Image Resizing & Cropping on the fly in .NET - high quality

While developing a CMS for small gallery websites, I decided to store only the original uploaded image and not to create a thumbnail upon uploading.

I didn't want, of course, to use the html resizing feature, especially not while uploading high res images.

Instead, I created a generic handler that creates the thumbnail image on the fly.

Not only this handler knows how to resize the image, but also to crop it, in case I want all the images to be on a fixed ratio of width and height.

So, how is it done?

There are many blog posts of resizing images on the fly.

where you'll find a code similar to this:

public static Bitmap CreateThumbnail(Bitmap loBMP, int lnWidth, int lnHeight)

{

System.Drawing.Bitmap bmpOut = null;

try

{

ImageFormat loFormat = loBMP.RawFormat;

decimal lnRatio;

int lnNewWidth = 0;

int lnNewHeight = 0;

//*** If the image is smaller than a thumbnail just return it

if (loBMP.Width <>

return loBMP;

if (loBMP.Width > loBMP.Height)

{

lnRatio = (decimal)lnWidth / loBMP.Width;

lnNewWidth = lnWidth;

decimal lnTemp = loBMP.Height * lnRatio;

lnNewHeight = (int)lnTemp;

}

else

{

lnRatio = (decimal)lnHeight / loBMP.Height;

lnNewHeight = lnHeight;

decimal lnTemp = loBMP.Width * lnRatio;

lnNewWidth = (int)lnTemp;

}

// *** This code creates cleaner (though bigger) thumbnails and properly

// *** and handles GIF files better by generating a white background for

// *** transparent images (as opposed to black)

bmpOut = new Bitmap(lnNewWidth, lnNewHeight);

Graphics g = Graphics.FromImage(bmpOut);

g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;

g.DrawImage(loBMP, 0, 0, lnNewWidth, lnNewHeight);

loBMP.Dispose();

}

catch

{

return null;

}

return bmpOut;

}

So, this works, and works pretty well.

And as for the cropping?

For that there is no other way but to use twice the

g.DrawImage(bmpTemp, new Rectangle(0, 0, width, height));

once for the size, and another time for the cropping...

And last, but not least, THE QUALITY of the jpeg is determined while outputing the image like here:

context.Response.ContentType = "image/jpeg";

EncoderParameters eps = new EncoderParameters(1);

eps.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, 100L);

ImageCodecInfo ici = GetEncoderInfo("image/jpeg");

bmp.Save(context.Response.OutputStream, ici, eps);

so there you have it.

cropping, resizing, and good quality output, on the fly.

now, when doing these actions, the server has to 'work hard' on it for every photo, so a good way will be to cache it REAL GOOD...

context.Response.Cache.SetExpires(DateTime.Now.AddYears(1));

context.Response.CacheControl = "public";

context.Response.Expires = 20160; // two weeks

context.Response.Cache.SetCacheability(HttpCacheability.Public);

context.Response.Cache.SetMaxAge(new TimeSpan(1, 0, 0));

context.Response.Cache.SetValidUntilExpires(true);

well, that's it.

enjoy the code...