Particle Vertex Array

bonanza

Unregistered

Post: #1

Jun 19, 2008 — 06:52 AM

Hi,

how i can render particles, with Vertex arrays. I have this simple structur

Code:

typedef struct _particle

{    

    float        finalColor[4];

    float        vertices[12];

.

.

.

}Particle;

init

Code:

_particles[i].vertices[0] =_particles[i].position[0]-(_particles[i].size/2);

_particles[i].vertices[1] =_particles[i].position[1]-(_particles[i].size/2);

_particles[i].vertices[2] =_particles[i].position[2];

_particles[i].vertices[3] =_particles[i].position[0]+(_particles[i].size/2);

_particles[i].vertices[4] =_particles[i].position[1]-(_particles[i].size/2);

_particles[i].vertices[5] =_particles[i].position[2];

_particles[i].vertices[6] =_particles[i].position[0]+(_particles[i].size/2);

_particles[i].vertices[7] =_particles[i].position[1]+(_particles[i].size/2);

_particles[i].vertices[8] =_particles[i].position[2];

_particles[i].vertices[9] =_particles[i].position[0]-(_particles[i].size/2);

_particles[i].vertices[10] =_particles[i].position[1]+(_particles[i].size/2);

_particles[i].vertices[11] =_particles[i].position[2];

and i like to render the particles like this:

Code:

glVertexPointer(3, GL_FLOAT, sizeof(Particle), &_particles[0].vertices);

glDrawArrays(GL_QUADS, 0, NUM_PARTICLES);

but, that seems to be wrong. Someone an idea?

ThemsAllTook

Moderator

Posts: 1,552
Joined: 2003.10

Post: #2

Jun 19, 2008 — 08:11 AM

Looks like there are two problems here:

The stride parameter to glVertexPointer is the number of bytes from the end of one vertex to the beginning of the next, as opposed to the number of bytes from the beginning of one vertex to the beginning of the next. This tripped me up, too. You'll want something like sizeof(struct _particle) - sizeof(_particles[0].vertices).
...However, this won't actually work, because the stride specifies the bytes between vertices, as opposed to the space between primitives. You may need to reorganize your data so that the space between each set of vertex coordinates is consistent. As it's currently set up, you have a 16-byte chunk between each quad, but the vertices in the quad itself are tightly packed.

bonanza

Unregistered

Post: #3

Jun 19, 2008 — 08:36 AM

Thanks,
but how can i do this.

Thats why i ask.

ThemsAllTook

Moderator

Posts: 1,552
Joined: 2003.10

Post: #4

Jun 19, 2008 — 10:36 AM

You'll need to model the data in your application in some way that allows lists of vertices to either be contiguous across the entire array you're drawing, or to have uniform byte spacing between each vertex across the entire array. How you accomplish this is really up to you; do whatever makes the most sense to fit into your current data model given the above constraints.

TomorrowPlusX

Sage

Posts: 1,199
Joined: 2004.10

Post: #5

Jun 19, 2008 — 11:31 AM

Here's how I arranged my data, for a foliage system ( a specialized particle system ).

Code:

struct foliage_vertex

{

    static const int INTERLEAVED_FORMAT = GL_T2F_C3F_V3F;

    inline foliage_vertex( void ) {}

    inline foliage_vertex( const foliage_vertex &c ):

        texcoord( c.texcoord ),

        color( c.color ),

        position( c.position )

    {}

    inline foliage_vertex( const vec2 &tc, const vec3 &c, const vec3 &p ):

        texcoord( tc ),

        color( c ),

        position( p )

    {}

    inline foliage_vertex &operator = ( const foliage_vertex &c )

    {

        texcoord = c.texcoord;

        color = c.color;

        position = c.position;

        return *this;

    }

    vec2 texcoord;

    vec3 color, position;

};

struct foliage_quad {

    foliage_vertex a,b,c,d;

    inline foliage_quad( void ) {}

    inline foliage_quad( const foliage_quad &f ):

        a( f.a ),

        b( f.b ),

        c( f.c ),

        d( f.d )

    {}

    inline foliage_quad &operator = ( const foliage_quad &f )

    {

        a = f.a;

        b = f.b;

        c = f.c;

        d = f.d;

        return *this;

    }

};

... later

std::vector< foliage_quad > _foliage;

And to submit it to GL:

Code:

glInterleavedArrays( foliage_vertex::INTERLEAVED_FORMAT, 0, &(_foliage.front()) );

glDrawArrays( GL_QUADS, 0, _numQuads * 4 );

glDisableClientState( GL_VERTEX_ARRAY );

glDisableClientState( GL_COLOR_ARRAY );

glDisableClientState( GL_TEXTURE_COORD_ARRAY );

Works for me!

