While happily exceeding the bandwidth limit on my 200 MB 3G data plan with AT&T, it occurred to me that a large chunk of that bandwidth has been devoted to uploading photos. I decided to look into if/how the iPhone stores photos and how they’re compressed, and found some details on music compression along the way.
Music compression
Over at Just Another iPhone Blog, @ragart details the new option in iTunes 9.1 that allows music compression on the iPhone. This is pretty straightforward: it just lowers the bitrate of your music and saves about 40% disk space in the process. You’ll notice the lower quality if you’re using decent earbuds, headphones or a stereo setup.
Image compression
To see how compression works, I did some very informal testing. I uploaded three Photos to MobileMe: one is a portrait indoors, one is a sunset photo, and one is a poster outside. I uploaded them to my MobileMe account and according to MobileMe, the original photos are 2592×1936 pixels and average 1.5 MB each.
The “actual size” photos on MobileMe are scaled down to 764×1024 pixels and average 149 KB each. This means that the scaled-down photos are 10% the disk space of the originals. Running jpegoptim on all the photos resulted in savings of only 2 KB per image, so MobileMe is doing a pretty good job at compression.
Which leads me to this question: Why should I be uploading 1.5 MB photos over 3G when all I’ll ever be viewing are the scaled-down images at 10% the file size? Other popular services are scaling down too, think Facebook and Twitter image services.
I’ll post my answer to that question another time. Until then, let me hear your thoughts.
I did a bit of googling and found this: http://osxdaily.com/2010/07/20/compress-photos-with-iphone-os-4/
I suppose a sort of jenky workaround could be to use email services for all your stuff: twitter, facebook, etc, but I don’t see why there aren’t just image compression apps (that I could find anyway).
Nice find. If the API that Mail uses to compress those photos isn’t private, it *should* be trivial to compress photos inside of other apps.
I emailed a photo a few times. The “small” and “medium” settings are close to useless; “large” is the 764×1024 image and was 1/5th the size of the original photo for me, but still had plenty of detail. So it wasn’t as I suspected – there’s no technical reason to *not* compress photos before upload. There was a slight (1-2 second) delay when emailing the large photo, but I think that’s worth the bandwidth saved.
I can’t figure out if the Twitter and Facebook apps compress photos before uploading. The fullsize image on yfrog is scaled down so it looks like the Twitter app does.
Okay, so for sure the Twitter app scales images down before uploading. Still don’t know about the Facebook app.