#define PACK_LUMINANCE(COUNT, RGBA, DST, FORMAT, TYPE, CONVERT)              \
   do {                                                                      \
      int i;                                                                 \
      int bits = sizeof(TYPE) * 8;                                           \
      TYPE *luminance = (TYPE *) DST;                                        \
      switch (FORMAT) {                                                      \
      case GL_LUMINANCE_INTEGER_EXT:                                         \
         for (i = 0; i < COUNT; i++) {                                       \
            luminance[i] =                                                   \
               CONVERT(RGBA[i][RCOMP] + RGBA[i][GCOMP] + RGBA[i][BCOMP], bits); \
         }                                                                   \
         return;                                                             \
      case GL_LUMINANCE_ALPHA_INTEGER_EXT:                                   \
         for (i = 0; i < COUNT; i++) {                                       \
            luminance[2*i] =                                                 \
               CONVERT(RGBA[i][RCOMP] + RGBA[i][GCOMP] + RGBA[i][BCOMP], bits); \
            luminance[2*i+1] = CONVERT(RGBA[i][ACOMP], bits);                \
         }                                                                   \
      }                                                                      \
   } while(0)

void
_mesa_pack_luminance_from_rgba_integer(GLuint n,
                                       GLuint rgba[][4], bool rgba_is_signed,
                                       GLvoid *dstAddr,
                                       GLenum dst_format,
                                       GLenum dst_type)
{
   assert(dst_format == GL_LUMINANCE_INTEGER_EXT ||
          dst_format == GL_LUMINANCE_ALPHA_INTEGER_EXT);

   switch (dst_type) {
   case GL_BYTE:
      if (rgba_is_signed) {
         PACK_LUMINANCE(n, rgba, dstAddr, dst_format, GLbyte,
                        _mesa_signed_to_signed);
      } else {
         PACK_LUMINANCE(n, rgba, dstAddr, dst_format, GLbyte,
                        _mesa_unsigned_to_signed);
      }
      return;
   case GL_UNSIGNED_BYTE:
      if (rgba_is_signed) {
         PACK_LUMINANCE(n, rgba, dstAddr, dst_format, GLubyte,
                        _mesa_signed_to_unsigned);
      } else {
         PACK_LUMINANCE(n, rgba, dstAddr, dst_format, GLubyte,
                        _mesa_unsigned_to_unsigned);
      }
      return;
   case GL_SHORT:
      if (rgba_is_signed) {
         PACK_LUMINANCE(n, rgba, dstAddr, dst_format, GLshort,
                        _mesa_signed_to_signed);
      } else {
         PACK_LUMINANCE(n, rgba, dstAddr, dst_format, GLshort,
                        _mesa_unsigned_to_signed);
      }
      return;
   case GL_UNSIGNED_SHORT:
      if (rgba_is_signed) {
         PACK_LUMINANCE(n, rgba, dstAddr, dst_format, GLushort,
                        _mesa_signed_to_unsigned);
      } else {
         PACK_LUMINANCE(n, rgba, dstAddr, dst_format, GLushort,
                        _mesa_unsigned_to_unsigned);
      }
      return;
   case GL_INT:
      if (rgba_is_signed) {
         PACK_LUMINANCE(n, rgba, dstAddr, dst_format, GLint,
                        _mesa_signed_to_signed);
      } else {
         PACK_LUMINANCE(n, rgba, dstAddr, dst_format, GLint,
                        _mesa_unsigned_to_signed);
      }
      return;
   case GL_UNSIGNED_INT:
      if (rgba_is_signed) {
         PACK_LUMINANCE(n, rgba, dstAddr, dst_format, GLuint,
                        _mesa_signed_to_unsigned);
      } else {
         PACK_LUMINANCE(n, rgba, dstAddr, dst_format, GLuint,
                        _mesa_unsigned_to_unsigned);
      }
      return;
   default:
      assert(!"Unsupported type");
   }
}