Better compression of edited PNG files about qimgv HOT 9 CLOSED

I seem to have found the issue and it looks like a Qt bug. Setting aside that it equals Quality with Compression (probably influenced by imagemagick), it has a wrong assumption: more quality == less compression, or they read the libpng API wrong.

This is what the libpng headers say about setting the compression level.:

/* Set the library compression level.  Currently, valid values range from
 * 0 - 9, corresponding directly to the zlib compression levels 0 - 9
 * (0 - no compression, 9 - "maximal" compression).  Note that tests have
 * shown that zlib compression levels 3-6 usually perform as well as level 9
 * for PNG images, and do considerably fewer caclulations.  In the future,
 * these values may not correspond directly to the zlib compression levels.
 */

This is the problem code in qpnghandler.cpp:

static bool write_png_image(const QImage &image, QIODevice *device,
                            int quality, float gamma, const QString &description)
{
    QPNGImageWriter writer(device);
    if (quality >= 0) {
        quality = qMin(quality, 100);
        quality = (100-quality) * 9 / 91; // map [0,100] -> [9,0]
    }
    writer.setGamma(gamma);
    return writer.writeImage(image, quality, description);
}

It reverses the magnitude of the specified quality/compression setting, from 0 to 9 and from 100 to 0. Thus PNG images set to be saved with the highest quality are saved with the least compression.

from qimgv.

And it's been fixed for Qt 5.12 but you must set compression option and must not set quality for saving PNG images. The behavior in Qt is to map 0 quality to the highest compression of PNG images and 100 to the least compression.

from qimgv.

This is the current dev branch code in qpnghandler.cpp:

static bool write_png_image(const QImage &image, QIODevice *device,
                            int compression, int quality, float gamma, const QString &description)
{
    // quality is used for backward compatibility, maps to compression

    QPNGImageWriter writer(device);
    if (compression >= 0)
        compression = qMin(compression, 100);
    else if (quality >= 0)                                                                                                                                                                                                                            
        compression = 100 - qMin(quality, 100);

    if (compression >= 0)
        compression = (compression * 9) / 91; // map [0,100] -> [0,9]

    writer.setGamma(gamma);
    return writer.writeImage(image, compression, description);
}

from qimgv.

So if you could detect the Qt version at runtime and the file format you could handle both Qt behaviors and fix this bug.

from qimgv.

Or just use the correct old quality behavior when saving PNG images. No Qt version detection needed.

from qimgv.

Just now I noticed there is a dev branch and it hardcodes quality to 0. Are you sure it won't save JPEG files with lower quality?

from qimgv.

Just now I noticed there is a dev branch and it hardcodes quality to 0. Are you sure it won't save JPEG files with lower quality?

Yeah it will corrupt jpegs. That's a leftover code from when i was testing different compression / quality values.

And it's been fixed for Qt 5.12

Could you please post a link to a bugreport? I wanna see what they did

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Also do you think i should present a compression quality option in ui (for lossy formats) or leave it hardcoded at some value?

from qimgv.

There doesn't seem to be a bug report but a direct to review patch
https://code.qt.io/cgit/qt/qtbase.git/commit/?id=1192c463db34e3f69be4d9ef976246ce265143a3

I think a sensible hardcoded value is fine for PNG but not so much for JPEG. On the other hand if quality or file size is a concern then the user should use a different tool for image editing. Considering these I think no option is needed. Gwenview doesn't have it and many other image viewers don't either.

from qimgv.