On 07/26/18 08:59, Rodolfo Medina via gimp-user-list wrote:

Hi all Gimp users.

I'm new to Gimp. I need rescaling and resizing a jpeg picture. But, after rescaling and resizing it with Gimp, and after exporting it as jpeg format, the size of the output file is much smaller: the original 12MB become 300K... Is it possible, and how, to preserve the original quality?

Thanks for any help.

"Quality" is two things:

* amount of "detail". This is directly related to the size of the image in pixels. If you take a 4000x3000 picture from a camera and scale it down to 1000*750, you have lost detail. Given identical compression settings, the file size is roughly proportional to the pixel count (so the 1000*750 is 16x smaller than the 4000*3000).

* lack of "distortion" due to compression. To compress images, the JPEG format drops some of the detail where is is supposed to be not visible, this is the JPEG "quality factor". Between 100 and 85, the lost is minimal, under 60, compression artifacts cna be a bit too noticeable.

In both cases, quality is related to the amount of information in the file, and even with the best compression algorithms, this is related to the file size. If you want to keep the quality, you have to keep the size. But there are plenty of places where top quality isn't required, if the picture is meant to be displayed on a HD screen it doesn't need to be more than 1080px high, for instance.

On 07/29/2018 05:45 AM, Ofnuts wrote:

On 07/26/18 08:59, Rodolfo Medina via gimp-user-list wrote:

Hi all Gimp users.

I'm new to Gimp. I need rescaling and resizing a jpeg picture. But, after
rescaling and resizing it with Gimp, and after exporting it as jpeg format, the
size of the output file is much smaller: the original 12MB become 300K... Is it possible, and how, to preserve the original quality?

Thanks for any help.

"Quality" is two things:

[ etc ]

Here's some more pointers...

In the present context, "quality" has two factors, resolution and compression.

Resolution:

Resolution is measured in DPI or "dots per inch" which means pixels per inch (or other unit of measurement) in your image editor or on-screen display.

72 DPI is a common default resolution for exported digital images. I would call that a "legacy" setting, and a lower limit for most uses.

96 DPI images look good on computer screens, and most website maintainers avoid higher resolutions (= larger file sizes) for most images, to save bandwidth. For quite a while 96 DPI was the native resolution of most monitors.

Most word processor documents will be viewed on computer monitors and/or printed with office laser printers. 150 DPI usually looks good in both of these contexts. Since word processor documents are often transmitted via e-mail, and stored in bulk on desktop computers, higher resolution images may be undesirable, again due to file size issues affecting bandwidth and storage space.

For color printing on glossy paper, 300 DPI resolution is a well established convention. Resolutions higher than 300 DPI will produce larger file sizes, but usually not result in a visible difference in the finished product. The user will be aware of higher resolution needs on any occasion where they turn up.

Compression:

The JPG format uses lossy compression; the compression algorithm 'guesses' what information can be discarded without a major impact on the appearance of the image, resulting in very substantial reductions in file size. Note that a JPG compression setting of "100%" quality still uses lossy compression, producing larger file sizes with little or no visible difference from smaller files compressed at, for instance, 85% quality.

Another factor to consider: When a JPG file is opened, edited, and re-saved as JPG, another round of lossy compression 'on top of' the already lossy compression from the original image file results in further loss of detail. This is one of the reasons why programs like the GIMP "save" images in the editor's native lossless format, and "export" images to other formats including JPG.

Where higher quality matters more than lower file size, for instance images intended for color printing on glossy stock, the lossless PNG format will usually be more appropriate than JPG. The output files are substantially larger, but with PNG what you save is what you get, with no loss of detail except due to scaling the image up or down in size while editing.

:o)

To add to Steve Kinney's excellent summaries, note also that PNGs will export (i.e. maintain) transparency while JPGs will not.

R. Strong

-----Original Message----- From: Steve Kinney
Sent: Sunday, July 29, 2018 11:36 PM To: gimp-user-list@gnome.org
Subject: Re: [Gimp-user] How to export jpeg without loss of quality?

clip...

Where higher quality matters more than lower file size, for instance images intended for color printing on glossy stock, the lossless PNG format will usually be more appropriate than JPG. The output files are substantially larger, but with PNG what you save is what you get, with no loss of detail except due to scaling the image up or down in size while editing.

