C# change dpi of an uploaded image

I've got to following function which is called to change the resolution of an image. I want to do this so uploaded image with for example 300dpi will be modified to 72dpi (for web). This question is related to another question here on SO where i'm working on.

I'm creation an extension method for this to be able to use this function on more places in my application, instead of only when uploading new files. (See above mentioned question)

public static byte[] SetDpiTo72(this byte[] imageToFit, string mimeType, Size newSize)
{    
    using (MemoryStream memoryStream = new MemoryStream(), newMemoryStream = new MemoryStream())
    {
        memoryStream.Write(imageToFit, 0, imageToFit.Length);
        var originalImage = new Bitmap(memoryStream);

        using (var canvas = Graphics.FromImage(originalImage))
        {
            canvas.SmoothingMode = SmoothingMode.AntiAlias;
            canvas.InterpolationMode = InterpolationMode.HighQualityBicubic;
            canvas.PixelOffsetMode = PixelOffsetMode.HighQuality;
            canvas.DrawImage((Image)originalImage,0,0, newSize.Width, newSize.Height);

            newBitmap.SetResolution(72, 72);
            newBitmap.Save(newMemoryStream, ImageFunctions.GetEncoderInfo(mimeType), null);
        }
        return newMemoryStream.ToArray();
    }
}

The mentioned extension methode is being called in a function similar to the situation below;

if (newSize.Width > originalImage.Width && newSize.Height > originalImage.Height)
{
     newSize.Width = originalImage.Width;
     newSize.Height = originalImage.Height;

     uploadedFileBuffer = uploadedFileBuffer.SetDpiTo72(uploadedFile.ContentType, newSize);

     return CreateFile(newSize, uploadedFile, uploadedFileBuffer);
}

The bytearray coming in is the file as an bytearray. It already has the correct size, but I want to change the resolution to 72dpi. However after exectution and saving the image the resolution is still the originale entered resolution, which is 300dpi. How can I do this?

UPDATE AFTER SEVERAL ANSWERS:

public static byte[] SetDpiTo72(this byte[] imageToFit, string mimeType, Size newSize)
        {
            using (MemoryStream memoryStream = new MemoryStream(), newMemoryStream = new MemoryStream())
            {
                memoryStream.Write(imageToFit, 0, imageToFit.Length);
                var originalImage = new Bitmap(memoryStream);

                using (var canvas = Graphics.FromImage(originalImage))
                {
                    canvas.SmoothingMode = SmoothingMode.AntiAlias;
                    canvas.InterpolationMode = InterpolationMode.HighQualityBicubic;
                    canvas.PixelOffsetMode = PixelOffsetMode.HighQuality;
                    canvas.DrawImage((Image)originalImage,0,0, newSize.Width, newSize.Height);

                    originalImage.SetResolution(72, 72);

                    var epQuality = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, 75);
                    var epParameters = new EncoderParameters(1);
                    epParameters.Param[0] = epQuality;

                    Image newimg = Image.FromStream(memoryStream);

                    //Getting an GDI+ exception after the execution of this line.
                    newimg.Save("C:\\test1234.jpg", ImageFunctions.GetEncoderInfo(mimeType), epParameters);

                    originalImage.Save("test.jpg", ImageFormat.Jpeg);

                    //This line give me an Argumentexception - Parameter is not valid.
                    //originalImage.Save(newMemoryStream, ImageFunctions.GetEncoderInfo(mimeType), epParameters);
                    //newMemoryStream.Close();
                }
             开发者_开发百科   return newMemoryStream.ToArray();
            }
        }

The stackstrace which comes with the exception is telling me the following;

   at System.Drawing.Image.Save(String filename, ImageCodecInfo encoder, EncoderParameters encoderParams)
   at Extensions.ByteArrayExtensions.SetDpiTo72(Byte[] imageToFit, String mimeType, Size newSize) in C:\Website\Project\Extensions\ByteArrayExtensions.cs:line 356
   at CMS.Presentation.FileFunctions.CreateFullsizeImage(HttpPostedFileBase uploadedFile, Size newSize, Byte[] uploadedFileBuffer) in C:\Website\Project\CMS.Presentation\FileFunctions.cs:line 197
   at CMS.Presentation.FileFunctions.CreateFile(HttpPostedFileBase uploadedFile, INodeService nodeservice, Guid userId, Node parentNode) in C:\Website\Project\CMS.Presentation\FileFunctions.cs:line 53

In the mean time I've also developed another function (see below) resizing just a bitmap. And this seem to work correctly. I can't use this function with my current implementation though because it returns just an Bitmap. Or should i change everything to work with bitmaps?

private static Bitmap ResizeImage(Image image, int width, int height)
        {
            var frameCount = image.GetFrameCount(new FrameDimension(image.FrameDimensionsList[0]));
            var newDimensions = ImageFunctions.GenerateImageDimensions(image.Width, image.Height, width, height);
            Bitmap resizedImage;

            if (frameCount > 1)
            {
                //we have a animated GIF
                resizedImage = ResizeAnimatedGifImage(image, width, height);
            }
            else
            {
                resizedImage = (Bitmap)image.GetThumbnailImage(newDimensions.Width, newDimensions.Height, null, IntPtr.Zero);
            }

            resizedImage.SetResolution(72,72);

            return resizedImage;
        }

Ok, I tried it only on files on harddrive, but it should work with streams too.

