Terminology used is picture sizing

[willythecat]

I'm a bit confused about picture sizing regarding the terminology used.
Hyperthetically, let's say l have a jpeg picture with a size of 1500kb,  and l want to reduce this to 1024kb.
Now every freebee resizing programs l've found say to change the "size" l have to type in new dimensions (nnn x nnn), which is what they refer to as size and not the kb value. - I'm confused!
How would l know what dimensions to type in to get it down to 1024kb, or is it just hit and miss?
Is there some formula then that l should be using relating to the kb and dimensions?
Although l've reduced pictures before using smaller dimensions, am curious as to how to do this using the kb value alone.
Thanks, and hope the above makes sense.



 

[patio]

Forget about the file size of the edited image...
Just concentrate on making it the size you want.
Different image editors will yeild different file size results...

[willythecat]

Thanks Patio m8, helpful as always.
I normally do just use the file dimensions themselves while tinkering.
But let's say l have a file size of 3072kb (if that is at all possible) and l want to mail it, but the limit is 2048kb. and winrar wont compress it enough?
This is all hyperthetical, and l'm probably talking a load of bow locks, but am just curious.
Or should l just retire while the going is good?
Regards
WTC

[BC_Programmer]

Thanks Patio m8, helpful as always.
I normally do just use the file dimensions themselves while tinkering.
But let's say l have a file size of 3072kb (if that is at all possible) and l want to mail it, but the limit is 2048kb. and winrar wont compress it enough?

You have to reduce to total number of pixels by approximately 1 third.  Just divide the x and y widths by 2.83, that reduces it by (approximately) a total of 1/3rd.

[rthompson80819]

I'm not sure what software you are using, but what I use is just the basic stuff that came with my HP camera.  In addition to changing brightness, colors, etc., one of the options is how much compression you want to use.  The more compression, the smaller the file.  You can play around with that until you get a file size that fits your email.

[Salmon Trout]

How would l know what dimensions to type in to get it down to 1024kb, or is it just hit and miss?
Is there some formula then that l should be using relating to the kb and dimensions?
Although l've reduced pictures before using smaller dimensions, am curious as to how to do this using the kb value alone.

With uncompressed BMPs you can calculate the size: size in bytes = (width x height) x bytes-per-pixel. Plus a few bytes for the file header. However jpeg files are compressed so that there is no way to calculate the final file size resulting from a resize. You would just have to try various sizes until you got to or below your target file size. Also with jpeg there is another setting called "quality" which can be set from 0 to 99 usually. Jpeg compression is "lossy" that is it works by throwing away detail. The more you throw away the smaller the final file size. Higher number here means larger file size.

[willythecat]

Thanks everyone, always get an answer on here.
Will probably never do this via the "kb" route but now get the general idea between dimensions and size.
Thanks again
Regards