Will epub devices automatically scale large images down

authorava · 01-30-2015, 01:02 PM

I've been reading the threads about recommended cover images, scaling illustrations and my head is spinning.

My question is what should the recommended size be for an image that is similar to a cover in that it sits alone on it's own page and should (mostly) fill the screen. I create a title page for my books that looks a little like what you'd see in a print book.

I've attached a small version here so you can see what I'm talking about. This image is 344 x650 at 72dpi and it's fairly small even in my Kindle Fire. I'd like to make them larger, but I don't want them to stray outside the edges of devices. I've just started making my own epubs with Sigil but I'm not really sure how to accurately preview for Nook or iPad. I don't care if it is smaller and doesn't quite fill screen, but I don't want it to be too big and spread off the reader's device. Do I even need to worry about that? Will epub readers automatically scale it down to fit in the device? I know that's a vague question but I'm really curious to hear what the experts say

Thanks,
Ava

DaleDe · 01-30-2015, 04:14 PM

Generally speaking large images will automatically be scaled by default. I don't know what will happen is explicit sizing is used. I would not recommend exceeding 4 Megapixel images in any event. Some readers don't have enough memory to deal with really large images.

Dale

JSWolf · 01-30-2015, 04:26 PM

Quote:

Originally Posted by DaleDe

Generally speaking large images will automatically be scaled by default. I don't know what will happen is explicit sizing is used. I would not recommend exceeding 4 Megapixel images in any event. Some readers don't have enough memory to deal with really large images.

Dale

That depends on the code used. I've seen plenty of images that are only partially displayed on screen because of poor coding.

An SVG wrapper can do the scaling and an img display can as well as long as you specify the size in a percent of width and/or height.

But one thing that can be a major issue for images. Screen resolution/size. You do want high resolution images because devices have differing resolutions and screen sizes. There is the Kindle, PW2, Voyage all with different resolutions. Then there is the Kobo Aura HD and H2O with 6.8" screen and high resolutions.

Jellby · 01-31-2015, 02:43 AM

Devices should start implementing some kind of "ppi setting", that would control how images with no explicit size are displayed. Your 200px maps are unreadable in the newest extra-hyper-high-resolution screen? - adjust the ppi slider and all images grow.

DaleDe · 01-31-2015, 01:38 PM

Interesting but not likely to happen. Device manufacture's are unlikely to do anything uniformly and particularly if it is not in the spec.

Dale

Notjohn · 02-02-2015, 04:02 PM

Quote:

Originally Posted by DaleDe

I don't know what will happen is explicit sizing is used.

That gives me an excuse to ask a question that has long puzzled me. In Sigil, when I insert a cover, this is the code that Sigil generates:

<svg xmlns="http://www.w3.org/2000/svg" height="100%" preserveAspectRatio="xMidYMid meet" version="1.1" viewBox="0 0 800 1169" width="100%" xmlns:xlink="http://www.w3.org/1999/xlink"><image height="1169" width="800" xlink:href="../Images/myimage.jpg"/></svg>

Why use both a relative and an absolute dimension? If the absolute dimensions (1169x800) are merely informative, why use them at all?

JSWolf · 02-02-2015, 04:31 PM

Quote:

Originally Posted by Jellby

Devices should start implementing some kind of "ppi setting", that would control how images with no explicit size are displayed. Your 200px maps are unreadable in the newest extra-hyper-high-resolution screen? - adjust the ppi slider and all images grow.

A 200px map is going to be unreadable on high resolution eInk screen. Especially if it's meant to be displayed full screen.

The problem is too many eBooks use images that are just too small. But what can be done about it? If you make the images high enough resolution, and you don't want them to be full screen, they'll be a bit too large on a non-high resolution eInk screen. What code can be used so it works on a regular Reader, a high resolutions screen, and an ultra-high resolution screen?

