MagickNet PNG with Transparency to JPEG

Question

I am using MagickNet for image manipulation in my ASP.NET C# project. My issue is that I am uploading a PNG image with transparency and when I convert it to JPEG, I get a black background with some white spots instead of a white background for the transparent part.

  Stream su = upload.FileContent;

MagickNet.Image testimage = new MagickNet.Image(su);

testimage.Filter = FilterType.LanczosFilter;
testimage.Compression = CompressionType.JPEGCompression;
testimage.QuantizeDither = false;  
testimage.BackgroundColor = new Color(System.Drawing.Color.White);

testimage.Resize( new System.Drawing.Size(Convert.ToInt32(testimage.Size.Width * 0.4), Convert.ToInt32(testimage.Size.Height * 0.4)));
testimage.Write(System.Web.HttpContext.Current.Server.MapPath(".") + "\\temp\\" + DateTime.Now.Hour +  "-"  +DateTime.Now.Minute + "-" + DateTime.Now.Second + ".jpg");
su.Close();
su.Dispose();

testimage.Dispose();
Magick.Term();

I played with it and always get the wrong result that I am after. Sometimes I get a transparent background but some parts of the image at the outer region have white dots. I also resize the image to be smaller than what it is. I think the re-sizing it causing the issue.

update: this is caused because of the resizing for some reason. Without resizing it works. Having said that, I need to resize, so I need it to work with it.

Thanks.

Do you resize before converting to jpg or after. Also jpg is lossy so you'll never get pure white where the png was transparent. — Karlth, Mar 07 '13 at 10:53
must be a way to flatten the image, a function of that sort. Without the resize it works, and I do get pure white because it's 100% transparent without color in those specific transparent areas — Liron Harel, Mar 07 '13 at 11:13
Is the resizing algorithm set for interpolation? Then that could be your problem. Try specifying (if possible) "nearest neighbor" instead. — Karlth, Mar 07 '13 at 15:01

score 0 · Answer 1 · answered Aug 10 '15 at 17:53

0

Try to composite onto a white background image.

Image bg = new Image(testimage.Size, new ColorRGB(255, 255, 255));
testimage = bg.Composite(testimage, 0, 0);

answered Aug 10 '15 at 17:53

Lou Franco

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This solution isn't compatible with MagickNet library – Otto Kanellis Nov 08 '19 at 12:16

score 0 · Answer 2 · answered Jul 22 '16 at 16:56

First of all, it is better to create your MagickImage object with the desirable size the speed of reading the file/stream with the required size can in some situation 100 times faster. You may not have that error.

using(var testimage = new MagickImage(yourstream/yourFileAddress, width, height)
{
....
}

But if you convert the MagickImage to Bitmap and then save the bitmap as jpg you can see that the image has a white background

using (var testBitmap = testimage.ToBitmap())
{
     testBitmap.Save(@"d:\temp.jpg"); 
}

also using is much better that calling dispose member function. Because if your code throw exception before it reaches the dispose call your object will remain in the memory. But with using, if program jump out of the block the object will be disposed.

MagickNet PNG with Transparency to JPEG

2 Answers2