anchor.diff:                  for py fixnum from yp by 3
                   for pu fixnum = (1+ py)
                   for pv fixnum = (plus py 2)
  -                for yy fixnum = (svref buffer py)
  -                for cb fixnum = (svref buffer pu)
  -                for cr fixnum = (svref buffer pv) do
  -                (setf (svref buffer py) ; BLUE
  -                      (the fixnum (limit (plus yy (svref *cb-b-tab* cb)))))
  -                (setf (svref buffer pu) ; GREEN
  -                      (the fixnum (limit (plus yy (ash (plus
  -                                                        (svref *cb-g-tab* cb)
  -                                                        (svref *cr-g-tab* cr))
  -                                                       (- shift))))))
  -                (setf (svref buffer pv) ; RED
  -                      (the fixnum (limit (plus yy (svref *cr-r-tab* cr)))))))))
  +                for yy fixnum = (aref buffer py)
  +                for cb fixnum = (aref buffer pu)
  +                for cr fixnum = (aref buffer pv) do
  +                (setf (aref buffer py) ; BLUE
  +                      (the uint8 (limit (plus yy (svref *cb-b-tab* cb)))))
  +                (setf (aref buffer pu) ; GREEN
  +                      (the uint8 (limit (plus yy (ash (plus
  +						       (svref *cb-g-tab* cb)
  +						       (svref *cr-g-tab* cr))
  +						      (- shift))))))
  +                (setf (aref buffer pv) ; RED
  +                      (the uint8 (limit (plus yy (svref *cr-r-tab* cr)))))))))
   
   (defun decode-frame-beginning (image s)
     (read-byte s) ; length
  
positive.code: (defconstant +FIX-3-072711026+ (round (+ (* 3.072711026 (ash 1 dct-shift)) 0.5)))
  
  ;;; Post-IDCT limiting array
  (defvar *idct-limit-array* (make-array 512 :initial-element 0))
  (loop for n from 0
        for i from 128 to 383 do
        (setf (svref *idct-limit-array* i) n))
  (loop for i from 384 to 511 do
        (setf (svref *idct-limit-array* i) 255))
  
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;; Encoder part
  
negative.code:   (loop for row across buffer
          for z-row across +zigzag-index+ do
          (loop for x fixnum from 0 to 7 do
                (setf (aref *zz-result* (aref z-row x))
                      (the fixnum (aref row x)))))
    *zz-result*)
  
  ;;; Writes frame header
  