How can i pick items by using glReadPixels ?
Is any other way to pick except glupickMatrix ?

  1. disable lighting and texturing, enable depth test.

  2. draw all elements in the back buffer,
    giving each element a unique color.

    using 24-bit color mode makes it easier to encode
    the element into a color value.

    red = element_id & 0xff;
    green = (element_id / 256) & 0xff;
    blue = (element_id / 65536) & 0xff;

    glColor3ub(red, green, blue);

    do NOT use the float/short/int version of glColor,
    because the red/green/blue values may be internally rounded.

  3. use glFlush, do NOT swap buffers. read the back buffer at pointer position.
    the color value tells you which element you
    clicked on.

    assuming you read the pixel color into a field col[3] you get the element id

    element_id = col[0]&0xff + 256*(col[1]&0xff) + 65536*(col[2]&0xff);