# OpenGL 3D algorithm

This topic is 2230 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

## Recommended Posts

Hey everyone, has been a while I've programmed but I'm back.

I know that re-writing existing code is useless. But I don't mind to re-write some code, I want to learn how it's done.
I want to make a 3D polygon and draw it in C++, without using DirectX or OpenGL.
I've been looking around after source codes or tutorials but haven't found a single tutorial that helps me.

Very simple example of what I want to do:
 m_Polygon = new Polygon(vertex1, vertex2, vertex3, vertex4); // where the vertexes have a X,Y,Z m_Polygon->Draw(); 

So I want to draw the 3D polygon in my 2D screen. Can someone help me with this.

Thank you and kind regards,
Jonathan

##### Share on other sites
Look up coordinate transformations, perspective projection, and rasterization. Basically, you'll be reimplementing parts of the GPU pipeline, in the form of a software renderer. Knowledge of linear algebra helps a lot if you want to understand half of the code you're writing.

It typically goes roughly like this:
- transform each vertex to world coordinates, if it's not already done, this is useful if you want to have multiple instances of the same mesh without duplicating vertices, for instance suppose you want to have two bunnies side to side, you could duplicate each vertex to make two separate bunnies, or you could instead just reuse the same mesh, but simply moving each vertex in the second bunny off to the side (translation)
- transform each vertex to camera coordinates (rotating the world around the camera, so that you're facing what you want to face)
- transform each of those vertices to perspective coordinates (so that vertices further away from the camera tend to the point at infinity, giving the illusion of depth)
- from this, work out where each vertex would appear on the screen, in normalized screen coordinates (with coordinates ranging from -1 to +1 in X and Y dimensions) this is the step where you project your 3D vertices on your screen
- upscale these vertex locations to your desired resolution (e.g. 1024 * 768)
- for each pixel on the screen, work out which triangle it belongs to (depending on how you defined triangles, e.g. triangle list or triangle strip, or even using indices) this is the rasterization step
- shade the pixel (often you interpolate important data like normals, etc.. from the three vertices of the triangle)

As you can see, it can be a lot of work, and that's a very high-level overview (I ignored the depth and stencil buffers, as well as a lot of other stuff) but it's a good learning exercise. Make sure to take it step by step so that you don't get overwhelmed! Edited by Bacterius

##### Share on other sites
A billion percent thank you!

In Win32 there are some standard functions for example: SetWorldTransform, SetGrapicsMode, etc. and structs like XFORM.
Should I make any use of those functions and structs or should I rather make my own ones?

Kind regards,
Jonathan

##### Share on other sites

In Win32 there are some standard functions for example: SetWorldTransform, SetGrapicsMode, etc. and structs like XFORM.
Should I make any use of those functions and structs or should I rather make my own ones?

I remember those functions, they seem to be remnants of some past era of software rendering using GDI (or they might be hardware-accelerated and used to draw desktop controls, I'm not sure). I wouldn't use them especially if you want to learn how transformation matrices work, since they hide all of that. All you really need is a form to display the results on, and a 2D array of pixels to render to, but of course you can use them if you find they are helpful. But you can absolutely do 100% of the rendering and computations without using built-in functions. You can even make your own vector and matrix class and roll with that, and I feel this is important if you've never done it before and are interested in low-level rendering.

It's up to you how deep you want to go. If you want to build on Win32 GDI to create your software renderer, you can absolutely do that and it's still fun (though you'll have to deal with the API, and you might not find as many tutorials as you'd like), or if you prefer to do everything from scratch, that's fun too but be ready to do a lot of linear algebra math!

By the way, re-writing existing code is not useless. It's important if you want to learn. Nobody would tell a beginner not to code Pong because it's already been done! It's only bad to reinvent the wheel if you want to get stuff done quickly, like a game. But If you're doing it for fun or for experience, reinventing the wheel is the right thing to do.

EDIT: The GDI API seems to be limited to 2D, so you might not find much use for GDI beyond accelerated pixel manipulation. I could be wrong though. Edited by Bacterius

##### Share on other sites
I'm looking up some information what GDI exactly is and this is a quote from wikipedia.
Simple games that do not require fast graphics rendering use GDI. However, GDI is relatively hard to use for advanced animation, and lacks a notion for synchronizing with individual video frames in the video card, lacks hardware rasterization for 3D, etc. Modern games usually use DirectX or OpenGL instead, which let programmers exploit the features of modern hardware.[/quote]

What makes DirectX a fast graphics renderer? How you decide the speed of rendering?
I just want to get low-level what programming concerns. I really want to get the knowledge into my head how it's done.
GDI only limited to 2D, how do I get the 3D in my program then? I'm assuming I need to calculate every pixel and draw them manually, please correct me if I'm wrong.

