Sign in to follow this  
Kwizatz

OpenGL OpenGL Maximum 2D acuracy?

Recommended Posts

Hello, I am coding what I hope will become a multi platform, multi API GUI library, and I am writting the OpenGL portion of it. To make the story short I will just say that I need to implement some basic drawing functions such as DrawPixel, DrawLine, DrawRect and DrawFilledRect, I started implementing these for SDL, and sure enought when I do a DrawPixel(x,y) with SDL the pixel shows up at x,y, when I do DrawLine(x1,y1,x2,y2), the line goes from x1,y1 to x2,y2 inclusive, everything is fine. But now that I am implementing the same stuff on OGL, which is supposed to be easier, it isn't! when I say DrawPixel(10,10) a pixel gets drawn at 10,10, but when I say DrawLine(10,0,10,200) for example, the line gets drawn at (9,0,9,200), if I try to draw a pixel at 0,0 it doesnt show, but if I try it at 1,1 it shows and its in the spot it should be! this is driving me crazy. Now, I am getting into Ortho mode with this code:
  glMatrixMode(GL_PROJECTION);
  glPushMatrix();
  glLoadIdentity();
  glOrtho(0,640,480,0, 0, 1);
  glMatrixMode(GL_MODELVIEW);
  glPushMatrix();  
  glLoadIdentity();
  glBlendFunc(GL_SRC_ALPHA,GL_ONE);
  glEnable(GL_BLEND);
I tried changing glOrtho(0,640,480,0, 0, 1) to glOrtho(0,639,479,0, 0, 1),glOrtho(-1,639,479,0, 0, 1) and glOrtho(-1,639,479,-1, 0, 1), no luck. Then line and pixel functions (GUIColor is a union which holds the ARGB values for a color):
void DrawPixel(Sint16 x, Sint16 y,GUIColor color)
{
  glDisable(GL_TEXTURE_2D);
  glDisable(GL_BLEND);
  glColor4ub(color.color.R,
	     color.color.G,
	     color.color.B,
	     color.color.A);
  glBegin(GL_POINTS);
  {
    glVertex2i(x, y);
  };
  glEnd();
  glEnable(GL_BLEND);
  glEnable(GL_TEXTURE_2D);
};
 
void DrawLine(Sint16 x1, Sint16 y1, Sint16 x2, Sint16 y2,GUIColor color)
{
  glDisable(GL_TEXTURE_2D);
  glDisable(GL_BLEND);
  glColor4ub(color.color.R,
	     color.color.G,
	     color.color.B,
	     color.color.A);
  glBegin(GL_LINES);
  {
    glVertex2i(x1,y1);
    glVertex2i(x2,y2);
  };
  glEnd();
  glEnable(GL_BLEND);
  glEnable(GL_TEXTURE_2D);
};



Any Ideas? [Edited by - Kwizatz on September 21, 2004 8:16:43 PM]

Share this post


Link to post
Share on other sites
There should be a way, I could hardcode everything, but its a GUI, the window size and resolution should be expected to be constantly changing from App to App, I do require pixel perfect rendering [sad], I guess I'll have to do implement the line algo using GL_POINTS, I wonder how much would that slow everything else.

Share this post


Link to post
Share on other sites
Not pixel perfect rendering, but I need reliable primitive drawing functions, IE a hipotetical SDL_DrawLine function should plot the same pixels as a hipotetical OGL_DrawLine Function if given the same arguments, such that later a DDraw_DrawLine or D3D_DrawLine function would also plot the same pixels.

I require this because like I said, I am coding a multi platform, multi API GUI, I need same results on different graphic APIs.

Anyway I think I found the solution:

glOrtho(0,640,479,-1, 0, 1);

Now I just need to find out if the same call (glOrtho(0,XResolution,YResolution-1,-1,0,1);) holds true for all cases.

[Edited by - Kwizatz on September 21, 2004 8:47:42 PM]

Share this post


Link to post
Share on other sites
The problem is how OpenGL fills pixels. What's happening is that you're trying to draw lines abd points on the edge between adjacent pixels, and rounding errors causes wrong pixels to be filled.

You should offset the projection matrix by 0.5 to make integer coordinates end up in the center of the pixels, instead of on the edge.

glOrtho(0, window_width, window_height, 0, 0, 1);
glTranslatef(0.5, 0.5, 0);

Share this post


Link to post
Share on other sites
Quote:
Original post by Brother Bob
Never heard about that. Do you have an example of a driver with a setting like that?

Sure. In Windows, you just need to open advanced display settings and go to the vendor specific tabs. You'll find the settings in current Catalyst™ drivers "3D" tab by clicking the "compatibility" button. IIRC, NVidia's ForceWare™ drivers offer a similar option (cannot tell exactly where since I currently don't have my old GeForce3 installed).

I guess that these settings are available on Linux systems, too (I don't use X on my penguin-powered box so have no clue about 3D driver settings on Linux).

Cheers,
Pat

Share this post


Link to post
Share on other sites
So I guess I have to search for someone with one of those drivers and have a look for myself (have a GF2 myself, and never seen anything there). To me at least, it sounds quite strange to have an option like that. Unless it was a broken feature in the past, so the user can have the option to go back to a previous broken behaviour.

But I had a look in the display properties user guide you can download at the same place you download NV-drivers, and I couldn't find anything in there about this setting.

Share this post


Link to post
Share on other sites
I haven't looked at the drivers, but that is most likely a Direct3D setting. D3D didn't spec sampling point location for pixels from the start so old legacy apps might rely on the pixel smapling point being different. OpenGL speced it from the start so any driver with a checkbox like this would brake ALL OpenGL apps which would be pretty stupid.

Share this post


Link to post
Share on other sites
Quote:
Original post by GameCat
I haven't looked at the drivers, but that is most likely a Direct3D setting.

Took a second look at it. It is indeed a D3D setting, so just ignore what I said before [smile].


Share this post


Link to post
Share on other sites
Actually, its good to know, as I plan to have a D3D interface in the future, say wouldnt:

glOrtho(0.5f, window_width+0.5f, window_height+0.5f, 0.5f, 0, 1);

have the same result as

glOrtho(0, window_width, window_height, 0, 0, 1);
glTranslatef(0.5, 0.5, 0);

?

Thanks for your help. [smile]

Share this post


Link to post
Share on other sites
I had the exact same problems with lines and rectangles. And however I tried, I couldn't get it right. So what I did was draw a line as a rectangle instead. This way I'm totally sure what happens. However, I'm only using this for vertical and horizontal lines at the moment.

Share this post


Link to post
Share on other sites
I wrote a GUI for my project to school and I've had same problem. My buttons were looking good on my ATI, but in software or NVidia there were errors and extra pixels on lines.
I think, NVidia gpus draw lines like you want (including border pixels), but my card draws without left pixel (upper-left pixel in rectangle was missing).
I made gui to look good on my card, but on nvidia it looked a bit weird (with extra pixel in upper-left corner). I was thinking about detecting vendor and change rendering according to that, but it is pretty stupid and it will make my nice code ugly :)
Since my app will work only in windows, I will probably rewrite gui to render woth gdi to texture and render that texture.
Maybe you can use something like drawing your gui to texture and render that, but I don't know how sdl handles this.

Share this post


Link to post
Share on other sites
@b2b3

I used the Render to "SDL_Surface-Use as texture-Render Quad" on aedGUI, my framerate droped to its knees (went from 210 to barelly 60), so I am not doing that again.

Anyway I think the 0.5f trick works so far, I may post some screen shots later, this library is meant to be LGPL, and useful for all game devs [smile]

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Announcements

  • Forum Statistics

    • Total Topics
      628383
    • Total Posts
      2982384
  • Similar Content

    • By test opty
      Hi all,
       
      I'm starting OpenGL using a tut on the Web. But at this point I would like to know the primitives needed for creating a window using OpenGL. So on Windows and using MS VS 2017, what is the simplest code required to render a window with the title of "First Rectangle", please?
       
       
    • By DejayHextrix
      Hi, New here. 
      I need some help. My fiance and I like to play this mobile game online that goes by real time. Her and I are always working but when we have free time we like to play this game. We don't always got time throughout the day to Queue Buildings, troops, Upgrades....etc.... 
      I was told to look into DLL Injection and OpenGL/DirectX Hooking. Is this true? Is this what I need to learn? 
      How do I read the Android files, or modify the files, or get the in-game tags/variables for the game I want? 
      Any assistance on this would be most appreciated. I been everywhere and seems no one knows or is to lazy to help me out. It would be nice to have assistance for once. I don't know what I need to learn. 
      So links of topics I need to learn within the comment section would be SOOOOO.....Helpful. Anything to just get me started. 
      Thanks, 
      Dejay Hextrix 
    • 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.
  • Popular Now