CODEWITHSUNDEEP
apiopenglopengl-esopengl-es-2.0
Arne
Arne

Reputation: 8509

what is the opengl current vertex?

this is what I found in the opengl documentation:

void glGetVertexAttribfv(GLuint index,  GLenum pname,  GLfloat *params);

GL_CURRENT_VERTEX_ATTRIB
             params returns four
        values that represent the current value for the
        generic vertex attribute specified by index. Generic
        vertex attribute 0 is unique in that it has no
        current state, so an error will be generated if
        index is 0. The initial value
        for all other generic vertex attributes is
        (0,0,0,1).
            glGetVertexAttribdv and glGetVertexAttribfv
            return the current attribute values as four single-precision floating-point values;
            glGetVertexAttribiv reads them as floating-point values and
            converts them to four integer values; glGetVertexAttribIiv and
            glGetVertexAttribIuiv read and return them as signed or unsigned
            integer values, respectively; glGetVertexAttribLdv reads and returns
            them as four double-precision floating-point values.

but my problem is, I have no idea what the current vertex is. How do I set the current vertex? Can I use this to test weather my attributes that I've sent to opengl contain correct data?

Upvotes: 2

Views: 817

Answers (1)

radical7
radical7

Reputation: 9144

glGetVertexAttrib returns the value associated with a generic vertex attribute, which can be set with the glVertexAttrib function.

In more detail, there are two types of attributes available in OpenGL:

  1. those that are updated for each vertex from the enabled vertex arrays that are processed when a draw call (e.g., glDrawArrays) is executed.
  2. those that sort of act like shader uniforms, which are used to provide values for attributes that don't have an associated, enabled set vertex array.

For example, let's say you enable two arrays for use as vertex attributes using glVertexAttribPointer, and glEnableVertexAttribArray, and then you render by calling glDrawArrays( GL_POINTS, 0, NumPoints ). In this case, the bound vertex shader will be executed NumPoints times, each time a new value read from each of the two arrays.

However, if you only enabled on array (say, vertex array zero), you could set an attribute value for attribute one using of the glVertexAttrib functions. If you once again rendering by calling glDrawArrays( GL_POINTS, 0, NumPoints ), the vertex shader would again be executed NumPoints times, with attribute zero's values being update with values from the enabled vertex attribute array, but attribute one's values being "constant", and set to the value provided by glVertexAttrib.

Upvotes: 3

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