MagicNET convert PDF to black and white (1bit) PNG

Question

One of our customers need to convert PDF shipping labels to PNG images. The PDF images need to be 300 DPI and have a bit depth of 1 (pure black and white without grayscale).

I have got this working but with some issues that i can not find any solution for.

My Code

MagickNET.SetGhostscriptDirectory(_ghostscriptPath);
            var settings = new MagickReadSettings();
            settings.Density = new Density(_dpi);
            //settings.ColorType = ColorType.Bilevel;                 // Not working
            //settings.Depth = 1;                                     // Not working
            //settings.SetDefine(MagickFormat.Png, "Bit-depth", "1"); // Not working

            using (var images = new MagickImageCollection())
            {
                images.Read(sourceFilePath, settings);
                using (var horizontal = images.AppendHorizontally())
                {
                    //horizontal.Density = new Density(_dpi);         // Not working
                    horizontal.BitDepth(1);                           // Testing (Sometimes commented out)
                    horizontal.Write(targetPath_working);
                }
            }

Result

When this code is run with the following setup (BitDepth(1) + 300 DPI) the PNG image is 1169x2303 and (4Bit depth).

When this code is run with the following setup (Removed BitDepth(1) + 300 DPI) the PNG image is 1169x2303 and (32Bit depth).

This gives me 2 primary issues, when BitDepth is set to 1 why is the PNG image still 4bit? Second the quality of that 4bit image is horrible and unreadable by a barcode scanner. It feels like the image is somehow being resized during the writing process. I would need to have the "sharpness" of the 32 bit image but as 1 bit.

Could someone point me in the right direction here, feels like i am lacking the knowhow of image conversion.

Thank you!

PS: I'm using Magick.NET-Q8-AnyCPU (7.23.3)

Test suggestion 1

Results in image with black lines as borders for everything, none of the text is filled with black however the image is now 1bit as expected.

MagickNET.SetGhostscriptDirectory(_ghostscriptPath);
            var settings = new MagickReadSettings();
            settings.Density = new Density(_dpi);
            settings.SetDefine(MagickFormat.Png, "Bit-depth", "1");

            using (var images = new MagickImageCollection())
            {
                images.Read(sourceFilePath, settings);
                using (var horizontal = images.AppendHorizontally())
                {
                    horizontal.AutoLevel();
                    horizontal.Threshold(new Percentage(50));
                    horizontal.Write(targetPath_working);
                }
            }

Try using `SetDefine(MagickFormat.Png, "Bit-depth", "1")` and including thresholding to 50% to ensure the image is bi-level because the PNG writer will not honour your `defines` if it believes that would cause a loss of quality. — Mark Setchell, Jun 24 '21 at 08:42
Edited my original post, with that code i get a picture that looks like black border lines on text and rest is white. There is no black inside the text. — Carl, Jun 24 '21 at 09:13
Ok, try using `auto-level` first in order to move your dark greys to solid black and your light greys to pure white before doing the thresholding. Explanation: If your blacks have brightness 3 and your greys have brightness 15, then thresholding them at 50% (i.e. 127 out of a possible 255) will make everything black - so make your whites white and your blacks black, then threshold. — Mark Setchell, Jun 24 '21 at 10:29
Adding `horizontal.AutoLevel();` before `horizontal.Threshold(new Percentage(50));` had no effect. I still get the white with black borders on everything sadly. — Carl, Jun 24 '21 at 20:14

score 0 · Accepted Answer · answered Jun 29 '21 at 11:16

Found solution for my issue see code below. Had to use a number of different settings in combination for a good end result.

MagickNET.SetGhostscriptDirectory(_ghostscriptPath);
            var settings = new MagickReadSettings();
            settings.Density = new Density(_dpi);
            settings.SetDefine(MagickFormat.Png, "Bit-depth", "1");

            using (var images = new MagickImageCollection())
            {
                images.Read(sourceFilePath, settings);
                using (var horizontal = images.AppendHorizontally())
                {
                    horizontal.AutoLevel();
                    horizontal.Threshold(new Percentage(50));
                    horizontal.ColorType = ColorType.Bilevel;
                    horizontal.Format = MagickFormat.Png8;
                    horizontal.Write(targetPath_working);
                }
            }

MagicNET convert PDF to black and white (1bit) PNG

1 Answers1