tenderlove · December 12, 2020 14:09 · Dec 9, 2020 · Dec 8, 2020
diff --git a/ngpng.rb b/ngpng.rb
@@ -68,8 +68,12 @@ def save filename
   # v needs to be a palette index
   def []= x, y, v
     index = (@stride * y) + (x / 2) + 1
-    v <<= 4 if x % 2 == 0
-    @buffer.setbyte(index, @buffer.getbyte(index) | v)
+    mask = 0xF
+    if x % 2 == 0
+      mask <<= 4
+      v <<= 4
+    end
+    @buffer.setbyte(index, (@buffer.getbyte(index) & ~mask) | v)
   end
 
   def write_chunk file, name, content
@@ -88,4 +92,4 @@ def write_chunk file, name, content
     png[x, y] = rand(3)
   end
 end
-png.save "out.png"
+png.save "out.png"
diff --git a/ngpng.rb b/ngpng.rb
@@ -0,0 +1,91 @@
+# Not Great PNG class.  This is a very simple example of writing a PNG.  It
+# only supports colors from the color palette stored in `@palette`.  This is
+# meant to be example code, but I am using it in a program for visualizing
+# heap dumps from Ruby.
+#
+# This is free and unencumbered software released into the public domain.
+#
+# Anyone is free to copy, modify, publish, use, compile, sell, or
+# distribute this software, either in source code form or as a compiled
+# binary, for any purpose, commercial or non-commercial, and by any
+# means.
+#
+# In jurisdictions that recognize copyright laws, the author or authors
+# of this software dedicate any and all copyright interest in the
+# software to the public domain. We make this dedication for the benefit
+# of the public at large and to the detriment of our heirs and
+# successors. We intend this dedication to be an overt act of
+# relinquishment in perpetuity of all present and future rights to this
+# software under copyright law.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+# IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+# OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+# ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+# OTHER DEALINGS IN THE SOFTWARE.
+#
+# For more information, please refer to <https://unlicense.org>
+
+require "zlib"
+
+class NGPNG
+  HEADER = [137, 80, 78, 71, 13, 10, 26, 10].pack("C*")
+
+  def initialize width, height
+    @width  = width
+    @height = height
+    # Using half a byte, so everything needs to be divided by 2
+    @bits       = 4 # 4 bit depth
+    @color_type = 3 # only use palette colors
+    @palette = [
+      [0xFF, 0xFF, 0xFF], # default color is white
+      [0xFF, 0x00, 0x00],
+      [0x00, 0x66, 0x00],
+      [0x00, 0x00, 0x00],
+    ]
+    # add one for the filter. First 0 is always the filter
+    @stride = (width / 2) + (width % 2).clamp(0, 1) + 1
+    @buffer = "\0".b * (@stride * height)
+  end
+
+  def write_to file
+    file.write HEADER
+    ihdr = [@width, @height, @bits, @color_type, 0, 0, 0].pack("NNC5")
+    write_chunk file, "IHDR", ihdr
+    plte = @palette.flatten.pack("C*")
+    write_chunk file, "PLTE", plte
+    idat = Zlib::Deflate.deflate(@buffer, Zlib::DEFAULT_COMPRESSION)
+    write_chunk file, "IDAT", idat
+    write_chunk file, "IEND", "".b
+  end
+
+  def save filename
+    File.open(filename, "wb") { |f| write_to f }
+  end
+
+  # v needs to be a palette index
+  def []= x, y, v
+    index = (@stride * y) + (x / 2) + 1
+    v <<= 4 if x % 2 == 0
+    @buffer.setbyte(index, @buffer.getbyte(index) | v)
+  end
+
+  def write_chunk file, name, content
+    file.write [content.bytesize].pack("N")
+    file.write name
+    file.write content
+    file.write [Zlib.crc32(content, Zlib.crc32(name))].pack("N")
+  end
+end
+
+# Sample usage
+height = width = 100
+png = NGPNG.new height, width
+width.times do |x|
+  height.times do |y|
+    png[x, y] = rand(3)
+  end
+end
+png.save "out.png"