Fix issue #58 : Add PNG to the mix

_episodes/02-image-basics.md

@@ -390,6 +390,9 @@ plt.imshow(three_colours,cmap=plt.cm.gray)
 
 Above we have exactly the same underying data matrix, but in greyscale.
 Zero maps to black, 255 maps to white, and 128 maps to medium grey.
+Here we only have a single channel in the data and utilize a grayscale color map
+to represent the luminance, or intensity of the data and correspondingly
+this channel is referred to as the luminance channel.
 
 ## Even More Colours
 
@@ -928,6 +931,12 @@ For example,if your images are stored in the cloud and therefore
 must be downloaded to your system before you use them,
 you may wish to use a compressed image format to speed up file transfer time.
 
+## PNG
+
+PNG images are well suited for storing diagrams. It uses a lossless compression and is hence often used 
+in web applications for non-photographic images. The format is able to store RGB and plain luminance (single channel, without an associated color) data, among others. Image data is stored row-wise and then, per row, a simple filter, like taking the difference of adjacent pixels, can be applied to
+increase the compressability of the data. The filtered data is then compressed in the next step and written out to the disk.
+
 ## TIFF
 
 TIFF images are popular with publishers, graphics designers, and photographers.

_episodes/03-skimage-images.md

@@ -163,7 +163,7 @@ In this case, the `.tif` extension causes the image to be saved as a TIFF.
 > the dimensions we want the new image to have, `new_shape`.
 >
 > Image files on disk are normally stored as whole numbers for space efficiency,
-> but tranformations and other math operations often result in
+> but transformations and other math operations often result in
 > conversion to floating point numbers.
 > Using the `skimage.img_as_ubyte()` method converts it back to whole numbers
 > before we save it back to disk.