File size of 2 images

[frostyboy]

Hi,
I hope someone may be able to explain an issue I'm facing.
Before covid I made posters for my small theatre sized at A4 x 300dpi. A typical file size saved as psp file would be 3.5Mb
After covid (now) I create the same sized image A4 and reduce it to 200dpi and still struggle to cope with it slowing down my computer. The size is typically 1.25Gb
I am struggling to see the difference in the images yet the file sizes are incredibly different. I'm perplexed!

[JoeB]

First things first. You cannot change DPI (which is Dots Per Inch and a printer software function only and eals with printer settings like Draft, Normal, High, Photo, etc.) in PSP or any other graphic program. You can only change PPI (Pixels Per Inch) as you can see if you look at the lines in the images of the dialogue boxes you posted (Pixels Per Inch - 300 and Pixels Per Inch - 200 - no DPI in sight).

That said, the difference in file sizes is because of the difference in the pixel sizes of the two images.

The first image shows a setting of 300 Pixels Per Inch (PPI). That tells the printer to print 300 pixels of your image to each inch of printed paper. That first image contains 2480 pixels on one side and 3508 pixels on the other. 2480 divided by 300 gives you 8.67 inches of print with 300 pixels of the image printed on each inch of paper. 3508 divided by 300 equals 11.69 inches of print with 300 pixels of that side of the image printed on each inch of paper.

With the second image you are telling the printer to print only 200 pixels of the image per each inch of print. Divide 200 pixels per inch into each of those dimensions shown will give you the same print size as the first image. That's because the printer is now being instructed to print fewer (one third fewer) of your image's pixels per each inch of print. And fewer pixels means a smaller file size.

As to why either of those file sizes should slow down your computer, I'm not sure what you mean by that. In what way, and during what part of the process, is your computer slowing down when working with these images?

[LeviFiction]

@frostyboy - care to share the freally large file? Or run this script - https://forum.corel.com/viewtopic.php?p=360536

that script will remove all of the history information that's saved in the PSPImage file. When PSP saves history it essentially saves a script of everything done to the image, and when you use patterns or textures the entire image data file gets saved in the history. Test a lot of patterns or textures or brush strokes, and that really begins to add up slowing PSP down.

It's best to turn off saving history in the preferences.

@JoeB they mentions that the smaller image, the one with fewer pixels, has a file size of over 1GB while the larger one is only . Which you can see in the File Size on Disk.

NOTE: for the numbers below I'm dividing by 1024 as you're supposed to do, when converting from KB to MB and again from MB to GB.

200ppi - 1653x2339. If you assume 32-bit per pixel (RGBA full raster layer) you're looking at an uncompressed size of 117MB per layer without transparency. Maybe. But the numbers in the screenshots don't show that reality at all.

On Disk it shows - 1027584k = 0.97 GB
In RAM 65984 - 64.43MB

300ppi - 2480x3508 - 32-bit = 265MB uncompressed per layer.
On Disk it shows 3487k = 3.40MB
In RAM 136364k - 133MB

These numbers don't really make sense on their own with what we know of the size of the image.

I'm assuming, this has to do with either # of layers used, or, most likely, saved history.

[frostyboy]

I'm assuming, this has to do with either # of layers used, or, most likely, saved history.

Thank you LeviFiction for your response.
You were correct with the saved history. I can't remember if I set that, or if it is set by default but I switched it off, replicated the image by recreating it (at 300ppi actually) and the problem had gone.
As you could see from my original post, the original 300ppi image had 9 layers and the 200ppi image only 8 layers. The 200ppi image file size, however, was 294 times larger.
The recreated image was only 12.6Mb in size.
When I create a new poster or play image, I often use a previous psp image with multiple layers which I then edit and modify to suit the new requirements. Little did I realise that ALL the previous editing history of the image was being stored and creating 'bloatware' so to speak.
I'm very grateful for the time taken to educate me. Thank you.

[JoeB]

@Levifiction: You are correct, of course. I didn't read that info in the screenshots correctly! For some reason I read it as 1027K - must be oldtimers setting in - or perhaps time to see an optometrist again!