[davidfor]

Originally Posted by Linton

Thanks for your interest David. I have investigated further and it seems that with my books (kepubs) and e-reader (Aura HD) pictures are displayed at 2/3 the linear size of picrures on the IPAD Kobo App compared with the page size.

For example look at a preview of "The Irrationals" by Julian Havil, a few pages in from the start of Chapter 1 (page 12 of 86 with my font size) there is a timeline diagram (Figure 1.1). On an IPAD this diagram occupies the full width of the displayed page. On the Aura it takes up about 2/3 of the page width. So its 2/3X2/3 =4/9 the size, which makes it total unreadable.

Another example - "Infinite series" by James Hyslop. Around page 3 or 4 of chapter 1 there is the formula for a function (its a picture). The IPAD version is much larger on the page (its actually too large to fit) whereas the Aura version is less than half a page wide.

I have only had a look at the first one so far. From searching the Kobo shop, I found it as https://store.kobobooks.com/en-us/ebook/the-irrationals.

I have had a look at the preview on my Glo HD, Aura H2O, Touch, iPad, Android phone and the Windows desktop application. From what I can see, the iPad app is the one that is out of step. The Android app might be as well, but the size of the screen might mean it handles things differently. Loading it as an epub in the calibre viewer and on my Aura H2O gives similar results.

Firstly, the image mentioned above is called "c01f001.jpg" and has size 596 x 123 pixels. The code that displays this is:

<p id="ch1fig1" class="imagef"><a id="page_15"></a><span xmlns="http://www.w3.org/1999/xhtml" class="koboSpan" id="kobo.118.1"><img src="../images/c01f001.jpg" alt="c01f001"/></span></p>

The relevant elements of the styles are:

body
{
	font-family:"Times New Roman", serif;
}

.imagef
{
	font-family:"Times New Roman", serif;
	font-size: 100%;
	margin-top:1em;
	margin-bottom:0.6em;
	text-align:center;
}
  div#book-inner p, div#book-inner div {
    font-size: 1em;
  }
  div#book-inner * {
    margin-top: 0 !important;
    margin-bottom: 0 !important;
  }

In all cases other than the iPad, the image is displayed at it's natural size on the screen. As the image is 596 pixels wide, that means on the Touch, which has a 600 pixel wide screen, a little is missed from the end because a few pixels are used by the margins. On the Aura H2O and the Glo HD, the image is centred at it's full size and appears to cover about half the width of the screen. Same for the desktop application. Changing the font size on the device doesn't change anything about the image.

The iPad app is the different one. It seems to resize the image a bit. I have an iPad Air with resolution of 2048x1536. But, in portrait mode, the image appears to be full width of the book area of the app. As there are large margins, that would make it appear about 1200 pixels wide.

As there is nothing in the book code or styles that would resize the image to fit the screen, this all seems to be working correctly. My guess is that the iOS app developers decided to magnify all the images a bit, but all the others are using the "natural" size.

I have no idea what the solution to this is. There is probably some code that can be used to maximise the image but keep the aspect ratio. That would work for images between paragraphs, but for in-line images, such as some of the equations, it doesn't work. They need to resize with the font size. But, it also means the book designer needed to get all this right in the first place. And as GeoffR said, this is a common mistake.