DaleDe · 02-02-2015, 07:04 PM

Quote:

Originally Posted by Notjohn

That gives me an excuse to ask a question that has long puzzled me. In Sigil, when I insert a cover, this is the code that Sigil generates:

<svg xmlns="http://www.w3.org/2000/svg" height="100%" preserveAspectRatio="xMidYMid meet" version="1.1" viewBox="0 0 800 1169" width="100%" xmlns:xlink="http://www.w3.org/1999/xlink"><image height="1169" width="800" xlink:href="../Images/myimage.jpg"/></svg>

Why use both a relative and an absolute dimension? If the absolute dimensions (1169x800) are merely informative, why use them at all?

SVG is a different kettle of fish than raster images. SVG builds a canvas to place the image on. The canvas defines the screen space used and is a scale factor for the image. The image is defined on the canvas. So in effect the image takes exactly the amount of space referenced to the canvas but the canvas itself is scaled to the screen. Hence height and width are defined to 100% of the screen and the canvas is the viewBox which defines the pixel size that represents 100% and then the image fits on the canvas. The image, even tho it is raster is being displayed on a SVG generated canvas.

Dale

DaleDe · 02-02-2015, 07:11 PM

Quote:

Originally Posted by JSWolf

A 200px map is going to be unreadable on high resolution eInk screen. Especially if it's meant to be displayed full screen.

The problem is too many eBooks use images that are just too small. But what can be done about it? If you make the images high enough resolution, and you don't want them to be full screen, they'll be a bit too large on a non-high resolution eInk screen. What code can be used so it works on a regular Reader, a high resolutions screen, and an ultra-high resolution screen?

The latest CSS has tries to fix this problem by defining a CSS pixel (see our wiki page on CSS pixel.

A CSS pixel is defined as the size of a pixel on a 96 dpi screen (typical computer monitor). When CSS encounters an explicit px being used in a page it interprets the px not as an absolute vale but rather as a scaled value. Thus an image marked to be 96 pixels wide should be 1 inch wide on the screen. Older devices not not support this but is is coming in HTML5 and ePub3.

Dale

Notjohn · 02-03-2015, 12:42 PM

Quote:

Originally Posted by DaleDe

SVG is a different kettle of fish than raster images. SVG builds a canvas to place the image on. The canvas defines the screen space used and is a scale factor for the image. The image is defined on the canvas. So in effect the image takes exactly the amount of space referenced to the canvas but the canvas itself is scaled to the screen. Hence height and width are defined to 100% of the screen and the canvas is the viewBox which defines the pixel size that represents 100% and then the image fits on the canvas. The image, even tho it is raster is being displayed on a SVG generated canvas.

Dale

Another day, another revelation! Thank you.

Jellby · 02-04-2015, 04:03 AM

Quote:

Originally Posted by JSWolf

A 200px map is going to be unreadable on high resolution eInk screen. Especially if it's meant to be displayed full screen.

If it's readable in a low-resolution screen, it should be readable in a high-resolution one, if it't enlarged. The only degradation in quality (with respect to the low-res screen) would be due to aliasing or faults in the scaling algorithm.

JSWolf · 02-05-2015, 09:27 PM

Quote:

Originally Posted by Jellby

If it's readable in a low-resolution screen, it should be readable in a high-resolution one, if it't enlarged. The only degradation in quality (with respect to the low-res screen) would be due to aliasing or faults in the scaling algorithm.

So how would you take an image and size it on a 650, a PW2, an H2O, and a Voyage so it's the same basic size? It won't be full screen. But if you take a ruler to the image, that image will measure the same and just make it a little bit harder, toss in Mobi to the four different Kindle screen sizes (Kindle, DX, PW2, Voyage).