I want to achieve real-time rendering. Dragging a 3D model into your window and be able to translate, scale and rotate in your project.
But that is not for now, first the baby steps. But I can't do this alone sadly enough. Need some help from people who have experience in this because I can't find almost anything that helps me. maybe I'm not looking in the right place.

Kind regards,
Jonathan

##### Share on other sites
Your graphics card already has all the steps I highlighted in my first post implemented, in hardware (not software). This means you can pump ridiculous amounts of triangles in it, and shade them in complex ways with multiple texturing and stuff and it'll still run in realtime. Furthermore, your graphics card is the component that's connected to your display monitor, and since the frame that's about to be rendered exists on the graphics card, it's very easy to send it to the screen. On the other hand, if you're using software rendering, then the frame is in system memory, and must be sent to the graphics card before being displayed, which is quite expensive (CPU to GPU transfers and vice versa are quite slow).

If you want to achieve realtime rendering, that is possible to some extent on a software renderer, but you certainly will not achieve the same amount of speed that can be achieved on a dedicated graphics card, and not with ease. The hardware is just superior in terms of performance. Unfortunately, you cannot really learn from hardware, so if you really want to know how it's done, you want to look at software rendering, which is great for learning, but remember it'll probably be quite slow without using a lot of optimization tricks and not using too many triangles.

Now it depends on your definition of "real-time" and "3D model". If you're talking about white triangles on black screen, on a model with like 10000 triangles, then sure - easily real-time. But if you want to draw a nice 1M polygon model with texturing and lighting... it's going to be harder for your software renderer than for the graphics card.

This is not to discourage you - graphics cards are designed with graphics performance in mind and are meant to be faster than processors for these kinds of operations, that's why you don't see a lot of software rendering around. But to learn the inner workings of graphics cards, there's nothing better.

##### Share on other sites
I would like to write a small copy of DirectX. How do they handle the vertices and rendering of 3D models? How should it be handled for an optimal framework/engine?
If my GPU can handle all the math, how can I render my 3D model with it? I still need to use some math from your first post or don't I?

I'm looking up coordinate transformations and it's quite fun.
And how about particles? Like when I have 1 million particles. How should I best handle them? Do I need to do all calculations and rendering on the GPU? Or are there better/faster ways?

You are helping me already but I'm still a bit confused about how it all works with the gpu / cpu thing. I want to get an optimal render system for low poly models with materials, shaders, lightning, bouncings, etc. I know the pipeline for the graphics rendering but I can't go any further than the first step at the moment. For example I want to be able to load a whole environment like COD with all players, bullets, effects in it.

Kind regards,
Jonathan

##### Share on other sites

I would like to write a small copy of DirectX. How do they handle the vertices and rendering of 3D models? How should it be handled for an optimal framework/engine?
If my GPU can handle all the math, how can I render my 3D model with it? I still need to use some math from your first post or don't I?

I'm looking up coordinate transformations and it's quite fun.
And how about particles? Like when I have 1 million particles. How should I best handle them? Do I need to do all calculations and rendering on the GPU? Or are there better/faster ways?

You are helping me already but I'm still a bit confused about how it all works with the gpu / cpu thing. I want to get an optimal render system for low poly models with materials, shaders, lightning, bouncings, etc. I know the pipeline for the graphics rendering but I can't go any further than the first step at the moment. For example I want to be able to load a whole environment like COD with all players, bullets, effects in it.

Kind regards,
Jonathan

So basically you want to write a software renderer which runs on the GPU right?
Remember that DirectX and OpenGL can communicate with your graphics driver, and I'm afraid you as a developer won't have that luxury (if you want to call it that). This only leaves you with the option to resort to GPGPU solutions, and while it's completely possible to write a software renderer with these, you probably won't be able to beat or even come near the performance a library like DirectX or OpenGL will give you. The reason for this is that DirectX and OpenGL are able to use the rasterizer hardware available on your GPU, while GPGPU solutions aren't able to do so.

Now the essential question here (before I ramble on) is: Why do you want do write your own renderer?

If this is just for learning I'd say try to do a very simple CPU-based renderer and leave the GPU out of the picture (except for maybe presenting your final result to the screen), this will teach you a lot about how the entire rasterization process works without having to worry about CPU-GPU communication and all that stuff. Try to implement each of the steps Bacterius laid out for you in his first post completely by yourself, from writing your own structures for managing vertices and indices to writing systems which can transform and shade these for you to get a final texture which you can then present to the screen.
Writing a simple software renderer which can do all of this is a very rewarding and fun experience which will teach you a lot. If you implement this well you could even use it do some very basic 3D games.

