Image quality and file size hacks

[ordreams]

GIMP Version: 2.8.14
Operating System: Mac OS
GIMP Experience: Basic Level

List any relevant plug-ins or scripts:
I plugged in the computer cable to the wall...?

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Hi all,

My goal:

- Load artwork scans to a commercial site.

Restrictions:

- RGB
- A JPEG file
- At least 1200 pixels x 1500 pixels
- Less than 50MB

My current method:

- Open original .tiff as .png in GIMP
- Export as .jpg and adjust resolution until it is under 50MB

Concern:

- Some scans are large files. This means a fairly big reduction in resolution.

Query:

- Are there any methods to maintain adherence to the site´s restrictions, but obtain the maximum quality of image?

Final message:

- Woo hoo! in anticipation of replies.

[Wallace]

Have you tried file compression on the .tiff, before opening with Gimp.

[rich2005]

50 MB for a jpeg is very generous. There is a big drop-off in file size with jpeg quality between 100 and 85 Vary rarely worth using 100 Below a quality setting of 85 there is a big drop off in quality versus file size.



The subsampling value also has an effect on file size. For example the previous Gimp 2.8 (edit: or was it Gimp 2.6 - memory fails) default jpeg setting was 85 and 4:2:2 against Gimp 2.10 95 and 4:4:4

When you export a jpeg if you enable "Show preview.." You get an estimate of file size This an A4 page size @ 300 ppi suitable for printing.

[ofnuts]

Assuming that "resolution" is actually the JPEG compression factor....

The biggest bang for the buck in terms of maximum size reduction with little quality impact is reducing the chroma subsampling (use "Chroma quartered", see the advanced JPEG options.

This said, the 50MB jpeg images in my collection are 100-megapixels monsters at Q99, so with 50MB you can encode fairly big images with good quality. If 50M isn't enough for you, you could be overdoing it. What is the size of the image in pixels, what is the intended purpose, and if this is for print, what is the printed size?

[ordreams]

Thanks everyone. I am a bit brain dead at the moment, but will consider the posts tomorrow.

[rich2005]

Here is a comparison using Gimp 2.8 which you are using ( but linux not Mac ) Larger image 8000x6600 pix prints @300 ppi 68 x 56 cm - just too large for an A2 sheet

Easy to see drop in file size with just a small decease in jpeg quality setting.

example.jpg (310.42 KiB) Viewed 4510 times

[ordreams]

Hello again,

The recommendations that you folks have provided appear to have worked.

Thanks!

Here is the info so far:

Original:

- .tiff
- 14414 x 14199 pixels
- 614 MB


Exported as:

- .jpg
- 87% resolution
- 4:2:0 subsampling
- 48.5 MB

Does this all seem like the way forward? In reference to:

Desired print size:

- art print
- 61 x 61cm

Yay.

[rich2005]

With that information 61 cm = 24 in (because ppi (also dpi) is usually stated in pixel-per-inch)

24 inch x 600 ppi = 14,400 pix about the pixel size of the tif.

600 ppi is well over the top for printing. Next time you scan, use 300 ppi maximum. Then it will be easy to get the required jpeg size.

Even that is maximum for an art print which typically is viewed from a distance

see: http://www.northlight-images.co.uk/what ... -distance/

[ofnuts]

see: http://www.northlight-images.co.uk/what ... -distance/

Yes, and since the distance increase linearly with image size, and the required definition decreases at the same time, the relative resolution is constant, and this relative resolution is the size in pixels. So the size in pixels is the same, whatever the physical print size. In practice, anything above 12Mpix is overkill.

[ordreams]

All very interesting and anxiety reducing! Thanks so much!

Just to double check; In general the reduction of the subsampling to 4:2:0 will be maintain image quality, more than reduction of the quality of the jpeg?

Eg: 4:2:0 at 97% quality should be predicted to yield better print quality than say 4:4:4 at 85%?