wolfenstein affine floor texturing

;; so this is code I wrote to draw the floor and ceiling
;; in an old wolfenstein game I made
;; I couldn't figure out how to get perspective correct texture
;; sampling to work, so I ended up throwing this together

;; basically, it walks down from the bottom of the wall on each pixel
;; column , and draws multiple smaller polygons to distract from the
;; texture distortion that you get with affine texture sampling.

;; e.g. instead of drawing the floor here as one polygon

|------|
| wall |
--------
\ floor|
 \     |
  \    |
   -----

;; it split the vertical space on the screen up into mutiple polygons
|------|
| wall |
--------
\------|
 \-----|
  \----|
   -----

;; This is also done on a per vertical pixel column basis, so it's fairly slow

(expand-samples
 ;; expand samples takes the gl:tex-coord and gl:vertex commands verbatim and splices them into
 ;; a single gl:begin block.  The last chunk passed in is an epilogue.  I
 ;; don't remember why it works that way



 ;;;   loop from bottom of wall on this pixel column down to the bottom of the screen
 ;;; if inc is smaller, the texture sampling will be more accurate, but slower
 (loop for y from start-floor by inc
       for i from 1
    while (<= i (1- floor-samples)) collect
    ;; loop from i until we reach the maximum number of samples allowed

    ;; variable bindings (with type declarations)

             ;; distance to current pixel?
      (*let ((cur-dist single-float (calc-dist y height))
             ;; i don't remember what the weight was
             (cur-weight single-float (/ cur-dist wall-dist))
             ;; floor position
             (cur-floor-x single-float (floor-texel cur-weight floor-x-wall pos-x))
             (cur-floor-y single-float (floor-texel cur-weight floor-y-wall pos-y))
             ;; affine texture coords
             (cur-tex-x integer (tex-coord cur-floor-x +texture-width+))
             (cur-tex-y integer (tex-coord cur-floor-y +texture-height+)))

            (prog2
                (list
                 ;; floor samples
                 (gl:tex-coord prev-tex-x prev-tex-y)
                 (gl:vertex x prev-pix-y)
                 (gl:tex-coord cur-tex-x cur-tex-y)
                 (gl:vertex x y)
                 ;; ceiling samples
                 (gl:tex-coord prev-tex-x prev-tex-y)
                 (gl:vertex x (- height prev-pix-y))
                 (gl:tex-coord cur-tex-x cur-tex-y)
                 (gl:vertex x (- height y)))
                ;; move texture positions for next polygon
                (setf prev-tex-x cur-tex-x
                      prev-tex-y cur-tex-y
                      prev-pix-y y))))
 ;; epilogue
 ;;floor
 ((gl:tex-coord prev-tex-x prev-tex-y)
  (gl:vertex x prev-pix-y)
  (gl:tex-coord near-tex-x near-tex-y)
  (gl:vertex x (- height 1))
  ;; ceiling
  (gl:tex-coord prev-tex-x prev-tex-y)
  (gl:vertex x (- height prev-pix-y))
  (gl:tex-coord near-tex-x near-tex-y)
  (gl:vertex x 1)))

;; even after that, the are some really nasty artifacts at the edges of two floor chunks
                                               /
|------|                                      |
| wall |  ------------------------------------|  side-wall
--------   no-wall, draw floor from horizon   |
\      | <- nasty texture mis-alignment       |
 \     |    on this edge here                 |
  \    |                                       \
   \----                                         \