I have been working on the line drawings and maps in "Seven Pillars of Wisdom" by T. E. Lawrence. I tried the epub version by HarryT on my Aura HD and the illustrations were far too small to view comfortably. I looked around for replacement images on the web and was only able to find photos of a couple of the maps and the images in the HTML version at gutenberg.net.au, which looks to be the source for HarryT's images. I contacted gutenberg.net.au and was informed that they have no other resolutions, and do not know of any.
I tried doubling the resolution of the images, but the JPEG artifacts and low resolution made that unworkable, so I obtained 1200dpi 8 bit scans of the line drawings and maps and got to work on making the best figures I could for a number of resolutions to match readers of interest to me. The larger versions are also suitable to use for reference on tablets, notebooks, or desktops while reading the text on an e-reader. This included stretching the contrast, converting to 1 bit, erasing specks, filling in gaps, and shrinking to various sized 4 bit images. I hope to convince HarryT to select one of my image sets to replace the maps and line drawings in his version of the book. Then I would like to upload the other resolutions to a post in the thread for the book so that people can customize the book to suit their device and taste or download as reference images.
PNG has a number of clear advantages over JPEG for this type of image and the dimensions of the Project Gutenberg / HarryT images are small, even for a 600x800 pixel display. (They are fine for the common pixel densities of 2001 when the Gutenberg version was produced, and not bad on a 100dpi display, provided you don't actually want to be able to read all the text on the maps.) The figures and maps in the book have the same width as the text in the book, so it would be consistent for them to use the full width of the reader screen, less the text margins. (On smaller screens, I think the maps should get full portrait width.)
There is also the matter of the "Forced Landing" figure. In the book, the figure is split onto facing pages, but the left figure is lower on the page. I believe the web page and epub should show the right image above the left image instead of the current order. While working on this figure, I noticed that the 3 flowers in the upper right of the camel image match those on the lower left of the other one (more evidence that the camel image is meant to be below). So, I have made versions of the figure as a single image. The version at the same scale as the Gutenberg / HarryT images fits on a 600 pixel wide screen.
I have made PNGs of each map and line drawing at the original scale, ans sized for screens 600, 758, 820, 1000, 1250, and 1550 pixels wide in portrait orientation and used them to replace the maps and line drawings in copies of the original epub. I only made the original size images for comparing size and other properties. They looked very nice on a Story HD (758), Onyx M92 (820), Nook HD (820), Aura HD (1000), Nook HD+ (1250), and borrowed Note 10.1 and Ipad retina (1550). Now we get to a point where I may need to open threads on the workshop and/or epub forum. When oversize images are present, some devices shrink the image, some crop it, and others display garbage. Also, the Aura HD reader could not display a 4 bit PNG with color present along with black and white. It also degraded 4 bit grayscale only images (full white parts of the image were darker than full white). Most devices handled the 4 bit images fine and the 8 bit images use far more bytes than the 4 bit images.
All that is needed to substitute my images for the original images is unzip and zip or anything with equivalent functionality. No epub specific tools or epub knowledge are required. If HarryT changes to PNG for maps and line drawings, it is almost trivial. If not, it is a bit more complicated. Many devices do not care about the details of image file names and rely on content.opf or the contents of the image file itself to determine file type. Some even cope with an incorrect content.opf. I have separate zip files for maps and drawings. Each has its images in a directory named images. When
https://www.mobileread.com/forums/showthread.php?t=78198 is updated to use PNG, all that is needed is to copy the epub to say, SPoW0820w.epub in some working directory that also has the zip files, then:
unzip SPoW0820w_ld
unzip SPoW0820w_map
zip -ru SPoW0820w.epub images
For the current version of the epub, the images need to be renamed from png to jpg and content.opf may or may not need to have the media-type changed for the new images. For strict correctness, instead of renaming the images, content.opf and the referring html files would need to be edited, which would be a bit much to expect each downloader to be able and willing to do.
I gather that it might be possible to use large images and use the HTML to direct that they be scaled to fit the screen, but that is more project than I can or care to take on at this time.
The gutenberg.net.au version is at:
http://gutenberg.net.au/ebooks01/0100111h.html
I am attaching a modified content.opf
I'm sure there is much I have left out, and equally sure I have rambled on too long.