2019 day 8: parts 1 & 2

2019/day08/README.org

@@ -43,10 +43,57 @@ would like you to find the layer that contains the /fewest =0= digits/.
 On that layer, what is /the number of =1= digits multiplied by the
 number of =2= digits?/
 
-To begin, [[file:8/input][get your puzzle input]].
+Your puzzle answer was =2250=.
 
-Answer:
+** --- Part Two ---
+Now you're ready to decode the image. The image is rendered by stacking
+the layers and aligning the pixels with the same positions in each
+layer. The digits indicate the color of the corresponding pixel: =0= is
+black, =1= is white, and =2= is transparent.
 
-You can also [Shareon
-[[https://twitter.com/intent/tweet?text=%22Space+Image+Format%22+%2D+Day+8+%2D+Advent+of+Code+2019&url=https%3A%2F%2Fadventofcode%2Ecom%2F2019%2Fday%2F8&related=ericwastl&hashtags=AdventOfCode][Twitter]]
-[[javascript:void(0);][Mastodon]]] this puzzle.
+The layers are rendered with the first layer in front and the last layer
+in back. So, if a given position has a transparent pixel in the first
+and second layers, a black pixel in the third layer, and a white pixel
+in the fourth layer, the final image would have a /black/ pixel at that
+position.
+
+For example, given an image =2= pixels wide and =2= pixels tall, the
+image data =0222112222120000= corresponds to the following image layers:
+
+#+BEGIN_EXAMPLE
+  Layer 1: 02
+           22
+
+  Layer 2: 11
+           22
+
+  Layer 3: 22
+           12
+
+  Layer 4: 00
+           00
+#+END_EXAMPLE
+
+Then, the full image can be found by determining the top visible pixel
+in each position:
+
+- The top-left pixel is /black/ because the top layer is =0=.
+- The top-right pixel is /white/ because the top layer is =2=
+  (transparent), but the second layer is =1=.
+- The bottom-left pixel is /white/ because the top two layers are =2=,
+  but the third layer is =1=.
+- The bottom-right pixel is /black/ because the only visible pixel in
+  that position is =0= (from layer 4).
+
+So, the final image looks like this:
+
+#+BEGIN_EXAMPLE
+  01
+  10
+#+END_EXAMPLE
+
+/What message is produced after decoding your image?/
+
+Your puzzle answer was =FHJUL=.
+
+Both parts of this puzzle are complete! They provide two gold stars: **