# OpenGL OpenGL / HUD - Computation of a good initial value for scale line

## Recommended Posts

I try to find the good initial value at the start of simulation. I took on the following image a value equal to 100 kpc (kpc is the unity used in my code for the positions of each particles) Edited by youpi1

##### Share on other sites

Firstly, the main problem is that you are using a perspective transform instead of an orthogonal transform.  In a perspective transform, only the particles with exactly the correct Z distances will be scaled correctly according your scale.

The first fix is to use an orthogonal transform.

Once that is done you will probably notice that everything gets small.

That is because now one pixel = one unit.

So if a particle has traveled 100 pixels on the screen it is trivial to calculate how many world units that means.  And the inverse of such operation (taking 100 world units and showing that as pixels on a graph/bar) is also trivial.

I leave this up to the reader who shows some self-help efforts.

L. Spiro

Edited by L. Spiro

##### Share on other sites

I don't understand why I have to use Ortho projection, Perspective allows me to make diplay  particles smaller/bigger when they get far away/closer (with zoom out/in that I have implemented) while Ortho is for 2D stuff.

##### Share on other sites

Well on that example picture you are watching the galaxy (which is very flat) from a much higher distance from far above it, so you gain not very much from perspective transform, but that distance-meter only works on 1 distance and is difficult to calculate correctly.

If you need to watch it from the side and at much lower distance the distance meter gets useless as the user cannot know if it applies to foreground or background objects for which it would have to be different. So you have to decide whats more important to your users, a useful distance meter with ortho or correct perspective drawing for minimally better visualization.

##### Share on other sites
I am not sure I have all understood. There are 2 things :

- the line I draw is done in "headupdisplay" function by :

//Setup for 2D
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glOrtho(0, w_width, w_height, 0, -1, 1);
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
// begin draw scale line
glDisable(GL_TEXTURE_2D);
glLineWidth(2.0f);
glBegin(GL_LINES);
glVertex2d(350, 12);
glVertex2d(450, 12);
glVertex2d(350, 9);
glVertex2d(350, 15);
glVertex2d(450, 9);
glVertex2d(450, 15);
glEnd();
glEnable(GL_TEXTURE_2D);
// end draw
glMatrixMode( GL_PROJECTION );
glPopMatrix();
glMatrixMode( GL_MODELVIEW );
glPopMatrix();
// end for 2D

So, this line has 100 pixel length.

- I want the value right to this line to represent the distance of the foreground plane, i.e the 2D projection of the 3D scene.

For example, I show you the result when I zoom (the value equals to 45.8 kpc) :

[attachment=13275:test3.png]

Now another picture with the same zoom but with also a rotation by mouse, this is a view by side of the galaxy :

[attachment=13276:test4.png]

wintertime, you say this distance is difficult to calculate correctly with perspective projection, could you give me some clue ?

I just want to get a value corresponding to the foreground objects, i.e the 2D projection in (xy) plane of the 3D scene without taking into account of the z coordinates. That's why you advise me to use Ortho projection ?

Thanks Edited by youpi1

##### Share on other sites

Perspective projection is like watching a pyramid from above its tip and the further away the wider it gets. Its impossible to give one number for how wide it is without a distance.

Orthographic projection is like looking at a box, its got same width everywhere. Its easy to give one number then and you can also just use the same projection for the graphics and the measuring line so its got same unit length.

##### Share on other sites
Its impossible to give one number for how wide it is without a distance.

So when I do :

gluPerspective(45.0f, (float)w_width / w_height, g_nearPlane, g_farPlane);"
gluLookAt (0.0, 0.0, 3.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);

I can't say that the width and height are equals to : width = height = 2 *tan(45) * distance ;  with distance = 3 ?

