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ecco_the_dolphin

OpenGL is it legal to pass invalid pointer to glTexCoordPointer

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I have very big array of vertex data (around 5 mb). I want to draw textured lines. But I dont want to store texture coordinates. I can calculate them during runtime. I do the following:
void __fastcall CGraphicCore::_SurfaceDrawTextureLine
                                           //Pointer to primitive
                                           (const Primitive_In * ptr_prim
                                           //Some additional info
                                           ,CDrawRule * rule)
{
    _ASSERTE(ptr_prim->PointsCount()>1);
    size_t first_point_index = ptr_prim->GetFirstPointIndex();
    size_t points_number     = ptr_prim->PointsCount();
    //rule->first_point_ptr - it is a pointer to the first point in vertex array
    const fvec2_s * cur_pnt_ptr    = rule->first_point_ptr + first_point_index;
    const fvec2_s * border_pnt_ptr = cur_pnt_ptr + points_number;

    double * tex_mem;
    //this is function for getting memory from buffer (actually template function)
    //it calls realloc(buffer_data_,sizeof(double)*elements_number)
    rule->additional_buffer_prt->GetRawMemory(points_number,&tex_mem);
    double * ptr_tex_storage = tex_mem;

    double tex_crd = 0;
    fvec2_s prev_pnt;
    fvec2_s cur_pnt;
    prev_pnt = *cur_pnt_ptr;
    cur_pnt  = *cur_pnt_ptr;    
    while (cur_pnt_ptr!=border_pnt_ptr)
    {
        cur_pnt = *cur_pnt_ptr;
        tex_crd = //do some calculations
        *ptr_tex_storage = tex_crd;
        ++ptr_tex_storage;
        ++cur_pnt_ptr;
        prev_pnt = cur_pnt;
    }
    //HACK: possible solution: modify Vertexpointer and in Deinit function reset it to first element.
    glTexCoordPointer(1,GL_DOUBLE,0,tex_mem - first_point_index);
    glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    //render primitive
    glDrawArrays(GL_LINE_STRIP,first_point_index,points_number);
    glDisableClientState(GL_TEXTURE_COORD_ARRAY);
    
    NDebugTool::DetectGLError();
    return;
}





The question is: can I set glTexCoordPointer(1,GL_DOUBLE,0,tex_mem - first_point_index) with (tex_mem - first_point_index)? I mean is it legal to pass invalid pointer to glTexCoordPointer if ("illegal pointer" + offset) == valid pointer? This code is working for me, but maybe it will not work with some opengl implementation... [Edited by - ecco_the_dolphin on June 8, 2009 2:08:20 PM]

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It doesn't *sound* like a good idea. Why? Well, an implementation might well do something to optimise access like (say), the first time you access element N, upload 0..N so that it has it all in local memory. You can't tell if it'll do something like that, as far as I know.

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Thaks for reply.
You mean that implementation can cashe vertex coordinates (or texture coordinates) which I specify in glVertexPointer (or glTexCoordPointer)?

Then if i write something like this :


...
struct fvect2d
{
float x;
float y;
};
....
void DrawSomething ()
{
fvect2d vertex_data[3];

vertex_data[0].x = 0;
vertex_data[0].y = 0;

vertex_data[1].x = 1.f;
vertex_data[1].y = 1.f;

vertex_data[2].x = 1.f;
vertex_data[2].y = 0;

glVertexPointer(2,GL_FLOAT,0,&vertex_data[0].x);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLES,0,3);

//I modify vertex_data during enabled vertex array
vertex_data[1].x = -1.f;
vertex_data[1].y = -1.f;

glDrawArrays(GL_TRIANGLES,0,3);
glDisableClientState(GL_VERTEX_ARRAY);
}




then, some implementation can display only first triangle? And to draw 2 triangles I must Disable/Enable array every time i modify vertex data?

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Quote:
Original post by ecco_the_dolphin
I mean is it legal to pass invalid pointer to glTexCoordPointer if ("illegal pointer" + offset) == valid pointer?
No. Doing so is undefined behavior, technically. If you have a pointer to a block of allocated memory, you are only allowed to do pointer arithmetic within the range of the block of allocated memory (plus 1 past the end). There was a nice discussion of pointer arithmetic and the undefined-ness of making pointers point outside of this range.

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Quote:
No. Doing so is undefined behavior, technically. If you have a pointer to a block of allocated memory, you are only allowed to do pointer arithmetic within the range of the block of allocated memory (plus 1 past the end). There was a nice discussion of pointer arithmetic and the undefined-ness of making pointers point outside of this range.


Thanks a lot! I didn't knew that. According to ISO/IEC 14882:2003 (it is C++ standart) section 5.7 :
Quote:

...
5.
When an expression that has integral type is added to or subtracted from a pointer, the result has the type of
the pointer operand. If the pointer operand points to an element of an array object, and the array is large
enough, the result points to an element offset from the original element such that the difference of the subscripts
of the resulting and original array elements equals the integral expression. In other words, if the
expression P points to the i-th element of an array object, the expressions (P)+N (equivalently, N+(P))
and (P)-N (where N has the value n) point to, respectively, the i+n-th and i–n-th elements of the array
object, provided they exist. Moreover, if the expression P points to the last element of an array object, the
expression (P)+1 points one past the last element of the array object, and if the expression Q points one
past the last element of an array object, the expression (Q)-1 points to the last element of the array object.
If both the pointer operand and the result point to elements of the same array object, or one past the last element
of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined.
...


[Edited by - ecco_the_dolphin on June 9, 2009 8:15:45 PM]

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