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Mythomania

OpenGL Mapping DirectX to OpenGL

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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.

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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.

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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)?

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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.

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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 !!!

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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...

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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

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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?

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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(..)

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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

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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?


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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

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