State setting.

gistfile1.c

static int set_render_state(GDrawRenderState *r, /* ... */)
{
   static struct gdraw_gl_blendspec {
      GLboolean enable;
      GLenum src;
      GLenum dst;
   } blends[ASSERT_COUNT(GDRAW_BLEND__count, 6)] = {
      { GL_FALSE, GL_ONE,        GL_ZERO                },  // GDRAW_BLEND_none
      { GL_TRUE,  GL_ONE,        GL_ONE_MINUS_SRC_ALPHA },  // GDRAW_BLEND_alpha
      { GL_TRUE,  GL_DST_COLOR,  GL_ONE_MINUS_SRC_ALPHA },  // GDRAW_BLEND_multiply
      { GL_TRUE,  GL_ONE,        GL_ONE                 },  // GDRAW_BLEND_add

      { GL_FALSE, GL_ONE,        GL_ZERO                },  // GDRAW_BLEND_filter
      { GL_FALSE, GL_ONE,        GL_ZERO                },  // GDRAW_BLEND_special
   };

   int blend_mode = r->blend_mode;
   assert(blend_mode >= 0 && blend_mode < sizeof(blends)/sizeof(*blends));
   if (blends[blend_mode].enable) {
      glEnable(GL_BLEND);
      glBlendFunc(blends[blend_mode].src, blends[blend_mode].dst);
   } else
      glDisable(GL_BLEND);
      
   // and so forth.
}