The problem as I see it, it's going to only really work if the image is of very high resolution and it's set to be full screen (such as a map or a chart with words)

Jellby · 02-06-2015, 03:28 AM

I think we are talking about different things (and my point was about upscaling, rather than downscaling, which is the thread's subject)

JSWolf · 02-08-2015, 01:06 AM

Quote:

Originally Posted by Jellby

I think we are talking about different things (and my point was about upscaling, rather than downscaling, which is the thread's subject)

But the same situation remains. Can you come up with code that will make an image the same size on screen regardless of the resolution/screen size?

Jellby · 02-08-2015, 02:59 AM

No, that's why I said that "devices should start implementing" something to remedy that, at least for cases that were (implicitly) designed with a specific resolution in mind.

Otherwise, one can use % in width, or absolute units (cm, in), but it still depends on the screen size and the reader's implementation of the absolute units (which I'm afraid is often flawed)

01-30-2015, 01:02 PM	#1
authorava Junior Member Posts: 7 Karma: 10 Join Date: Jan 2015 Device: Kindle, Kindle Fire	Will epub devices automatically scale large images down I've been reading the threads about recommended cover images, scaling illustrations and my head is spinning. My question is what should the recommended size be for an image that is similar to a cover in that it sits alone on it's own page and should (mostly) fill the screen. I create a title page for my books that looks a little like what you'd see in a print book. I've attached a small version here so you can see what I'm talking about. This image is 344 x650 at 72dpi and it's fairly small even in my Kindle Fire. I'd like to make them larger, but I don't want them to stray outside the edges of devices. I've just started making my own epubs with Sigil but I'm not really sure how to accurately preview for Nook or iPad. I don't care if it is smaller and doesn't quite fill screen, but I don't want it to be too big and spread off the reader's device. Do I even need to worry about that? Will epub readers automatically scale it down to fit in the device? I know that's a vague question but I'm really curious to hear what the experts say Thanks, Ava Attached Thumbnails

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01-30-2015, 04:14 PM	#2
DaleDe Grand Sorcerer Posts: 11,470 Karma: 13095790 Join Date: Aug 2007 Location: Grass Valley, CA Device: EB 1150, EZ Reader, Literati, iPad 2 & Air 2, iPhone 7	Generally speaking large images will automatically be scaled by default. I don't know what will happen is explicit sizing is used. I would not recommend exceeding 4 Megapixel images in any event. Some readers don't have enough memory to deal with really large images. Dale

01-31-2015, 02:43 AM	#4
Jellby frumious Bandersnatch Posts: 7,515 Karma: 18512745 Join Date: Jan 2008 Location: Spaniard in Sweden Device: Cybook Orizon, Kobo Aura	Devices should start implementing some kind of "ppi setting", that would control how images with no explicit size are displayed. Your 200px maps are unreadable in the newest extra-hyper-high-resolution screen? - adjust the ppi slider and all images grow.

01-31-2015, 01:38 PM	#5
DaleDe Grand Sorcerer Posts: 11,470 Karma: 13095790 Join Date: Aug 2007 Location: Grass Valley, CA Device: EB 1150, EZ Reader, Literati, iPad 2 & Air 2, iPhone 7	Interesting but not likely to happen. Device manufacture's are unlikely to do anything uniformly and particularly if it is not in the spec. Dale

02-06-2015, 03:28 AM	#13
Jellby frumious Bandersnatch Posts: 7,515 Karma: 18512745 Join Date: Jan 2008 Location: Spaniard in Sweden Device: Cybook Orizon, Kobo Aura	I think we are talking about different things (and my point was about upscaling, rather than downscaling, which is the thread's subject)

02-08-2015, 02:59 AM	#15
Jellby frumious Bandersnatch Posts: 7,515 Karma: 18512745 Join Date: Jan 2008 Location: Spaniard in Sweden Device: Cybook Orizon, Kobo Aura	No, that's why I said that "devices should start implementing" something to remedy that, at least for cases that were (implicitly) designed with a specific resolution in mind. Otherwise, one can use % in width, or absolute units (cm, in), but it still depends on the screen size and the reader's implementation of the absolute units (which I'm afraid is often flawed)

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