Note: I'm billboarding in a vertex shader, so in principle I could use a VBO instead of a vertex array. BUT: I'm periodically depth-sorting the vertices, so I need to stream them. I could probably switch to a streaming or write-only vbo-format, but my foliage system allocation strategy is a little rough, and I don't want to blow out my vram.

kuon_

Apprentice

Posts: 9
Joined: 2008.01

Post: #6

Jun 19, 2008 — 07:31 PM

ThemsAllTook Wrote:Looks like there are two problems here:

The stride parameter to glVertexPointer is the number of bytes from the end of one vertex to the beginning of the next, as opposed to the number of bytes from the beginning of one vertex to the beginning of the next. This tripped me up, too. You'll want something like sizeof(struct _particle) - sizeof(_particles[0].vertices).

Err, are you sure about that? I just wrote some testing code to render a VBO and the stride is the number of byte from the start of a vertice to the next.

For example, if you have

Code:

// FIXME: dirty testing code

    size_t float_count = vertex_count * (3 + 3 + 2 + 2 + 1);

    buf_len = sizeof(GLfloat) * float_count;

    buf = malloc(buf_len);

    uint32_t i;

    for(i = 0; !feof(fd) && i < float_count;) {

        // FIXME: fscanf is dirty, only for testing, move to binary when working.

        vec3 v;

        // pos

        fscanf(fd, "%f %f %f\n", &v.f[0], &v.f[1], &v.f[2]);

        v = v_div3f(v, 100000);

        buf[i++] = v.f[0];

        buf[i++] = v.f[1];

        buf[i++] = v.f[2];

        // norm

        fscanf(fd, "%f %f %f\n", &v.f[0], &v.f[1], &v.f[2]);

        v = v_div3f(v, 100000);

        buf[i++] = v.f[0];

        buf[i++] = v.f[1];

        buf[i++] = v.f[2];

        // uv0

        fscanf(fd, "%f %f\n", &v.f[0], &v.f[1]);

        v = v_div3f(v, 100000);

        buf[i++] = v.f[0];

        buf[i++] = v.f[1];

        // uv1

        fscanf(fd, "%f %f\n", &v.f[0], &v.f[1]);

        v = v_div3f(v, 100000);

        buf[i++] = v.f[0];

        buf[i++] = v.f[1];

        // flags

        fscanf(fd, "%f\n", &v.f[0]);

        buf[i++] = v.f[0];

}

You will do something like:

Code:

#define glOffsetf(_x) ((const GLvoid *)(_x * sizeof(GLfloat)))

    #define glStridef(_x) (_x * sizeof(GLfloat))

    glEnableClientState(GL_VERTEX_ARRAY);

    glEnableClientState(GL_NORMAL_ARRAY);

    glEnableClientState(GL_TEXTURE_COORD_ARRAY);

    glBindBuffer(GL_ARRAY_BUFFER, vbo);

    glVertexPointer(3, GL_FLOAT, glStridef(11), glOffsetf(0));    

    glNormalPointer(GL_FLOAT, glStridef(11), glOffsetf(3));

    glClientActiveTexture(GL_TEXTURE0);

    glTexCoordPointer(2, GL_FLOAT, glStridef(11), glOffsetf(6));

    glClientActiveTexture(GL_TEXTURE1);

    glTexCoordPointer(2, GL_FLOAT, glStridef(11), glOffsetf(8));

    glEnableVertexAttribArray(attrib_loc);

    glVertexAttribPointer(attrib_loc, 1, GL_FLOAT, GL_FALSE, glStridef(11), glOffsetf(10));

    glDrawArrays(GL_TRIANGLES, 0, vertex_count);

Note that the stride is always 11 * sizeof(GLfloat).

This code is working properly (even if it's dirty).

OneSadCookie

Luminary

Posts: 5,125
Joined: 2002.04

Post: #7

Jun 19, 2008 — 07:54 PM

kuon is correct; stride is either 0 ("tightly packed") or number of bytes from the beginning of one vertex to the beginning of the next.

ThemsAllTook

Moderator

Posts: 1,552
Joined: 2003.10

Post: #8

Jun 19, 2008 — 10:37 PM

Hmm, my mistake. The man page isn't worded as clearly as it could be...

AnotherJake

Moderator

Posts: 3,554
Joined: 2003.06

Post: #9

Jun 19, 2008 — 11:41 PM

ThemsAllTook Wrote:The man page isn't worded as clearly as it could be...

... Nice save!

bonanza

Unregistered

Post: #10

Jun 27, 2008 — 07:46 AM

Thanks for suggestions

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