How to change resolution DPI of an image

Question

I have a JPEG picture with a DPI of 72  I want to change 72 dpi to 300 dpi   How could I change resolution of JPEG pictures using C

User · Answer

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.

User · Answer

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

User · Answer

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

User · Answer

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   gt  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  lt  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

User · Answer

This article talks about modifying the EXIF data without the re-saving re-compressing  and thus loss of information -- it actually uses a  trick   there may be more direct libraries  required by the SetResolution approach  This was found on a quick google search  but I wanted to point out that all you need to do is modify the stored EXIF data   Also   NET lib for EXIF modification and another SO question  Google owns when you know good search terms

[c#] How to change resolution (DPI) of an image?

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