File Size In Paint.net Is Different After Save

[Rokitman]

I'm trying to post an old photo to reddit to be repaired and restored. The image needs to be at the highest quality while still maintaining a strict size limit ~ 20mb. The problem I'm having is that when I save an image with Paint.net, the file size shown in that program and the saved file size once it's on my PC are different. Even if I maintain aspect ratio and adjust it to the size I need, it's always different after it is saved.

For instance. I loaded a photo into PDN that was 35.5mb. I clicked on Image > Resize. Checked By Absolute Size > Maintain Aspect Ratio. I reduced the pixels from 5100w to 1949w. Bringing the New Size to 19.9mb. But when I saved a new file to my desktop, did a right-click Properties, the new size was 5.96mb.

 

Can someone please tell me how to change the file size in mb's using PDN to the exact size I need?

[Pixey]

This tutorial may have the answer:

 

 

[MJW]

7 hours ago, Rokitman said:

For instance. I loaded a photo into PDN that was 35.5mb. I clicked on Image > Resize. Checked By Absolute Size > Maintain Aspect Ratio. I reduced the pixels from 5100w to 1949w. Bringing the New Size to 19.9mb. But when I saved a new file to my desktop, did a right-click Properties, the new size was 5.96mb.

 

Unless I misunderstand what you're saying, you seem to be confusing the image size with the file size. They're related, in the sense that a bigger image usually requires a bigger file, but they aren't the same. The image size is the number of bytes per pixels (which is four) multiplied by the width times the height. The file size in the number of bytes it takes to store the image on the storage device. Obviously the less space the file takes, the better, so considerable efforts are made to compress the information to make the file smaller. Resizing the image can affect how well the image can be compressed.

 

If you're worried the smaller size indicates a loss in quality, if you're saving in a lossless format like PNG, there's nothing to worry about; if you're saving in a lossy format like JPEG, set the quality to the maximum value when you save the image.

 

Perhaps more to the point, though, I think your calculations are off. 1949/5100 is about 0.382.  If the file size were exactly proportional to the image size, it should be reduced by that value squared, about 0.146. So the final image size would be 5.18 -- quite close to what it is. Even if you incorrectly assume a linear reduction, it would be about 13.6 MB, not 19.9. Maybe I'm misinterpreting what you're saying.

[otuncelli]

The value shown in Image->Resize panel is the raw size of the image without any compression. This can be calculated with this formula: Pixel Width x Pixel Height x 4 in bytes. You'll need that much amount of memory available to resize operation to be successful.

 

When you save an image, actual file size will differ from format to format and being lossy or lossless. This shows you the projected file size: