Depends on your output. What are you planning to use this for? Print? Web? Large banner? etc.
Is each small part a smart object with a larger than required resolution? Rasterize each smart object in that case. (in Layer-->Rasterize-->All Layers)
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If you are saving the master copy - don't reduce it, don't rasterize - just save as a *.psb complete with layers, smart objects etc. Disk space is relatively cheap - the ability to go back and edit a master file later - priceless. That said you can enable compression on the psb file in Preferences > File Handling and uncheck "Disable Compression of PSD and PSB Files". The compression is lossless and will reduce file size but at the expense of loading time.
If you are sending the file on - then send the minimum needed for that purpose. If no further editing will be done then flattening a copy will reduce file size considerably and you can again look at the compression options under the various file types
It will be an exterior map for PC game, made with same method as google maps)
I want to note, it is not necessary that image must be .dds format (because it 2d image, big, flat and work in game as it is)
But the photoshop allows me to choose only between .tiff or .dds, which one have less weight?
No, each small picture - is a layer. Full image is 141*141 layers, each layer over 430*430px size, it completely gray, have 8 bit channel, and .png format.
so, maby there is some lower formats or channels i can use to reduce it's weight?
ty Dave, i'm already have saved .psb file) time of loading anyway will be long, because each piece as layer taking ~1.5mb RAM (1.5*20000 = 30gb ), and i can't merge them, because I will lose the possibility to edit each layer individually)
Yep, i'm looking for minimum needed), didn't know about flattering, ty, will try it
Don't flatten the master - or you will lose the ability to edit !
Only flatten the copy
kk, let's play with formats and mathematics)
i have small .png image, gray-colored, size - 448kb.
if i save it as:
monocrome - quality completely lost, size 37.5kb = 1194.67 % win
16bit color - quality lost, size 150kb = 298.67 % win
256bit color - adds blue/yellow artifacts, size 301kb = 148.84 % win
24 depth - no visible changes, size 900kb = 49.78 % lose
JPEG - no visible changes, size 170kb = 263.53 % win (ave!)
gif - quality lost, size 122kb = 367.21 % win
tif - minor quality lost, size 516kb = 86.82 % lose
png - resave without alpha chanel, size 402kb = 111.44 % win
so, JPEG win
this way if i convert all 20000 pieces in jpeg, global image will be
25500mb (25.5gb) / 263% = 9695mb (9.6 gb)
not bad, but i need to reduce it more)