I am wondering what kind of methods are commonly used when we do zoom in/out.
In my current project, I have to display millions of 2D rectangles on the screen, and I am using a fixed viewport and changing glortho2D variables when I have to zoom in/out.
I am wondering if this is a good way of doing it and what other solution can I use.
I also have another question which I think it is related to
how should I do zoom in/out.
As I said, I am currenly using a fixed viewport and changing glortho2D variables in my code, and I assumed that opengl will be able to figure out which rectangles are out of the screen and not render them. However, it seems like opengl is redrawing all the rectangles again and again. The rendering time of viewing millions of rectangles (zoom out) is equal to vewing hundreds of rectangles (zoom into a particular area), which is opposite of what I expected. I am wondering if it is related to the zooming methods I used or am I missing something important.
ie . I am using VBO while rendering the rectangles.
and I assumed that opengl will be able to figure out which rectangles are out of the screen
You assumed wrong
and not render them.
OpenGL is a rather dumb drawing API. There's no such thing like a scene in OpenGL. All it does is coloring pixels on the framebuffer one point, line or triangle at a time. When geometry lies outside the viewport it still has to be processed up to the point it gets clipped (and then discarded).
Related
I have been following some OpenGL tutorials for an open world project i am currently working on where the goal is to have an Openworld Scene with several objects (mountains etc...) present and with a SkyBox where all the objects are placed inside it.
I would like to ask if there is any way of the camera freely moving inside the skybox, "interacting" with potential objects in it, but without actually getting out of the boundaries of the box. In the tutorials the translation of the camera is removed, so it can only look around without moving around.
Is it a common practice to actually move the camera inside the skybox, or should i somehow move the skybox along with the camera, thus never reaching the boundaries of the box?
Skybox is usually rendered without offset to camera because its content represent stuff very far away (many times bigger than actual camera movement) like stars or mountains that are many kilometers away. So even if you move like 100 m in any direction the rendered result is not changed at all (or very little that can not be recognized).
If your skybox contains stuff you want to move towards than is doable but you need to limit the movement so you not get too close as that would result in pixelation of the skybox and eventually even crossing it. That can be done by game terrain (you can not jump above boundary mountains or swim too far from an island etc.
Another option is to limit camera distance from skybox center to some safe distance. If more far then the limit move the skybox to match the distance again... that way you can come near/far to skybox up to a point (it gets bigger/smaller on the close/far side) and never cross it ... without any actual camera position restrictions...
First things first, when you are rendering a sky box, generally, you don't render an actual box.
The skybox contains stuff that generally never or only very slowly change and is so far away that the player will never reach. The skybox is stored in a cube map texture and rendered through a full screen rectangle. In the shader you use OpenGL's cubemap sampling by sampling with the eye vector into the map.
If the skybox is dynamic, like dynamic time of day, it is only re rendered every couple of frames or only when needed.
A while back I wrote an article on how to do it: GLSL Skybox (You will need to update the code to a modern OpenGL version through...)
Using OPENGL , I am making a simple animation where a small triangle will move through the path that I have created with mouse (glutMotionFunc).
So the problem is how can I animate a small triangle without redrawing the whole path using glutSwapBuffers();
And also ,how can I rotate that triangle only.
I don't want to use overlay as switching between these 2 layers takes much time.
If redrawing the whole path is really too expensive, you can do your rendering to an off-screen framebuffer. The mechanism to do this with OpenGL is called Frame Buffer Object (FBO)
Explaining how to use FBOs in detail is beyond the scope of an answer here, but you should be able to find tutorials. You will be using functions like:
glGenFramebuffers()
glBindFramebuffer()
glFramebufferRenderbuffer() or glFramebufferTexture()
This way, you can draw just the additional triangle to your FBO whenever a new triangle is added. To show your rendering on screen, you can copy the current content of the FBO to the primary framebuffer using glBlitFramebuffer().
You cant! Because it just does not makes sense!
The way computer screen work is the same as in films: fps! Frames per second. There is no thing as "animation" in screens, it is just a fast series of static images, but as our eyes cannot see things moving fast, it looks like it is moving.
This means that every time something changes in the thing you want to draw, you need to create a new "static image" of that stage, and that is done with all the glVertex and so pieces of code. Once you finish drawing you want to put it on the screen, so you swap your buffer.
