Optimizing images for use in Kindle

[Daithi]

One more shot.

[Daithi]

Woohoo! Here's the color version.

[RickyMaveety]

I tried mucking about with that image for a bit. It's hard to turn that into a good Kindle image because it's a little too murky. That looks fine in color, but it's a real bitch to turn into four levels of gray.

I'd post my results, but I don't think they are much (if any) better than yours.

[ProDigit]

I'd suggest doing the color resize to the perfect resolution, and then save that version into 4color Grey mode.

Keeping it in higher resolution always will cause an additional dithering to be done by the kindle.

So first resize, then b&W.

[wallcraft]

Originally Posted by ProDigit

So first resize, then b&W.

These are the correct resolution for the Kindle (525x640). The original color version looks better, to me, on the Kindle (1st screenshot) than the off-line B&W conversion (2nd sceenshot).

If you have a SD card on your Kindle, then <alt><shift>G creates a screenshot (like these) on the card.

[ProDigit]

the second one just looks darker to me.

[Daithi]

I was actually happy with the way the images turned out, and they do look better on the Kindle than they do on the computer screen. As wallcraft pointed out the images are at the maximum Kindle resolution (525x640). I hadn't even tried just loading the color version to the Kindle, so thanks wallcraft -- that will save me some time.

By the way, I was able to confirm definitively that 525x640 is the maximum size of an image inside of a Kindle ebook. I did so by creating two images. The first was a solid black line that was 800 pixels high and 3 pixels wide, and the second was a 600 pixels wide and 3 pixels high black line. I then made a mobi file that contained these images on seperate pages, and then I did the alt-shift-G thing to take snapshots of each image. Then I loaded the snapshots into photoshop and selected each line and cropped the image to what was selected. The 800 pixel high image had been resized to 640, and the 600 pixel wide image had been resized to 525, so that is definately the maximum size. (Most of you already know this, but I wanted to explicitly state it for new users coming to this site.)

I also used photoshop to examine the grayscale of the images produced by the Kindle. The four colors of gray are Black (#000000), Dark Gray (#555555), Light Gray (#AAAAAA), and White (#FFFFFF). It's my understanding that the Sony has a grayscale of eight instead of four and that the Kindle uses the same screen manufacture as Sony, so why the Kindle limits itself to 4 colors is kind of strange.

[wallcraft]

Originally Posted by Daithi

It's my understanding that the Sony has a grayscale of eight instead of four and that the Kindle uses the same screen manufacture as Sony, so why the Kindle limits itself to 4 colors is kind of strange.

The Sony PRS-505 does support 8 levels, but the Kindle uses a different (older) display controller than Sony and it only supports 4 levels of greyscale.

[ProDigit]

Testing out what the exact pagesize is, is pretty hard.
Best thing you can do, is create a 600x800 png, gif or BMP. If the kindle does not support these picture files your last resort is a JPG.
Then create on the picture a B&W GRID.

Say every even line is black, uneven lines are white.

Then save your picture as 600x800 pix.
Crop it, and save another copy as it's native resolution (say 595x795).
All the way down to a little below what you believe is 525x640 resolution (say 520x635 or so)

you might be able to discover the display resolution of the decoder before it upscales the picture.
When the Kindle displays a grey image you most likely are not on it's 'perfect' resolution.

When you can see more areas with Black & White lines than grey area, it means you're getting close!


Come to think of it, even if the 'perfect' resolution was 525x640, it will upscale that picture making it more blurry.

So instead of 1 line black, 1 line white,you'd be better off doing 2 lines black, and 3 white, or reverse,since I suspect 1/1B/W will always seem grey on the kindle.

[Daithi]

I hate to disagree with you, but finding the optimal image size is not that hard. To find find the optimal height create a solid black image that is larger in height than the screen display but thinner in width, and display that image on the Kindle. The Kindle will resize that image to its maximum height. Now save the Kindle image (alt-shift-G) and examine that image in a program like photoshop to determine the maximum height. Do the same thing to determine the maximum width. The optimal image size for the Kindle is 525x640.

[Daithi]

What do you mean when you say, "Come to think of it, even if the 'perfect' resolution was 525x640, it will upscale that picture making it more blurry."

If 525x640 is as large an image as can be displayed within an ebook then I'm not sure what you mean by upscale.

[ProDigit]

That is that the kindle's display is 800x600 pixels. Obviously there has to be some sort of 'upscaling' to be done to fill the remaining 75x160pix.

Your solution of creating a larger border might work if the kindle does not rescale the picture. The Sony for instance, always shows a picture fullscreen unless you zoom in.
Even if the image is smaller than the display (eg:320x240).

[wallcraft]

Originally Posted by ProDigit

That is that the kindle's display is 800x600 pixels. Obviously there has to be some sort of 'upscaling' to be done to fill the remaining 75x160pix

When images are in a MOBI/AZW ebook, their maximum size is 640x525. This is the size of the images in the screenshots of the Kindle screen in my post #35. You can see what the rest of the space is used for there (book title, progress bar, status bar and white space margins).

[ProDigit]

I do have to say that the e-ink displays I've seen, have an extremely high DPI ratio.

I could not distinguish individual pixels.
but 75 pixels vertically out of 600 would be about 12% of the display used for 'bars'?

I think there might be some softwarematic things happening under the hood, like resizes.
160pix would be 20% of the screensize. You should check (measure) out if the progress bar really takes up that much space (20%).

I guess I'll be testing out my Sony reader soon, to see what the exact resolutions are, regardless of what the ebook format resolutions are.

I'm only suggesting, but the Ebook format could be based on older devices supporting lower resolutions.

Either way, I can really not say about the Kindle, but I'll test out myself on the Sony.

[wallcraft]

Originally Posted by ProDigit

You should check (measure) out if the progress bar really takes up that much space (20%).

The "screenshots" of the Kindle are copies of the screen bitmap, so they are exactly 600x800 and the image part is exactly 525x640.

Having a screenshot capability (only currently on the iLiad and the Kindle) makes this kind of optimization much easier, because you can "measure" the screen using the screenshot and an image editor on a PC. All Desktop PCs have a screenshot capability, as does the iPhone, but if does not seem to have trickled down to most EInk devices.