clarify that header bytes are exact in comments, add 1x1 test

src/inline.jl

@@ -96,9 +96,10 @@ function get_ind(sub_arr, arr)
     return findfirst(i -> all(sub_arr .== arr[i:i+nsub-1]) , 1:search_end)
 end
 
-# get w, h from PNG base64
+# extract width and height from a PNG file header inside a base64 string
 function png_wh(img::String)
-    decoded = base64decode(img[1:32])  # you need 13 + 8 bytes, this is a bit extra
+    # PNG header is 8 bytes, 4 byte chunk size, 4 byte IHDR string, 8 bytes for w, h
+    decoded = base64decode(img[1:32]) 
     ihdr = get_ind(b"IHDR", decoded) + 4  # find the location of the header
     w, h = ntoh.(reinterpret(Int32, decoded[ihdr:ihdr+8-1]))  # get the 8 bytes after
     return w, h

test/inline.jl

@@ -25,6 +25,9 @@ import IJulia: InlineIOContext, png_wh
 
     # test that we can extract a PNG header
     @test png_wh(
-        "iVBORw0KGgoAAAANSUhEUgAAAAUAAAAMCAYAAACqYHctAAAAE0lEQVR42mNk+P+/ngENMI4MQQCgfR3py/"
-        * "xS9AAAAABJRU5ErkJggg==") == (5,12)
+        "iVBORw0KGgoAAAANSUhEUgAAAAUAAAAMCAYAAACqYHctAAAAE0lEQVR42mNk+P"
+        * "+/ngENMI4MQQCgfR3py/xS9AAAAABJRU5ErkJggg==") == (5,12)
+    @test png_wh(
+        "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mP8/x"
+        * "8AAwMCAO+ip1sAAAAASUVORK5CYII=") == (1,1)
 end