notes

pitfall/pitfall/png-reader.rkt

@@ -9,18 +9,19 @@ Grab key chunks from PNG. Doesn't require heavy lifting from libpng.
   ((or/c input-port? bytes?) . -> . hash?)
   (define png (make-hasheq))
   (parameterize ([current-input-port (if (input-port? ip-or-bytes)
-                 ip-or-bytes
-                 (open-input-bytes ip-or-bytes))])
+                                         ip-or-bytes
+                                         (open-input-bytes ip-or-bytes))])
     (define header (read-bytes 8))
     (let loop ()
       (cond
         [(eof-object? (peek-byte)) png]
         [else
-         (define chunk-size (readUInt32))
+         (define chunk-size (read-32bit-integer))
          (define chunk-name (read-bytes 4))
          (case chunk-name
-           [(#"IHDR") (hash-set*! png 'width (readUInt32)
-                                  'height (readUInt32)
+           [(#"IHDR") (hash-set*! png
+                                  'width (read-32bit-integer)
+                                  'height (read-32bit-integer)
                                   'bits (read-byte)
                                   'colorType (read-byte)
                                   'compressionMethod (read-byte)
@@ -28,28 +29,28 @@ Grab key chunks from PNG. Doesn't require heavy lifting from libpng.
                                   'interlaceMethod (read-byte))]
            [(#"PLTE") (hash-set*! png 'palette (read-bytes chunk-size))]
            [(#"IDAT") (hash-set*! png 'imgData (read-bytes chunk-size))]
-           [(#"tRNS") (read-bytes chunk-size)]
-           [(#"tEXt") (read-bytes chunk-size)]
-           [(#"IEND") (hash-set! png 'colors
-                                (case (hash-ref png 'colorType)
-                                  [(0 3 4) 1]
-                                  [(2 6) 3]))
-                     (hash-set! png 'hasAlphaChannel (member (hash-ref png 'colorType) '(4 6)))
-                     (define colors (+ (hash-ref png 'colors) (if (hash-ref png 'hasAlphaChannel) 1 0)))
-                     (hash-set! png 'pixelBitlength (* (hash-ref png 'bits) colors))
-                     (hash-set! png 'colorSpace
-                                (case (hash-ref png 'colors)
-                                  [(1) "DeviceGray"]
-                                  [(3) "DeviceRGB"]))]
+           [(#"tRNS") (read-bytes chunk-size)] ; todo implement this section
+           [(#"tEXt") (read-bytes chunk-size)] ; todo implement this section
+           [(#"IEND") (define color-value (case (hash-ref png 'colorType)
+                                            [(0 3 4) 1]
+                                            [(2 6) 3]))
+                      (define alpha-value (member (hash-ref png 'colorType) '(4 6)))
+                      (hash-set*! png
+                                  'colors color-value
+                                  'hasAlphaChannel alpha-value
+                                  'pixelBitlength (* (hash-ref png 'bits) (+ color-value (if alpha-value 1 0)))
+                                  'colorSpace (case color-value
+                                                [(1) "DeviceGray"]
+                                                [(3) "DeviceRGB"]))]
            [else (read-bytes chunk-size)])
          (read-bytes 4) ; skip crc
          (loop)]))))
 
-(define (readUInt32)
+(define (read-32bit-integer)
   (integer-bytes->integer (read-bytes 4) #t #t))
 
 (module+ test
   (require rackunit)
   (check-equal?
-  (read-png (open-input-file "test/assets/test.png"))
-  (read-png (file->bytes "test/assets/test.png"))))
+   (read-png (open-input-file "test/assets/test.png"))
+   (read-png (file->bytes "test/assets/test.png"))))