk. I'll move it all onto the matrix stack using push/pop, and will call Glortho every time I do the draw loop. But that will have to wait until I'm home :) Will post how it goes, thanks again.

Simon

Share this post


Link to post
Share on other sites
Thanks Jyk, that really helped clean up the code and make everything straight in my mind. It turned out that by doing this, I was forced to rethink the code a little and came across my bug.

I'll post it here incase anyone else has my issue

When zooming I needed to do this:
double xAdjust = (((get_width())*zoomFactor/2)+(-totalMovedX*zoomFactor));

(ie, I needed to account for the area that I had already moved by dragging the mouse, and add the difference to that due to the scale - that's the area following the +, this is what I didn't have before).

Share this post


Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
  • Advertisement
  • Popular Tags

  • Advertisement
  • Popular Now

  • Similar Content

    • By LifeArtist
      Good Evening,
      I want to make a 2D game which involves displaying some debug information. Especially for collision, enemy sights and so on ...
      First of I was thinking about all those shapes which I need will need for debugging purposes: circles, rectangles, lines, polygons.
      I am really stucked right now because of the fundamental question:
      Where do I store my vertices positions for each line (object)? Currently I am not using a model matrix because I am using orthographic projection and set the final position within the VBO. That means that if I add a new line I would have to expand the "points" array and re-upload (recall glBufferData) it every time. The other method would be to use a model matrix and a fixed vbo for a line but it would be also messy to exactly create a line from (0,0) to (100,20) calculating the rotation and scale to make it fit.
      If I proceed with option 1 "updating the array each frame" I was thinking of having 4 draw calls every frame for the lines vao, polygons vao and so on. 
      In addition to that I am planning to use some sort of ECS based architecture. So the other question would be:
      Should I treat those debug objects as entities/components?
      For me it would make sense to treat them as entities but that's creates a new issue with the previous array approach because it would have for example a transform and render component. A special render component for debug objects (no texture etc) ... For me the transform component is also just a matrix but how would I then define a line?
      Treating them as components would'nt be a good idea in my eyes because then I would always need an entity. Well entity is just an id !? So maybe its a component?
      Regards,
      LifeArtist
    • By QQemka
      Hello. I am coding a small thingy in my spare time. All i want to achieve is to load a heightmap (as the lowest possible walking terrain), some static meshes (elements of the environment) and a dynamic character (meaning i can move, collide with heightmap/static meshes and hold a varying item in a hand ). Got a bunch of questions, or rather problems i can't find solution to myself. Nearly all are deal with graphics/gpu, not the coding part. My c++ is on high enough level.
      Let's go:
      Heightmap - i obviously want it to be textured, size is hardcoded to 256x256 squares. I can't have one huge texture stretched over entire terrain cause every pixel would be enormous. Thats why i decided to use 2 specified textures. First will be a tileset consisting of 16 square tiles (u v range from 0 to 0.25 for first tile and so on) and second a 256x256 buffer with 0-15 value representing index of the tile from tileset for every heigtmap square. Problem is, how do i blend the edges nicely and make some computationally cheap changes so its not obvious there are only 16 tiles? Is it possible to generate such terrain with some existing program?
      Collisions - i want to use bounding sphere and aabb. But should i store them for a model or entity instance? Meaning i have 20 same trees spawned using the same tree model, but every entity got its own transformation (position, scale etc). Storing collision component per instance grats faster access + is precalculated and transformed (takes additional memory, but who cares?), so i stick with this, right? What should i do if object is dynamically rotated? The aabb is no longer aligned and calculating per vertex min/max everytime object rotates/scales is pretty expensive, right?
      Drawing aabb - problem similar to above (storing aabb data per instance or model). This time in my opinion per model is enough since every instance also does not have own vertex buffer but uses the shared one (so 20 trees share reference to one tree model). So rendering aabb is about taking the model's aabb, transforming with instance matrix and voila. What about aabb vertex buffer (this is more of a cosmetic question, just curious, bumped onto it in time of writing this). Is it better to make it as 8 points and index buffer (12 lines), or only 2 vertices with min/max x/y/z and having the shaders dynamically generate 6 other vertices and draw the box? Or maybe there should be just ONE 1x1x1 cube box template moved/scaled per entity?
      What if one model got a diffuse texture and a normal map, and other has only diffuse? Should i pass some bool flag to shader with that info, or just assume that my game supports only diffuse maps without fancy stuff?
      There were several more but i forgot/solved them at time of writing
      Thanks in advance
    • By RenanRR
      Hi All,
      I'm reading the tutorials from learnOpengl site (nice site) and I'm having a question on the camera (https://learnopengl.com/Getting-started/Camera).
      I always saw the camera being manipulated with the lookat, but in tutorial I saw the camera being changed through the MVP arrays, which do not seem to be camera, but rather the scene that changes:
      Vertex Shader:
      #version 330 core layout (location = 0) in vec3 aPos; layout (location = 1) in vec2 aTexCoord; out vec2 TexCoord; uniform mat4 model; uniform mat4 view; uniform mat4 projection; void main() { gl_Position = projection * view * model * vec4(aPos, 1.0f); TexCoord = vec2(aTexCoord.x, aTexCoord.y); } then, the matrix manipulated:
      ..... glm::mat4 projection = glm::perspective(glm::radians(fov), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f); ourShader.setMat4("projection", projection); .... glm::mat4 view = glm::lookAt(cameraPos, cameraPos + cameraFront, cameraUp); ourShader.setMat4("view", view); .... model = glm::rotate(model, glm::radians(angle), glm::vec3(1.0f, 0.3f, 0.5f)); ourShader.setMat4("model", model);  
      So, some doubts:
      - Why use it like that?
      - Is it okay to manipulate the camera that way?
      -in this way, are not the vertex's positions that changes instead of the camera?
      - I need to pass MVP to all shaders of object in my scenes ?
       
      What it seems, is that the camera stands still and the scenery that changes...
      it's right?
       
       
      Thank you
       
    • By dpadam450
      Sampling a floating point texture where the alpha channel holds 4-bytes of packed data into the float. I don't know how to cast the raw memory to treat it as an integer so I can perform bit-shifting operations.

      int rgbValue = int(textureSample.w);//4 bytes of data packed as color
      // algorithm might not be correct and endianness might need switching.
      vec3 extractedData = vec3(  rgbValue & 0xFF000000,  (rgbValue << 8) & 0xFF000000, (rgbValue << 16) & 0xFF000000);
      extractedData /= 255.0f;
    • By Devashish Khandelwal
      While writing a simple renderer using OpenGL, I faced an issue with the glGetUniformLocation function. For some reason, the location is coming to be -1.
      Anyone has any idea .. what should I do?
  • Advertisement