        Bitmap bitmap = new Bitmap(loadFrom);
        Bitmap newBitmap = new Bitmap(bitmap);
        newBitmap.SetResolution(72, 72);
        newBitmap.Save(saveTo);

Took me a while, but I finally found the problem! The problem lied in the ResizeImage function I used. In the 'GetThumbnailImage' to be specific. I ran into another problem with blurry images, which was explainable because GetThumbnailImage would stretch up the created ThumbNail to the desired size. And the resolution off the thumbnail never changes.

private static Bitmap ResizeImage(Image image, int width, int height)
        {
            var frameCount = image.GetFrameCount(new FrameDimension(image.FrameDimensionsList[0]));
            var newDimensions = ImageFunctions.GenerateImageDimensions(image.Width, image.Height, width, height);
            Bitmap resizedImage;

            if (frameCount > 1)
            {
                //we have a animated GIF
                resizedImage = ResizeAnimatedGifImage(image, width, height);
            }
            else
            {
                resizedImage = (Bitmap)image.GetThumbnailImage(newDimensions.Width, newDimensions.Height, null, IntPtr.Zero);
            }

            resizedImage.SetResolution(72,72);

            return resizedImage;
        }

By modifying the function above to the function below I was able to solve the problem using Graphics.DrawImage to redraw the new image before rendering it. Also the GenerateImageDimensions was slightly modified. This taken together the problem was solved.

private static Bitmap ResizeImage(Image image, int width, int height)
        {
            var frameCount = image.GetFrameCount(new FrameDimension(image.FrameDimensionsList[0]));
            var newDimensions = ImageFunctions.GenerateImageDimensions(image.Width, image.Height, width, height);

            var resizedImage = new Bitmap(newDimensions.Width, newDimensions.Height);
            if (frameCount > 1)
            {
                //we have a animated GIF
                resizedImage = ResizeAnimatedGifImage(image, width, height);
            }
            else
            {

                //we have a normal image
                using (var gfx = Graphics.FromImage(resizedImage))
                {
                    gfx.SmoothingMode = SmoothingMode.HighQuality;
                    gfx.InterpolationMode = InterpolationMode.HighQualityBicubic;
                    gfx.PixelOffsetMode = PixelOffsetMode.HighQuality;

                    var targRectangle = new Rectangle(0, 0, newDimensions.Width, newDimensions.Height);
                    var srcRectangle = new Rectangle(0, 0, image.Width, image.Height);

                    gfx.DrawImage(image, targRectangle, srcRectangle, GraphicsUnit.Pixel);
                }
            }

            return resizedImage;
        }

By "changing the resolution", do you actually mean you want to reduce the number of pixels in the image by 72/300? I.e. change a 4000x3000 image to 960x720?

If so, I can't see where your code actually does that. The overload of DrawImage() you're using does this:

Draws the specified image, using its original physical size, at the location specified by a coordinate pair.

Which is exactly what is happening.

Try one of the other overloads such as this one:

Draws the specified Image at the specified location and with the specified size.

for example:

// Create image.
Image newImage = Image.FromFile("SampImag.jpg");

// Create coordinates for upper-left corner of image and for size of image.
int x = 0;
int y = 0;
int width = 450;
int height = 150;

// Draw image to screen.
e.Graphics.DrawImage(newImage, x, y, width, height);

EDIT: per the comments, I understand the OP wants to reduce file size without reducing pixel count. Therefore the files must be recompressed.

I've borrowed some sample code from here:

ImageCodecInfo iciJpegCodec = null;

// This will specify the image quality to the encoder. Change the value of 75 from 0 to 100, where 100 is best quality, but highest file size.
EncoderParameter epQuality = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, 75);

// Get all image codecs that are available
ImageCodecInfo[] iciCodecs = ImageCodecInfo.GetImageEncoders();

// Store the quality parameter in the list of encoder parameters
EncoderParameters epParameters = new EncoderParameters(1);
epParameters.Param[0] = epQuality;

// Loop through all the image codecs
for (int i = 0; i < iciCodecs.Length; i++)
{
    // Until the one that we are interested in is found, which is image/jpeg
    if (iciCodecs[i].MimeType == "image/jpeg")
    {
        iciJpegCodec = iciCodecs[i];
        break;
    }
}

// Create a new Image object from the current file
Image newImage = Image.FromFile(strFile);

// Get the file information again, this time we want to find out the extension
FileInfo fiPicture = new FileInfo(strFile);

// Save the new file at the selected path with the specified encoder parameters, and reuse the same file name
newImage.Save(outputPath + "\\" + fiPicture.Name, iciJpegCodec, epParameters);

Rob, I believe that issue with your code is at saving the image - the actual digital image data would be certain number of dots/pixels i.e. (m x n) and setting resolution at bitmap wouldn't/shouldn't change the number dots (and hence physical byte size of image). The resolution information will be stored in the image header (to be used by programs while printing/editing images) - what happens if you store the new bitmap to file instead of mem stream

newBitmap.Save("c:\test.png", ImageFormat.Png);

Check dpi for above file from file -> properties -> summary (advanced). It should be 72 dpi.