Saving png images

HI, I am considering becoming a subscriber so I can download map images for my photo book that I'm going to print. The company told me I need map images at least 300dpi for the quality to be ok. If I just save the png images from the site will the quality be ok?

I'm no expert but...

dpi depends on how big you're going to print the image, as png isn't a vector based format, it's a bitmap with compression.

I've saved my travel map to see what outputs, the default view of the world saved at 1366x485 pixels and 24bit colour depth. I think that equates to around 72dpi at its default size, ie optimised for screen viewing not high quality printing, as any zooming in on screen immediately looked pixelated. You'd need to use it at a smaller size than the default view to get decent quality print from it. Sorry.

I don't know whether it would be worth trying an image upscaler? eg this free one https://icons8.com/upscaler

[ Edit: Edited on 23 Sep 2021, 08:52 GMT by AndyF ]

A few corrections and additions

Quoting AndyF

it's a bitmap with compression.

The benefit of PNG is actually that (unlike JPG) it's without compression.

Quoting AndyF

I've saved my travel map to see what outputs, the default view of the world saved at 1366x485 pixels and 24bit colour depth.

The size of the output of the export function unfortunately depends directly on your screen resolution. If I on my 1920x1080 monitor put my browser in full-screen mode, I get an image which is 1920x930 pixels. If you have a 4K monitor, it should be much larger still.

Quoting AndyF

I think that equates to around 72dpi at its default size

This is "wrong". As you wrote before, "dpi depends on how big you're going to print the image".
"dpi" stand for "dots per inch" The pixels are the dots, the inches are the size you print the image at. So a 1366px wide image at 300dpi could be printed 4.5 inch (11.43 cm) wide. My 1920px wide image would be 6.4 inches (16.2cm), which is a bit larger than standard photo-size, and probably should be acceptable for a book, though isn't good enough yet if you want the book to be coffee-table sized, and/or do a full spread. If you have an even larger monitor (or know someone who has one at whose computer you feel comfortable logging in), you can save and thus print it even larger.

[ Edit: Edited on 23 Sep 2021, 09:14 GMT by Sander ]

Thanks Sander for clarifying my inept wording.

If the site will supply the image at better resolution for better monitors, I wonder if there is any way to hack it to request a higher resolution? Presumably it is the browser which is requesting the image from the site, and in the process telling it what resolution to supply?

Oh, good one! Yes, you can actually do this by not putting your browser window fullscreen, but instead resizing it nearly as large as it goes, then dragging if off into a corner so only a small part shows, and the resizing it to nearly as large as it goes again (repeat as necessary), leading to a browser window which is much larger than your screen resolution.

This works, but it's problematic in that it becomes really hard to get a good idea of what the image you're saving will look like.

[ Edit: Edited on 23 Sep 2021, 10:35 GMT by Sander ]

In answer to the original question - yes, you can save your map image at 300dpi! It is intended precisely for the purpose of printing it. The code actually sets the browser window's device pixel ratio before saving out the image.

I will add a caveat though: it is slightly buggy. I have found that I need to hit the Save as PNG button twice to get the higher resolution. Probably it hasn't finished setting the ratio properly when it is triggering the save for some reason. For the second click it has.

So, step by step, this is what you do.

1. Go to the map
2. Click on the export tab
3. Line up the map so it is showing the framing you want to use. You can zoom into whatever level works best for you. You can also select to show all trips, or just one trip.
4. Click the Save as PNG button at top right. Save it.
5. Click the Save as PNG button AGAIN. Save it over the top.

Or save the second one with a different file name - you'll then be able to see how the second image is a lot larger. On my map it goes from a 600kb image to a 2.8mb image. The width of the image jumps from 2880 to 9000 pixels (this is on a fairly large monitor, so already large to start off with).

I hope it helps.

Thanks Peter. My 4k monitor seems to be able to create good definition images each about 2mb in size. Now a happy supporter.
Steve