api - what is the opengl current vertex? -


this found in opengl documentation:

void glgetvertexattribfv(gluint index,  glenum pname,  glfloat *params);  gl_current_vertex_attrib              params returns 4         values represent current value         generic vertex attribute specified index. generic         vertex attribute 0 unique in has no         current state, error generated if         index 0. initial value         other generic vertex attributes         (0,0,0,1).             glgetvertexattribdv , glgetvertexattribfv             return current attribute values 4 single-precision floating-point values;             glgetvertexattribiv reads them floating-point values ,             converts them 4 integer values; glgetvertexattribiiv ,             glgetvertexattribiuiv read , return them signed or unsigned             integer values, respectively; glgetvertexattribldv reads , returns             them 4 double-precision floating-point values.

but problem is, have no idea current vertex is. how set current vertex? can use test weather attributes i've sent opengl contain correct data?

glgetvertexattrib returns value associated generic vertex attribute, can set glvertexattrib function.

in more detail, there 2 types of attributes available in opengl:

  1. those updated each vertex enabled vertex arrays processed when draw call (e.g., gldrawarrays) executed.
  2. those sort of act shader uniforms, used provide values attributes don't have associated, enabled set vertex array.

for example, let's enable 2 arrays use vertex attributes using glvertexattribpointer, , glenablevertexattribarray, , render calling gldrawarrays( gl_points, 0, numpoints ). in case, bound vertex shader executed numpoints times, each time new value read each of 2 arrays.

however, if enabled on array (say, vertex array zero), set attribute value attribute 1 using of glvertexattrib functions. if once again rendering calling gldrawarrays( gl_points, 0, numpoints ), vertex shader again executed numpoints times, attribute zero's values being update values enabled vertex attribute array, attribute one's values being "constant", , set value provided glvertexattrib.


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