How to smooth out jagged lines in a bitmap

A reader asked for advice on using graphics software to smooth out lines in a bitmap. Many old, free images were originally digitized in true 1-bit bitmap format, meaning two colors – black and white. This clipart features jagged steps that will look bad on screen or print.

Contents
  1. Getting rid of bumps in Line Art
  2. Customize Paint.Net
  3. Run Gaussian Blur Filter
  4. Gaussian Blur 1 or 2 pixels
  5. Using Curves Adjustment
  6. Curve overview
  7. White Point Adjustment
  8. Black point adjustment
  9. Save custom image
  10. Optional: use levels instead of curves

Getting rid of bumps in Line Art

Luckily, you can use this little trick to smooth out those bumps pretty quickly. This tutorial uses the free Paint.NET photo editor, but works with most image editing programs. You can adapt it to any other image editor as long as it has a Gaussian blur filter and a tool for adjusting curves or levels. These are pretty standard tools in most image editors.

Save this sample image to your computer to follow the tutorial.

Customize Paint.Net

Start by opening Paint.NET and then click the button Open on the toolbar and open the preview image or any other image you want to work with. Paint.NET is designed to only work with 32-bit images, so any open image will be converted to 32-bit RGB color mode. If you are using another image editor and your image is in a reduced color format such as GIF or BMP, convert the image to an RGB color image first. Refer to your software’s help files for information on changing the color mode of an image.

Run Gaussian Blur Filter

After opening the image, go to the section Effects to fade Gaussian Blur

Gaussian Blur 1 or 2 pixels

to install Gaussian Blur Radius for 1 or 2 pixels, depending on the image. Use 1 pixel if you want to keep thinner lines in the final result. Use 2px for thicker lines. Click Okay

Using Curves Adjustment

Go to section Institutions curves

Curve overview

Drag Dialog curves sideways so you can see your image as you work. In the dialog box curves a graph is represented by a diagonal line from the bottom left to the top right. This chart shows all tonal values ​​in your image from pure black at the bottom left to pure white at the top right. All shades of gray in between are represented by a slanted line.

We want to increase the slope of this diagonal line to reduce the amount of change between pure white and pure black. This turns our image from blurry to sharp, reducing the amount of change between pure white and pure black. We don’t want to make the corner perfectly vertical, though, otherwise we’ll get the image back to the jagged look we started with.

White Point Adjustment

Click the point at the top right of the curve graph to adjust the curve. Drag it right to the left so that it is halfway between its original position and the next dotted line on the graph. The lines in the fish may disappear, but don’t worry – we’ll get them back in a minute.

Black point adjustment

Now drag the bottom left point to the right and keep it near the bottom edge of the graph. Notice how the lines in the image get thicker as you drag to the right. The jagged look will come back if you go too far, so stop at a point where the lines are smooth but not blurry. Take the time to experiment with the curve and see how it changes your image.

Save custom image

Click Okay and save the finished image by going to the section File Save as when you are satisfied with the setting.

Optional: use levels instead of curves

Find a resource Levels if you are working with a graphical editor that does not have a tool curves † You can manipulate the white, black, and midtone sliders as shown here to achieve a similar result.

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