Tiff Compression is different between versions

[AVO-SSA]

I am in the process of updating an old project that is using an outdated version of Magick.NET-Q8-AnyCPU, version 7.14.5. When updating to the latest version the compression behaves differently. The old version using Compression Group4 looks good in all scenarios, while the new version will sometimes have images that are blurry. This seems to happen if there is something written on top of the images, the background has some greys, or there are some blocky text/images on the page. Did something change between the versions that is causing this?

        private static readonly MagickReadSettings _settings = new()
        {
            UseMonochrome = true,
            Compression = CompressionMethod.Group4,
        };

            using MagickImageCollection col = new MagickImageCollection();
            for (int i = 0; i < pages; i++)
            {
                MagickImage mi = new MagickImage(stream, _settings)
                {
                    Format = MagickFormat.Tif
                };
                if (rotation != 0)
                    mi.Rotate(Convert.ToDouble(rotation));
                col.Add(mi);
            }

            return col.ToByteArray();

[dlemstra]

How do you expect me to answer your question? I have no clue what your new version is and you are not providing me with any images that show what is happening?

[AVO-SSA]

Apologies for omitting the information. I am using the latest version of Magick.NET, which is 14.5.0. Most of the images I deal with have sensitive information but hopefully you can get an idea of the issue with what I'm providing. The clearer image is 7.14.5 and the blurry one is 14.5.0. The clearer one also has a smaller file size than the blurry one. I can use a different compression with 14.5.0, like LZW, but it greatly increases the file size, which can cause issues. Please let me know if you need any additional info.

[dlemstra]

Thanks for the extra information, that makes it much easier to give you a better answer.

I am not getting the same pixelated image when I just set col[0].Settings.Compression = CompressionMethod.Group4; but a real monochrome image that only has two colors. And this would be the expected output when compressing a tiff image with Group4 compression. Maybe you could use Zip or LZMA compression instead? The latter doesn't work right now due to a bug but that should be fixed in the next release. And them I get the following 37kb file based on your input png file: lzma.zip

[AVO-SSA]

If this is the expected behavior when compressing tiffs with Group4 then something definitely changed between the versions. And if there isn't a way to get back to how it was then it seems I'll have to try a different compression method.

I've tried a few different methods and so far, JPEG seems like the best one. The quality is good enough compared to no compression and the file size is acceptable. For size I am testing with a 58-page document and I get the following results: Group4 9MB, JPEG 19MB, ZIP 39MB, None 46MB. You say LZMA is bugged; will the size / quality be comparable to ZIP when it is fixed?

[dlemstra]

I have no idea what the size will become so you will need to test that yourself after the next release. You could also experiment with reducing the colors of the image yourself before saving it with Group4 compression. I am unable to reproduce your issue but that might be related to the fact that you are using a different input file.

[AVO-SSA]

I was able to get back to the original size and quality by using "mi.Sharpen()". Unfortunately, it takes longer than I would like to process it. I wonder if the previous version somehow was doing this under the hood but more efficiently.