Cover Art: How to reduce PNG file size in GIMP?

I’m using GIMP.
My attempts to reduce the file size down to something around 2 MB are patchy at best.
One image file of 39MB made it into the CAA last time. I’m scanning at 600ppi in a sometimes futile attempt to reduce moire pattern.

GIMP actions:

  1. Rotate to fairly orthogonal. (Gimp does not provide a handy on-image right-angle to facilitate accuracy in this.)
  2. Cropping to outside of cover art image to leave maximal information for later users.
    3.The scans are at 600ppi
    I reduce the resolution to 300ppi using GIMP.
    Gimp just automatically increases the dimensions of the image when I do this. So the file size remains the same.
    Can anyone offer cookbook/recipe instructions on how to shrink the size of the exported PNG file when using GIMP?

And no I wont be doing anything else, like output to jpeg, color balancing, use ImageMagik, use the command line, etc, etc, etc. If you like doing that sort of thing email me and I’ll look at posting my cover art to you for only a minimal charge. :smiley:
Whip waving and stern voice can be arranged but do cost much more.

I’m thinking this should be quick and easy. If it isn’t then I’ve got other things to do.

GIMP is a bit funny with dpi. If I resize an image, I just change the size in pixels. You can also use percentage.

Apologies for these two remarks that I am fully aware are slightly off-topic and not a direct answer to your question.
So please ignore this completely if you wish.

  1. I personally would prefer a 600dpi jpg to a 300dpi png.
  2. for optimizing the file size of a png image, I can recommend PNGGauntlet.

Sorry, but I don’t have any suggestions for reducing the size of the PNG files, but…

For rotating, I use a plug-in called auto-rotate available here. You simply draw a path on the image marking the desired horizontal (or vertical) and then select Layer -> Auto-Rotate. It’s a great time saver, and makes precise rotating much easier. Articles describing the plug-in are available here and here.


Like mfmeulenbelt, I’ve been scaling down by pixels also, normally to a height of 1000. That’s giving me file that are mostly in the 1.5 - 2 MB range.

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Please leave the resolution at 600 dpi and save as high quality jpeg instead. A typical CD front cover will result in at 5–10 MB file. This gives a much better quality-per-filesize ratio. See discussion in this thread.

GIMP lets you do several different modifications to images, and it helps to be very clear about what modification you want to do. When you say, “reduce the resolution”, what about the image do you want to change, and what GIMP tool are you using to do it?

I use GIMP 2.8 on Mac OS X. GIMP offers me a “Canvas Size…” tool, a “Print Size…” tool, and a “Scale Image…” tool. GIMP uses terminology of “Image Size”, “Canvas Size”, and “X [or Y] resolution”. They are different things.

If you scan a 4.75" CD cover at 600ppi, the result should be an “image size” of about 2850 pixels x 2850 pixels.

Changing the “X [and Y] Resolution” from 600 pixels/in to 300 pixels/in using either the “Scale Image…” tool or the “Print Size…” tool will not change the image size. It just changes a pair of numbers tucked away in the image that say how many mm wide and high to make the image when printing.

To make the image have a smaller file size, the most fundamental change to make is to reduce the “image size”. For that, I would use the “Scale Image…” tool. I would set the “image size” units to “percent”. I would make sure that the “chain” symbols linked the “Width” and “Height” boxes, and the “X resolution” and “Y resolution” boxes. Then I would edit the “Width” and “Height” boxes to read 50.00 (as in 50% as many pixels in width and height). Then I would edit the numbers in the “X resolution” box to be a number half as large. If it originally read “600”, I would change it to “300”. Changing the value in the “X resolution” box should cause the value in the “Y resolution” box to change also. The result should be an image with 1/4 as many pixels, and so a file size 1/4 as large, but which prints to the same physical size in mm or inches.

GIMP can make things quick and easy, but only if you use it just right. For a simple scaling like this, I’d actually be more likely to use ImageMagick, but you seem to have ruled that out.

