How to resize Image File to asked file size without changing its width and height?

Question

How can I change an image file size to desired size ? Like an image of 800KB into 100KB ?

I have an image of 3MB Dimensions: (2560 X 1600) with 300 dpi and bit depth of 24. I simply read it in Bitmap change its resolution to 150 dpi and save with new name with thinking that may reduce it to almost half of its original but the new file keeps same size and dpi SetResolution donot create any effect.

Bitmap image = Bitmap.FromFile("myPic.jpeg");
image.SetResolution(96, 96);
image.Save("newPic.jpeg");

Then I used this code

// Reads Image
Bitmap image = Bitmap.FromFile("myPic.jpeg");

// Sets canvas with new dpi but same dimensions and color depth
Bitmap canvas = new Bitmap(image.Width, image.Height, PixelFormat.Format24bppRgb);
canvas.SetResolution(150, 150);

// Draw image on canvas through graphics
Graphics graphics = Graphics.FromImage(canvas);
graphics.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
graphics.DrawImage(image, new Rectangle(0, 0, image.Width, image.Height),
            new Rectangle(0, 0, image.Width, image.Height), GraphicsUnit.Pixel);

// Saved Image
bitmap.Save("newPic.jpeg");

Here dpi got changed for new file but file size jumps to 7.84MB

Where is the fault? Does dpi have any effect on file size ?

Thank you for your consideration. Stay Blessed

Guffa · Accepted Answer · 2013-03-17 13:30:54Z

2

The resolution (ppi / dpi) has no effect on the file size. It's only information about how large the image should be in physical dimensions, i.e. how to convert the pixel size into inches.

When you resave the image using a different resolution, the file will be completely identical, except for those two values.

When you create a new bitmap and draw the image onto that, the new bitmap object doesn't have a file type associated with it, so when you save it without specifying a file type, it will be saved as a PNG image instead of JPEG.

What you can do to change the file size without changing the pixel dimensions, is to change the compression level for the JPEG file.

MSDN: How to: Set JPEG Compression Level

answered Mar 17, 2013 at 13:30

Guffa

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3 Comments

Fahad Over a year ago

Thanks bro I thought default format for image saving is jpeg :O. I have used this(Set JPEG Compression Level) quality technique but there is some problem, if I save an image 75 percent quality if then I want to further compress(Lowering its size) that image than I have to use lower than 75 percent quality ie 70, 60 or 50 otherwise if I use 80 or 90 percent quality for already (75 percent) reduced quality image it causes the increase in size rather than decrease. Is there any way to compress image to desired size or near to it ?

Guffa Over a year ago

@Fahad: There is no support for specifying a target file size, but you can try different compression levels and save to a MemoryStream instead of file, and when the file size is close enough, just write the memory stream to the file.

Fahad Over a year ago

Umm thanks I was in search for short procedure cause compressing again and again is slower process :( any how thank you for your kindness stay blessed

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