:o)

Thanks all who kindly replied to my request of help... I understand there's a loss of the original size because of my resizing and scaling under Gimp.. But here we're talking of a 12MB file that becomes 380KB...! Isn't it too much also with those considerations?

Depends what you mean by "too much." A very large original source file would usually be a Good Thing, indicating a high resolution (high DPI) image. Squeeze 12MB of data into a 300KB container, and a LOT of information gets lost no matter what: More so, however, if you are using a lossy format for the output file.

As for the suggestion of using the png format, I have the original image in jpeg format... Are you suggesting of converting jpg to png before modifing it with Gimp? Or to save it, within Gimp, from jpg to png? The final use of this job is not for the web, but for printing, so preserving the original quality is important...

Normal procedure is to start with the largest available source image (usually the only one in the case at hand), open it with the GIMP, and save it as an XCF file. XCF uses lossless compression, and also saves all image layers, text layers, masks, etc. present in images in process. If you have to quit editing and resume later, the XCF file will still have all your work in progress.

Sometimes editing large images may be a slow-ish process, more or less so as the computer used has more or less system memory and a slower or faster CPU. Some operations such as rotating images, scaling images, or applying complex filters to whole images may take a while with an image that started as a 12MB JPG.

If possible, it's best to avoid scaling the original image down until you consider it print-ready. Then, save the XCF file one last time /before/ scaling down, just in case you have to go back and change something, or want to re-use all of part of the image in other projects. That done, calculate the X and Y dimensions of your print image in pixels, based on 300 per inch on the printed page (or other units per locale), and scale it down. Export the scaled image as PNG, and it's ready to import into a desktop publishing tool like Scribus or etc.

Your desktop publishing application will enable you to control the page margins, where the image (or images) appears on the page, and add lines, boxes, text or etc. as needed. Once your page is configured for print, save the desktop publisher's source file in its own native format (again, so you can change or reuse it later), then export the file to PDF, making sure the PDF document is set to 300 DPI. (Or other, if you are using higher or lower resolution images.)

PDF a.k.a. Portable Document Format, could also be called Printable Document Format, because that's what Adobe, working with printer manufacturers, designed the PDF format for all those years ago.

The first copy printed out will be your "proof." Examine it closely, with special attention to color: Printers and monitors have gotten a lot more uniform in recent years, and with any luck your image will look just right when printed. But if not, open up your XCF file again, save it with a new name (maybe add a version number to the original file name), and work on the new XCF file. Precise color adjustment methods are WAY out of scope for beginners, and for advice delivered here: Years of reading tutorials and watching videos on this subject have given me /some/ clues but I know I have miles left to go.

As a starting point, do Layers > New from visible, and work on the new layer rather than the "original" layers; that makes starting over as needed way easier. Or maybe your color problem only affects parts of the image on one layer; if so, duplicate that layer, and work on the duplicate. You may find the Hue-Saturation tool, the Levels and Curves tools useful; note that you can separately adjust the Red, Blue and Green channels with the Curves tool.

One gotcha: If your printed page has any text added with the desktop publishing application (not in the GIMP image or images), there will be print errors if the computer the printer is hooked to does not have the fonts you used installed on it: Typically, the program that's sending the job to the printer will substitute a generic font for the one you wanted.

:o)

On 08/13/2018 01:19 AM, Steve Kinney wrote:

That done, calculate the X and Y dimensions of your print image in pixels, based on 300 per inch on the printed page (or other units per locale), and scale it down. Export the scaled image as PNG, and it's ready to import into a desktop publishing tool like Scribus or etc.

Silly me. In Layers > Scale Layer, you can select a DPI value; so select 300. Then change the Units to inches (or etc) and set the dimensions you need for your print-ready image. Bang, zoom, no calculator needed.

:D

On 08/13/2018 01:23 AM, Steve Kinney wrote:

Silly me. In Layers > Scale Layer

Well dang it. I meant Image > Scale Image.

Getting late here... :D

On Mon, 2018-08-13 at 01:04 +0200, Rodolfo Medina via gimp-user-list wrote:

I'm
embedding that picture in a pdf file after converting into eps format. [...] The picture is a photo, that I have in its original jpg format..., 12MB big.

That's not very large for a print-resolution image. People who work with print files are used to dealing with 100MByte images or larger, and it's not a big deal.

Liam

Liam Quin - web slave for https://www.fromoldbooks.org/
(slave ankh on irc)