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Thread: How to create the best lo-res copy of a large (100 MB) graphic image?

  1. #1
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    Default How to create the best lo-res copy of a large (100 MB) graphic image?

    I have a bunch of very high resolution graphic images that were created for me by professional graphic artists. Several of them are photo collages containing 50-100 photographs. The master images are between 0.14 and 0.65 square meters (1.5 to 7 sq ft) and from 50 MB to almost 100 MB. Most are PNGs, some are JPGs. I would like to create low res images for distribution and other purposes. I am wondering what the best way might be to do that.

    I just tried opening one of them in IV and then saving it as a JPG, setting the file size to 5 KB. That seemed to work pretty well. On my computer screen, I can't tell which is which.

    Is there a better way?

    Thanks

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    Moderator Enterprise User Bhikkhu Pesala's Avatar
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    I find that hard to believe. If I save a 20Mbyte PNG image @ 5 Kbytes the quality is the lowest JPG value of 10, the image is 131Kbytes, and the image looks really bad.

    Please tell us the size of the images in pixels.

    The best method would be to resize them to the desired size in pixels first, e.g. to suit a standard monitor width of 1920 pixels, then save that image with JPG compression. Depending on the source image, the quality slider should be somewhere between 50 and 90 for acceptable quality. Any image with text or sharp contrast, e.g. images of machinery, should use the higher quality, while seascapes or landscapes often look fine at the lower quality setting of 50.

    Below is an image showing the quality/file size graphs for different sample images. Saving as PNG will work best for images of text or images with a small number of different colours colours, e.g. software interface dialogues, etc.

    Click image for larger version. 

Name:	Quality Compression Tradeoff.png 
Views:	7 
Size:	136.0 KB 
ID:	5719

    Saving as WebP may give better results. Again, the results will depend on the image content.
    Last edited by Bhikkhu Pesala; 15.01.2020 at 10:06 AM.

  3. #3
    Moderator Enterprise User Bhikkhu Pesala's Avatar
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    I have some WebP images on my website. For this Mahasi Gateway Image:

    1. The original 2,950 x 2,055 pixel PNG image was 8.29 Mbytes on disk
    2. The resized 800 x 557 pixel JPG image is 50.38 Kbytes on disk (save quality = 50)
    3. The 2,950 x 2,055 pixel WebP image is 334.52 Kbytes on disk (save quality = about 50). Click on the magnifying glass icon to view the image in your web browser.

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