You have to copy the bits over a new image with the target resolution, like this:
using (Bitmap bitmap = (Bitmap)Image.FromFile("file.jpg"))
{
using (Bitmap newBitmap = new Bitmap(bitmap))
{
newBitmap.SetResolution(300, 300);
newBitmap.Save("file300.jpg", ImageFormat.Jpeg);
}
}
DPI should not be stored in an bitmap image file, as most sources of data for bitmaps render it meaningless.
A bitmap image is stored as pixels. Pixels have no inherent size in any respect. It's only at render time - be it monitor, printer, or automated crossstitching machine - that DPI matters.
A 800x1000 pixel bitmap image, printed at 100 dpi, turns into a nice 8x10" photo. Printed at 200 dpi, the EXACT SAME bitmap image turns into a 4x5" photo.
Capture an image with a digital camera, and what does DPI mean? It's certainly not the size of the area focused onto the CCD imager - that depends on the distance, and with NASA returning images of galaxies that are 100,000 light years across, and 2 million light years apart, in the same field of view, what kind of DPI do you get from THAT information?
Don't fall victim to the idea of the DPI of a bitmap image - it's a mistake. A bitmap image has no physical dimensions (save for a few micrometers of storage space in RAM or hard drive). It's only a displayed image, or a printed image, that has a physical size in inches, or millimeters, or furlongs.
This code using merge and convert 200 dbi
static void Main(string[] args)
{ Path string Outputpath = @"C:\Users\MDASARATHAN\Desktop\TX_HARDIN_10-01-2016_K";
string[] TotalFiles = Directory.GetFiles(Outputpath, "*.tif", SearchOption.AllDirectories);
foreach (string filename in TotalFiles)
{
Bitmap bitmap = (Bitmap)Image.FromFile(filename);
string ExportFilename = string.Empty;
int Pagecount = 0;
bool bFirstImage = true;
bitmap.SetResolution(200, 200);
ExportFilename = Path.GetDirectoryName(filename) + "\\" + Path.GetFileName(filename)+"f";
MemoryStream byteStream = new MemoryStream();
Pagecount = bitmap.GetFrameCount(FrameDimension.Page);
if (bFirstImage)
{
bitmap.Save(byteStream, ImageFormat.Tiff);
bFirstImage = false;
} Image tiff = Image.FromStream(byteStream);
ImageCodecInfo encoderInfo = ImageCodecInfo.GetImageEncoders().First(i => i.MimeType == "image/tiff");
EncoderParameters encoderParams = new EncoderParameters(2);
EncoderParameter parameter = new EncoderParameter(System.Drawing.Imaging.Encoder.Compression, (long)EncoderValue.CompressionCCITT4);
encoderParams.Param[0] = parameter;
parameter = new EncoderParameter(System.Drawing.Imaging.Encoder.SaveFlag, (long)EncoderValue.MultiFrame);
encoderParams.Param[1] = parameter;
// bitmap.Dispose();
try
{
tiff.Save(ExportFilename, encoderInfo, encoderParams);
}
catch (Exception ex)
{
}
EncoderParameters EncoderParams = new EncoderParameters(2);
EncoderParameter SaveEncodeParam = new EncoderParameter(System.Drawing.Imaging.Encoder.SaveFlag, (long)EncoderValue.FrameDimensionPage);
EncoderParameter CompressionEncodeParam = new EncoderParameter(System.Drawing.Imaging.Encoder.Compression, (long)EncoderValue.CompressionCCITT4);
EncoderParams.Param[0] = CompressionEncodeParam;
EncoderParams.Param[1] = SaveEncodeParam;
if (bFirstImage == false)
{
for (int i = 1; i < Pagecount; i++)
{
//bitmap = (Bitmap)Image.FromFile(filenames);
byteStream = new MemoryStream();
bitmap.SelectActiveFrame(FrameDimension.Page, i);
bitmap.Save(byteStream, ImageFormat.Tiff);
bitmap.SetResolution(200, 200);
tiff.SaveAdd(bitmap, EncoderParams);
}
} SaveEncodeParam = new EncoderParameter(System.Drawing.Imaging.Encoder.SaveFlag, (long)EncoderValue.Flush);
EncoderParams = new EncoderParameters(1);
EncoderParams.Param[0] = SaveEncodeParam;
tiff.SaveAdd(EncoderParams);
tiff.Dispose();
bitmap.Dispose();
File.Delete(filename);
}
}
It's simply a matter of scaling the image width and height up by the correct ratio. Not all images formats support a DPI metatag, and when they do, all they're telling your graphics software to do is divide the image by the ratio supplied.
For example, if you export a 300dpi image from Photoshop to a JPEG, the image will appear to be very large when viewed in your picture viewing software. This is because the DPI information isn't supported in JPEG and is discarded when saved. This means your picture viewer doesn't know what ratio to divide the image by and instead displays the image at at 1:1 ratio.
To get the ratio you need to scale the image by, see the code below. Just remember, this will stretch the image, just like it would in Photoshop. You're essentially quadrupling the size of the image so it's going to stretch and may produce artifacts.
Pseudo code
ratio = 300.0 / 72.0 // 4.167
image.width * ratio
image.height * ratio
Source: Stackoverflow.com