Does this help?
—Jim DeLaHunt


Thanks to everyone.
And the cigar goes to Jim. :1st_place_medal:

I don’t have the technical know-how that would enable me to make an informed decision.
Which leaves me to look for authority.
US library archivers recommend PNG.
Their arguments are as convincing as yours.
Could they be wrong - sure - my local library system catalogues multi case CD releases with the one catalog number - which makes it impossible to order the exact CD I’m after, and they can’t see a problem.
If they’re wrong about PNG then there will be lots of wrongness.

You could also just upload the full-size PNGs to the Cover Art Archive. They have never complained about file sizes. Storage space is not very expensive anymore (and we’re talking about images, not video). Bandwidth is more expensive, but most uses for the cover art data use the smaller thumbnails that are automatically produced when images are uploaded anyway.

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The full sized scans are ~40MB.
Currently it would be possible to back up CAA on a single HDD according to my scrap paper calculation…
Which I do like the idea of.
40MB scans would seem to rapidly increase the number of HDD required.
But I probably will put in bigger than usual files for front covers.

Short general answer (IMHO)

PNG format is a lossless compression file format (archival - think FLAC)
It’s what was scanned, no image data loss. (Archivist choice)

JPG format is a lossy compressed file format (think MP3)
You make the decision on how much data is thrown away in order to get a smaller file size.

Historical documents should be in PNG so that the archivist can zoom in to see if it is a period “.” or a comma “,” (or maybe dirt or smudge).

Viewing a web page image, music player display, or some other image that is only need to convey the visual aspects of the image will generally be in JPG to keep the size smaller.

Everything else is really up to the user to decide.

I leave my JPG scans uncompressed, I do both 300 and 600 dpi depending on my desires (mostly 600dpi). Sometimes I am forced into 300dpi due to overall size, such as the folded out book cover I just loaded last night.

I never said that JPEG is better then PNG. Also, I’m pretty sure that US library never ever archives at 300 dpi…

Let me state it this way:

  • By far the most important decision is the resolution, since this makes a huge difference quality-wise. (Not only) my strong advice is not to go below 600 dpi for archival purposes.
    More precisely: The image resolution should be well above the original printing resolution.
  • Only now comes the decision is about the file compression. PNG is lossless, JPEG is lossy. For JPEG, only high quality (> 90%) makes sense. JPEG will give much smaller files (~factor 5), at the price that theoretically the image quality might suffer a bit from the lossy compression. In practice, I never experienced a visible artifact with JPEG, as long as the scanning resolution was high (see above) and the JPEG compression was set to > 90%. This decision is up to you, my personal preference goes to JPEG as the advantage (much smaller size) by far outweighs the disadvantage (the theoretical risk of visible compression artifacts).

Conclusion: Never go below 600 dpi. Pick PNG or hq JPEG, both are perfectly fine in this resolution range.


When I look I find they’re not recommending PNG but TIFF.

You and FADGI are both recommending 600 or higher ppi/dpi for highest quality archiving of photos. Which would apply to many front covers.

PNG and TIFF both are lossless formats, so there is no difference in data quality. In contrast to TIFF, PNG was designed as a free format from ground up, which makes it preferably over TIFF.

If I could please also request that you upload non-reduced 600dpi png’s to the database. If you’re interested in archival images, the big file size means you’re doing things right!

Of course you can always downscale images for your own use afterwards, but I would appreciate it if you upload the images before this step.

If you really want to spend 50 MB, 1200 dpi hq JPEG will give much better quality than 600 dpi PNG!

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If your scanner can handle 1200dpi, go for it. Why would you go for jpg over png @spitzwegerich ?

All moderately recent scanners can handle 1200 dpi.

As I wrote: Increasing the resolution gives a much better bargain in image quality than switching from lossy to lossless compression.

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That doesn’t answer my question at all. I’m saying that in that case, why not use PNG and bigger resolution?