PNG File Sizes???

[cobrien84]

While I freely admit to being an IrfanView newbie, I'm a bit perplexed on one point. I've taken a HUGE number of iPhone (5S) screenshots to document my progress with a particular application - and while the majority of these (PNG) screenshots are in the horizontal format, once in a great while, I'll get one {for SOME (unknown) reason} in the vertical format. And while I organize and view these images in Picasa, I prefer to use IrfanView to correct these "errors" because I can retain the original file date and time. EXCEPT - why would a vertical (1136 x 640) PNG that's 460,854 bytes "translate" to a horizontal (640 x 1136) PNG that's ONLY 284,184 bytes??? Why would the bytecount be different AT ALL?

[Bhikkhu Pesala]

It depends on PNG Compression. If the original PNG was not compressed at all the size difference could be much more than double. The best compression would likely be obtained with the PNGOUT plugin, but that is too slow to be worth using unless size is critical. The default compression of 6 is fine, but I use 9 as I save a lot of images for the web.

If you use FastStone Capture, you may find that your file sizes are smaller than whatever utility you are currently using for screen shots. It's compression is also very good.

Compare the four apparently identical attachments.

1. PNGOUT Compression = 29.33 KB (30,029 Bytes)
2. PNG Compression (9) = 30.27 KB (30,992 Bytes)
3. PNG Default Compression (6) = 31.70 KB (32,460 Bytes)
3. PNG No Compression (0) = 455.48 KB (466,413 Bytes)

Note that there is no difference at all in the pixels in the images — they are all identical to the original screen shot (PNG Compression). Only the compression is different.

[cobrien84]

I REALLY appreciate your time and detailed reply; knowing PNG files were "lossless," I never really considered the fact that they'd be compressed as well. (Just figured that if they were one size vertical, they'd be the same horizontal. I wonder if they're compressed "from" the iPhone?)

[Bhikkhu Pesala]

I would expect a mobile phone to compress images because memory is costly on mobile devices, and people like to take video as well as stills.

I would not expect the orientation to make any difference for otherwise identical images, but IrfanView might well compress the image more than the phone.

Two apparently similar images with the same pixel dimensions could be quite different sizes depending on how many different colours they contained. Large blocks of the same colour will compress extremely well.