If this is about writing a production-quality renderer I'm going to be harsh and say: don't bother.
You're talking about rendering millions of particles, and rendering a huge amount of objects, so I'm guessing that this is actually what you want to do. I know re-inventing the wheel can be a lot of fun sometimes, but trying to implement something like this will put you in a lot of nasty sticky situations, and in the end you'll have a system which tries to sub-optimally solve a problem for which we already have 2 fast and proven solutions (being DirectX and OpenGL), if you even manage to complete your renderer at all (and I'm going to be harsh again and say that this is very unlikely as you don't have any previous experience writing software renderers). Edited by Radikalizm

##### Share on other sites
If you want to do simple 3d lines and such, an easy way to get going is to try simple perspective. This is what I played around with before I went into matrix math. You will probably get into matrices at some point, but if you just want something basic, you can start with this.

3d without rotation or translation:

A single perspective transform is as easy as this:

//sw is screen width, sh is screen height
//zcut is z plane cutoff (don't render anything closer to or behind z) WHY? z=0 is forbidden. z > 1 means point is in front of camera. z=-1 means point is behind camera. zcutoff of .1, .01 etc are reasonable.

 int perspective(float x, float y, float z, int* sx, int* sy) { if (z < zcutoff) return 0; //refuse to transform sx = x* sw / z; //scale X by screen height and distance. //So, an object sw pixels wide at Z=1 is the width of the screen. //Z=2, half the widht of the screen. Z=.5, twice the width of the screen. etc sy = y* sh / z; return 1; } 

To draw a line in 3d:
 void line3d( float x, float y, float z, float x2, float y2, float z2) { int sx, sy, sx2, sy2; //only draws if both points are in front of camera //later, if you want to get fancy, if one is in front, and the other is behind, you clip at z=zcutoff if (perspective(x,y,z, &sx, &sy) && perspective(x2,y2,z2,&sx2, &sy2)) draw_line(sx,sy,sx2,sy2); } 

With the above snippets, you should be able to draw a 3d perspective object from the point of view of the origin.

To move the camera around, just subtract the camera position from the coordinates:

 int perspective(float x, float y, float z, int* sx, int* sy) { x -= camera_x; y -= camera_y z -= camera_z; if (z < zcutoff) return 0; //refuse to transform sx = x* sw / z; //scale X by screen height and distance. //So, an object sw pixels wide at Z=1 is the width of the screen. //Z=2, half the widht of the screen. Z=.5, twice the width of the screen. etc sy = y* sh / z; return 1; } 

With that, you should be able to move around the 3d environment, but view is constrained to always looking down the Z axis. But, its a start.

The last thing you can do, is allow camera rotation along the y axis (like wolf3d). It's been a while but if http://www.siggraph....tran/2drota.htm is correct, then:

 int perspective(float x, float y, float z, int* sx, int* sy) { float xr, yr, zr; //translate to camera position x -= camera_x; y-= camera_y z -= camera_z; //rotate 2D about y axis: xr = x * cos(camera_angle) - z * sin(camera_angle); zr = z* cos(camera_angle) + x * sin(camera_angle); yr = y; // height does not change if (zr < zcutoff) return 0; //refuse to transform sx = xr* sw / zr; //scale X by screen height and distance. //So, an object sw pixels wide at Z=1 is the width of the screen. //Z=2, half the widht of the screen. Z=.5, twice the width of the screen. etc sy = yr* sh / zr; return 1; } 

That should give you 5 degrees of freedom: You can move up/down, left/right, forward/back and rotate about Y. So, it's 'DOOM' controls. You can add another rotation to look up/down, but at the point you should consider trying to understand matrices. Edited by DracoLacertae

##### Share on other sites
I wrote a software renderer to the OpenGL 1 Spec last year, compiled it and linked it to QuakeGL. It worked pretty well.

This is the book that made software rendering click for me: http://www.amazon.com/Building-3D-Game-Engine-C/dp/0471123269/ref=sr_1_6?ie=UTF8&qid=1354824709&sr=8-6&keywords=3d+game+engine+programming+C%2B%2B

It's pretty concise and straight forward. Not one of those books that you want to copy the code verbatim out of due to it's age. You really need to read it and understand it. This is what i had after a few weeks with the book:

• ### What is your GameDev Story?

In 2019 we are celebrating 20 years of GameDev.net! Share your GameDev Story with us.

• 14
• 14
• 45
• 22
• 27
• ### Forum Statistics

• Total Topics
634043
• Total Posts
3015206
×