Resizing Image Quality Reduction

Answer 1

My first thought was that .NET's jpeg encoder uses chroma subsampling even at the highest quality setting, so color information is stored at half resolution. The setting is hard-coded as far as I can tell. But that wouldn't explain why you would get better quality in the second example. Unless maybe in the 2nd it didn't use antialiasing, giving a sharper (but lower quality) image and the artifacts went unnoticed.

Edit: The dst.Save(m, format); looks like your problem. You're encoding it as jpeg there, with default quality (not 100%), then immediately decoding it back to an image. dst is already an Image (Bitmap class inherits from Image), so you can just return it as-is

Answer 2

I found this example snippet of code (sorry I've forgotten the source) that might help you:

// Create parameters
EncoderParameters params = new EncoderParameters (1);

// Set quality (100)
params.Param[0] = new EncoderParameter(Encoder.Quality, 100);

// Create encoder info
ImageCodecInfo codec = null;
foreach (ImageCodecInfo codectemp in ImageCodecInfo.GetImageDecoders())
{
    if (codectemp.MimeType == "image/jpeg")
    {
        codec = codectemp;
        break;
    }
}

// Check
if (codec == null)
    throw new Exception("Codec not found for image/jpeg");

// Save
image.Save(filename, codec, params);