## Create an account

Register a new account

• ### Forum Statistics

• Total Topics
628305
• Total Posts
2981962
• ### Similar Content

• By mellinoe
Hi all,
First time poster here, although I've been reading posts here for quite a while. This place has been invaluable for learning graphics programming -- thanks for a great resource!
Right now, I'm working on a graphics abstraction layer for .NET which supports D3D11, Vulkan, and OpenGL at the moment. I have implemented most of my planned features already, and things are working well. Some remaining features that I am planning are Compute Shaders, and some flavor of read-write shader resources. At the moment, my shaders can just get simple read-only access to a uniform (or constant) buffer, a texture, or a sampler. Unfortunately, I'm having a tough time grasping the distinctions between all of the different kinds of read-write resources that are available. In D3D alone, there seem to be 5 or 6 different kinds of resources with similar but different characteristics. On top of that, I get the impression that some of them are more or less "obsoleted" by the newer kinds, and don't have much of a place in modern code. There seem to be a few pivots:
The data source/destination (buffer or texture) Read-write or read-only Structured or unstructured (?) Ordered vs unordered (?) These are just my observations based on a lot of MSDN and OpenGL doc reading. For my library, I'm not interested in exposing every possibility to the user -- just trying to find a good "middle-ground" that can be represented cleanly across API's which is good enough for common scenarios.
Can anyone give a sort of "overview" of the different options, and perhaps compare/contrast the concepts between Direct3D, OpenGL, and Vulkan? I'd also be very interested in hearing how other folks have abstracted these concepts in their libraries.
• By aejt
I recently started getting into graphics programming (2nd try, first try was many years ago) and I'm working on a 3d rendering engine which I hope to be able to make a 3D game with sooner or later. I have plenty of C++ experience, but not a lot when it comes to graphics, and while it's definitely going much better this time, I'm having trouble figuring out how assets are usually handled by engines.
I'm not having trouble with handling the GPU resources, but more so with how the resources should be defined and used in the system (materials, models, etc).
This is my plan now, I've implemented most of it except for the XML parts and factories and those are the ones I'm not sure of at all:
I have these classes:
For GPU resources:
Geometry: holds and manages everything needed to render a geometry: VAO, VBO, EBO. Texture: holds and manages a texture which is loaded into the GPU. Shader: holds and manages a shader which is loaded into the GPU. For assets relying on GPU resources:
Material: holds a shader resource, multiple texture resources, as well as uniform settings. Mesh: holds a geometry and a material. Model: holds multiple meshes, possibly in a tree structure to more easily support skinning later on? For handling GPU resources:
ResourceCache<T>: T can be any resource loaded into the GPU. It owns these resources and only hands out handles to them on request (currently string identifiers are used when requesting handles, but all resources are stored in a vector and each handle only contains resource's index in that vector) Resource<T>: The handles given out from ResourceCache. The handles are reference counted and to get the underlying resource you simply deference like with pointers (*handle).
And my plan is to define everything into these XML documents to abstract away files:
Resources.xml for ref-counted GPU resources (geometry, shaders, textures) Resources are assigned names/ids and resource files, and possibly some attributes (what vertex attributes does this geometry have? what vertex attributes does this shader expect? what uniforms does this shader use? and so on) Are reference counted using ResourceCache<T> Assets.xml for assets using the GPU resources (materials, meshes, models) Assets are not reference counted, but they hold handles to ref-counted resources. References the resources defined in Resources.xml by names/ids. The XMLs are loaded into some structure in memory which is then used for loading the resources/assets using factory classes:
Factory classes for resources:
For example, a texture factory could contain the texture definitions from the XML containing data about textures in the game, as well as a cache containing all loaded textures. This means it has mappings from each name/id to a file and when asked to load a texture with a name/id, it can look up its path and use a "BinaryLoader" to either load the file and create the resource directly, or asynchronously load the file's data into a queue which then can be read from later to create the resources synchronously in the GL context. These factories only return handles.
Factory classes for assets:
Much like for resources, these classes contain the definitions for the assets they can load. For example, with the definition the MaterialFactory will know which shader, textures and possibly uniform a certain material has, and with the help of TextureFactory and ShaderFactory, it can retrieve handles to the resources it needs (Shader + Textures), setup itself from XML data (uniform values), and return a created instance of requested material. These factories return actual instances, not handles (but the instances contain handles).

Is this a good or commonly used approach? Is this going to bite me in the ass later on? Are there other more preferable approaches? Is this outside of the scope of a 3d renderer and should be on the engine side? I'd love to receive and kind of advice or suggestions!
Thanks!
• By nedondev
I 'm learning how to create game by using opengl with c/c++ coding, so here is my fist game. In video description also have game contain in Dropbox. May be I will make it better in future.
Thanks.

• So I've recently started learning some GLSL and now I'm toying with a POM shader. I'm trying to optimize it and notice that it starts having issues at high texture sizes, especially with self-shadowing.
Now I know POM is expensive either way, but would pulling the heightmap out of the normalmap alpha channel and in it's own 8bit texture make doing all those dozens of texture fetches more cheap? Or is everything in the cache aligned to 32bit anyway? I haven't implemented texture compression yet, I think that would help? But regardless, should there be a performance boost from decoupling the heightmap? I could also keep it in a lower resolution than the normalmap if that would improve performance.
Any help is much appreciated, please keep in mind I'm somewhat of a newbie. Thanks!

• Hi,
I'm trying to learn OpenGL through a website and have proceeded until this page of it. The output is a simple triangle. The problem is the complexity.
I have read that page several times and tried to analyse the code but I haven't understood the code properly and completely yet. This is the code: