Archived

This topic is now archived and is closed to further replies.

Mythomania

OpenGL Mapping DirectX to OpenGL

Recommended Posts

Mythomania    122
Hello everybody, I''m developing a 3D engine using DirectX which I plan to port to other OS/APIs. Since I don''t know much about OpenGL I have problems designing my interface classes. e.g. I have a vertex buffer class which behaves much like the DX8 vertex buffers do. I read a bit about OpenGL, and specifying vertices seems to be different there. That confuses me because I''m pretty sure that the DX vertex buffers are laid out in the same way as in hardware. And because I want a fast engine (of course) I need to know how to treat the vertices in OpenGL. I hope you understand what the problem is, my description sounds a bit strange... And BTW: I need someone else to help with the engine since it''s quite a bit of work. If somebody''s interested, just send me an email and I''ll tell you more.

Share this post


Link to post
Share on other sites
masonium    118
DirectX uses vertex buffers and custom vertices, like you already know. You can specify OpenGL vertices in two different ways. YOu can either do vertex by vertex, like shown here:

glBegin(GL_TRIANGLES);
glTexCoord2f(0.0f, 0.0f); glVertex3f(0.0f, 0.0f, 0.0f);
glTexCoord2f(0.0f, 1.0f); glVertex3f(1.0f, 0.0f, 0.0f);
glTexCoord2f(1.0f, 0.0f); glVertex3f(0.0f, 1.0f, 0.0f);
glEnd();

or use vertex arrays. I''m not too familiar with vertex arrays, so I might give you the wrong info if I post some code. But, the basic concept is that you have an array of floats containing all of your vertex information or one containing the tex coords, one containing the locations, one containing the normals, etc. You specify the arrays through the function gl*Pointer, * representing Vertex, Normal, etc. Then you use either glDrawElements, glDrawRangeElements, or some other function.

Share this post


Link to post
Share on other sites
Mythomania    122
thanx maonium.
new to me is, that i can put all vertex data in one array. this is the way opengl (and i think hardware) does it. but i couldn''t find anything about that in the opengl 1.2 specification.
can anybody point me to some links where these kind of vertex arrays is specified (or maybe other links which explain how to do opengl things just like in dx/hardware)?

Share this post


Link to post
Share on other sites
Shag    122
have a look at Vertex Arrays here

http://ask.ii.uib.no/ebt-bin/nph-dweb/dynaweb/SGI_Developer/OpenGL_PG/@ebt-link;cs=fullhtml;pt=38709?target=%25N%14_4472_START_RESTART_N%25#X

That will give you the basix background to what you are trying to achieve, but note that this method has been superceded using OpenGL extensions ... nvidia for example use NV_VERTEX ARRAY_RANGE.

Share this post


Link to post
Share on other sites
Guest Anonymous Poster   
Guest Anonymous Poster
Humm....
Interesting link...

http://nehe.gamedev.net

Sure that after their 32 tutorials(the best I''ve seen),
you''ll be able to finish your project without
any prob''...

Go check it out !!!

Share this post


Link to post
Share on other sites
Mythomania    122
ok, another question:

how are more complex vertex formats achieved in ogl? as far as i can see only simple vertex types are supported...
what if i want to use an additional color(specular), several (3d)textures or indexed vertex blending?
ogl doesnt seem to support these things...

Share this post


Link to post
Share on other sites
zedzeek    529
EXT_secondary_color
EXT_separate_specular_color
3dtextures doesnt really make sense, do u mean 3d texture coordinates?
EXT_vertex_weighting

Share this post


Link to post
Share on other sites
meZmo    122
Hava a look at this for volumetric textures:

EXT_texture3D - http://oss.sgi.com/projects/ogl-sample/registry/EXT/texture3D.txt


Edited by - meZmo on November 12, 2001 2:57:31 PM

Share this post


Link to post
Share on other sites
Mythomania    122
thanx a lot everybody.
i''m slowly starting to understand how opengl works...
i just don''t like this extension stuff...do all the developers have different extensions each?

and im still not sure whether i have understood how vertex arrays work. let me summarize:

one can either store the different parts of one vertex in separate arrays OR store everything in one array. i can imagine how the first possibility would be coded, but from the docs i cant figure out the second one.
could somebody post a little example please?
and i would also like to know if its possible to add the extended vertex parts like vertex weighting into one array with the others or if i have to use separate arrays in that case?

Share this post


Link to post
Share on other sites
zedzeek    529
basically opengl unlike d3d is a state machine thus anything turned on will remain on until u turn it off,
typically all the vertex arrays stuff is off when u start.

*beware* the syntax below is most likely wrong
glEnableClientState( GL_VERTEX_ARRAY ); vertices ON
glEnableClientState( GL_TEXTURE_COORD_ARRAY ); texture coords on
glEnableClientState( GL_VERTEX_WEIGHTING_ARRAY ) vertex weighting ON

// where is all the info stored that u want to draw

VertexWeightPointer( ... , ptr_weightweightsdata );
TexCoordPointer( ... , ptr_texcoordsdata );
VertexArrayPointer( ... , ptr_verticesdata );

// draw it

glDrawArrays(.. ) or drawelements(..)

Share this post


Link to post
Share on other sites
a2k    124
i''m not sure of the syntax, but for having just one array, you''d be using glInterleavedArray or something like that. this means that the array contains data like vertex, normal, color, vertex, normal, color, vertex, normal, color.... etc. so, different types of data are thrown into one array, as opposed to several arrays of different data.

a2k

Share this post


Link to post
Share on other sites
Mythomania    122
I think I understand now. When I want to specify one vertex array like in D3D with a vertex format that contains position, normal and color I would do the following:

struct Vertex
{
float x, y, z;
float normalx, normaly, normalz;
DWord color;
};

Vertex vertices[100];

//fill vertices

//and now the important bit:

VertexPointer(3, FLOAT, sizeof(Vertex), vertices);
NormalPointer(FLOAT, sizeof(Vertex), vertices+12);
ColorPointer(4, BYTE, sizeof(Vertex), vertices+24);

This would work, right?

Now the last question: How does OGL work together with video and agp mem now? When is the data put into this mem? And how does OGL decide where?


Share this post


Link to post
Share on other sites
Vlion    151
The OpenGL Red Book has been placed online somewhere.
That would answer alot of your questions.
No, I don`t know where it is- does anybody else know ?
~V''lion

I came, I saw, I got programmers block.
~V''''lion

Share this post


Link to post
Share on other sites

  • Similar Content

    • By pseudomarvin
      I assumed that if a shader is computationally expensive then the execution is just slower. But running the following GLSL FS instead just crashes
      void main() { float x = 0; float y = 0; int sum = 0; for (float x = 0; x < 10; x += 0.00005) { for (float y = 0; y < 10; y += 0.00005) { sum++; } } fragColor = vec4(1, 1, 1 , 1.0); } with unhandled exception in nvoglv32.dll. Are there any hard limits on the number of steps/time that a shader can take before it is shut down? I was thinking about implementing some time intensive computation in shaders where it would take on the order of seconds to compute a frame, is that possible? Thanks.
    • By Arulbabu Donbosco
      There are studios selling applications which is just copying any 3Dgraphic content and regenerating into another new window. especially for CAVE Virtual reality experience. so that the user opens REvite or CAD or any other 3D applications and opens a model. then when the user selects the rendered window the VR application copies the 3D model information from the OpenGL window. 
      I got the clue that the VR application replaces the windows opengl32.dll file. how this is possible ... how can we copy the 3d content from the current OpenGL window.
      anyone, please help me .. how to go further... to create an application like VR CAVE. 
       
      Thanks
    • By cebugdev
      hi all,

      i am trying to build an OpenGL 2D GUI system, (yeah yeah, i know i should not be re inventing the wheel, but this is for educational and some other purpose only),
      i have built GUI system before using 2D systems such as that of HTML/JS canvas, but in 2D system, i can directly match a mouse coordinates to the actual graphic coordinates with additional computation for screen size/ratio/scale ofcourse.
      now i want to port it to OpenGL, i know that to render a 2D object in OpenGL we specify coordiantes in Clip space or use the orthographic projection, now heres what i need help about.
      1. what is the right way of rendering the GUI? is it thru drawing in clip space or switching to ortho projection?
      2. from screen coordinates (top left is 0,0 nd bottom right is width height), how can i map the mouse coordinates to OpenGL 2D so that mouse events such as button click works? In consideration ofcourse to the current screen/size dimension.
      3. when let say if the screen size/dimension is different, how to handle this? in my previous javascript 2D engine using canvas, i just have my working coordinates and then just perform the bitblk or copying my working canvas to screen canvas and scale the mouse coordinates from there, in OpenGL how to work on a multiple screen sizes (more like an OpenGL ES question).
      lastly, if you guys know any books, resources, links or tutorials that handle or discuss this, i found one with marekknows opengl game engine website but its not free,
      Just let me know. Did not have any luck finding resource in google for writing our own OpenGL GUI framework.
      IF there are no any available online, just let me know, what things do i need to look into for OpenGL and i will study them one by one to make it work.
      thank you, and looking forward to positive replies.
    • By fllwr0491
      I have a few beginner questions about tesselation that I really have no clue.
      The opengl wiki doesn't seem to talk anything about the details.
       
      What is the relationship between TCS layout out and TES layout in?
      How does the tesselator know how control points are organized?
          e.g. If TES input requests triangles, but TCS can output N vertices.
             What happens in this case?
      In this article,
      http://www.informit.com/articles/article.aspx?p=2120983
      the isoline example TCS out=4, but TES in=isoline.
      And gl_TessCoord is only a single one.
      So which ones are the control points?
      How are tesselator building primitives?
    • By Orella
      I've been developing a 2D Engine using SFML + ImGui.
      Here you can see an image
      The editor is rendered using ImGui and the scene window is a sf::RenderTexture where I draw the GameObjects and then is converted to ImGui::Image to render it in the editor.
      Now I need to create a 3D Engine during this year in my Bachelor Degree but using SDL2 + ImGui and I want to recreate what I did with the 2D Engine. 
      I've managed to render the editor like I did in the 2D Engine using this example that comes with ImGui. 
      3D Editor preview
      But I don't know how to create an equivalent of sf::RenderTexture in SDL2, so I can draw the 3D scene there and convert it to ImGui::Image to show it in the editor.
      If you can provide code will be better. And if you want me to provide any specific code tell me.
      Thanks!
  • Popular Now