ShaneHunterPhotography wrote:
I'm creating a Snapchat Geofilter for an event I'm doing for my senior models this weekend.  Snapchat's requirements are that the image be 1080x1920px, 300kb or smaller, saved as a PNG on a transparent background.  When I save my file, it is 2.3MB at 300dpi.  To decrease the file size in order to get it under the 300kb limit, what should I do?  Decrease dpi?  Thanks for any help, in advance!

DPI has no bearing on this. Is your graphic 1080x1920 pixels? Then forget the DPI.

How complex is the imagery you are using? PNG isn't designed for displaying high resolution images at a small file size. It's best used for simple graphics with fewer than 256 colors. The fewer the colors, the smaller the file size. Basically, it's like a GIF with transparency in that regard. It's best on vector images.

There are various tools to optimize PNG images while reducing file size.

http://how-to.wikia.com/wiki/How_to_red … a_PNG_file

https://superuser.com/questions/444859/ … ze-for-web

TMA Photo and Training wrote:
Saving your HD screen images at 72 dpi rather than 300dpi will make a  G R E A T difference in file size!

people may not realize that dpi has no relevance in screen size nor in file size; only in print output. please kindly do not confuse them with incorrect information.

a 600 pixel width image shows the same width on a screen and setting the image dpi has no bearing if it is set at 1 dpi or 3,000 dpi. the screen shows 600 pixels. once i scanned some 24mm x 36mm images at 4,000 dpi and the inexperienced printer didn't know how to print a 1 inch image ten inches wide on the page at 300 dpi.

dpi setting is only information indicated inside the file for reference (like color space, f-stop and other metadata). some applications have no use for it and completely ignor it without any consequences.

TMA Photo and Training wrote:
I'm thinking that for use on the web... that you should use 72 or 96 dpi for images that show up on a screen.

This is meaningless because you don't know what device the image will be displayed on. Consider also resolution independence.

TMA Photo and Training wrote:
I usually only use 300 dpi for images that I am going to print out on photographic paper.

Saving your HD screen images at 72 dpi rather than 300dpi will make a  G R E A T difference in file size!

Most all imaging programs will allow you to create or to save your images at different dots per inch.

I usually create my images at 300dpi in case I want a high res print capability later...but export them using the "save as" or "Export"  command to create the 72 dpi versions for the web.

That's off-topic but still: 300 dpi is not high resolution printing. Not in 2018.

TMA Photo and Training wrote:
Be sure to also create or save your image as an "s-RGB color space" if you are able to... so your colors show up perfectly on screen in a browser or app.

1. There is no such thing as "s-RGB". You have a tendency to punctuate everything you write in a strange way.

2. There is no such thing as "save your image as ... color space". One can assign (or "tag") an image a proper color space. This is metadata, so you are not actually saving the image (the main data) as that.

3. Technically there is no such thing as "perfect show up of colors" because there is no such thing as perfect color. There is only level of color accuracy.

3. How a CMS (if there is one) handles color rendering is beyond the topic of this thread. In any case assigning a color profile is not a guarantee for accurate color reproduction (or perception).

If you really are an educator as you advertise you should be much more careful about how you say things and about their validity. Giving a lot of information does not necessarily mean it is the right, the correct or the requested information.

Leonard Gee Photography wrote:
people may not realize that dpi has no relevance in screen size nor in file size; only in print output. please kindly do not confuse them with incorrect information.

Indeed. But still some marketers use the DPI instead of PPI when listing (e.g. smartphone) specs.

a 600 pixel width image shows the same width on a screen and setting the image dpi has no bearing if it is set at 1 dpi or 3,000 dpi. the screen shows 600 pixels.

Only if the displaying software uses resolution independence.

In any case the question about file size comes down to how data is stored on a computer:

memory_to_store_image = width_px * height_px * bits_per_pixel (+additional meta data for headers etc)

A file can contain that compressed (losslessly or otherwise).