I need some help in surface area selection on a 3d model rendered in opengl by picking points through mouse. I know how to get a point in world coordinate but cant find a way to select an area. Later I need to remesh that selected area and map an image over it which I know.
Well, OpenGL by itself can't help you there. OpenGL is a drawing API. You draw things, but once the drawing commands have been executed all that's left are pixels in a framebuffer and OpenGL has no recollection about the geometry whatsoever.
You can use OpenGL to implement image based area selection algorithms, for example by drawing each face with a unique index color into an off screen framebuffer. Then by looking at what values can be found therein you know which faces are present in a given area.
Later I need to remesh
This is called topology modification and is completely outside the scope of OpenGL.
that selected area and map an image over it which I know
You can use a image based approach for this again, however you must know in which way you want to make images to faces first. If you want to unwrap the mesh, then OpenGL is of no help. However if you want the user to be able to "directly draw" onto the mesh, this can be done by drawing texture coordinates into another off screen framebuffer and by this reverse mapping screen coordinates to texture coordinates.
I am writing a small program in OpenGL on my Mac. I have a question considering subwindows. I have created two subwindows in my main window. Everything works fine to that point. I can draw in both of them. But i want something different. I want to draw (with my mouse) in one window and simultanously get a drawing in the other window.
Now its not even possible to get the same drawing in both windows. If i want to see something i always have to draw in that one particular window.
Do you have some ideas maybe how to do this. Or do you have an example perhaps. Unfortunately i couldnt find much information on the topic "subwindows".
OpenGL is not a scene graph. All what it gives you are the computer equivalents of pencils and crayons. So you draw something to a framebuffer, it will show up in only that one framebuffer.
You want to draw a scene from multiple vantage points? Then you have to draw that scene multiple times from those choosen vantage points to the designated viewports.
I have a simple solid modeling application in which I want to implement several "navigation modes", ways for the user to navigate the camera through 3d space. One of them is the ubiquitous 'drag and pan/rotate' that is used in SketchUp, Blender etc.; I also want to implement something that is more relevant to my specific application. Specifically, I want to implement a mode where the camera floats on a 'ring' above the object being modeled (a building), and always looks at the center of the model; this way, a user can easily 'circle' around the object, a common operation in my application.
So, what I want to do is render the building in my view, and display a torus in the top right of the view, with a small sphere on the torus to represent the camera location. There would be a north arrow in the torus, and the user would drag the camera around the model object by dragging the sphere; moving the sphere would reposition the camera and redraw the scene.
It looks like what I should do is the following: render the 'main view', i.e. the building; then render the torus and sphere (with different perspective settings and lighting) to an offscreen buffer, and blit it from there to my main view.
Then however I get to the hit testing. I want to detect if the user clicks on the sphere, or the torus; from what I understand from OpenGL picking (it seems to be a hard subject :/ ), all picking methods apply only for selecting in one 'scene'. Apart from that, I still want to detect 'normal' picking operations in the building model, obviously.
So, my questions:
How do I render to an offscreen buffer and blit into another OpenGL context (with alpha blending & transparence like for the center of the torus)?
How do I do hit testing in the described scenario?
I don't think you need to do off-screen rendering for this. You should be able to just re-set the camera and viewport and render the overlay after the main scene. You might have issues with Z-ordering and/or buffering, but perhaps the "sub-scene" is simple enough for that not to matter, or you could of course just clear the Z buffer before rendering it.
As far as drawing the torus/sphere goes, create a separate class for that and implement a "draw" method. Have the class contain the location of both the sphere and torus and have draw() render those things on the screen.
Then just call myRing.draw() in your main drawing method and you'll have a sphere and torus!
If you mean you want to have a a circle/ring rendered in 2D (which might be easier) in the top right corner of the window, then the same sort of idea would apply as in your hitbox post (except without that annoying projection calculation!)
Lastly, I'd consider using a function key in combination with mouse drags to implement the functionality you want... E.g. the user holds "shift" and then click-drags the mouse across the screen. These mouse events are caught and the x-delta is used to compute the angle of rotation. The camera's location is updated as this happens and you get a smooth sliding motion :)
I agree with #unwind; you don't need an offscreen buffer. If you want to anyway, search for "render-to-texture".
As for hit testing, The OpenGL FAQ has an entry on it. It describes several solutions: using GL_SELECTION render mode, using gluUnproject() to get a 3D collision ray and a simple 2D solution using unique colors.