GIMP Tutorials

Intention

In this tutorial I'll show you how to do gaussian blur overlays using GIMP. This is an interesting technique that intensifies and saturates the colors in the image, increases contrast, and adds a slightly hazy, "dreamy" feel to the image.

The basic technique is to create a duplicate layer in the image, lighten it and blur it, and combine it using a layer mode with the original.

Giving credit where credit is due: I did not come up with this method. I adapted it for GIMP from a Photoshop tutorial on the luminous-landscape.com web site (great photography web site BTW; I recommend it).

The Procedure

Here is the original example image, loaded into GIMP.

Step 1

Open the Layers dialog. Right-click on the Background layer and select Duplicate (there is also a button for this in the bottom button bar of the Layers dialog ().

Step 2

Now double-click on the duplicate layer and rename the new layer "Blur Overlay". This step is not strictly necessary, but it is helpful to prevent confusion about what is on each layer, especially if you add some additional layers for other editing purposes, or more importantly, if you save the file with layers and open it six months later.

Step 3

In the Layers dialog, select the Blur Overlay layer. In the "Mode" drop-down box, select "Overlay".
Now go back to the image window and apply a Levels ( Image -> Colors -> Levels) or Curves ( Image -> Colors -> Curves) and adjust it until the overall image has the proper brightness. You'll usually find it necessary to adjust the gamma slider (middle slider in Levels) down. You are only adjusting the upper layer, but you are viewing the cumulative effect of the layer blend.

Tip: If you don't get a good effect with Overlay mode, try Multiply mode (you can even change this while the Levels dialog is active).

Tip #2: while the Levels dialog is active you can toggle visibility of the upper layer to see the original image and compare to the blend. Just click on the "eye" next to the upper layer.

Step 4

Go back to the image window and right click, selecting Filters -> Blur -> Gaussian Blur. You will need to experiment to find the best value, but typically a value between 10 and 30 will do nicely.

Voila! If you don't like the effect, you can undo the blur (Ctrl+Z) and redo it (Shift+Alt+F) with a different value.

Click on the "eye" next to the Blur Overlay layer in the Layers dialog to rapidly compare the image with and without the overlay. Similarly, turn off the Background layer if you want to view the overlay to do further work on it.

Tip: Protecting Highlights with a Layer Mask

Although I like the effect, there is one problem with this technique and that is that it also increases contrast: the shadows get darker and the highlights get lighter.

You might be able to apply a contrast mask to counteract this effect, but in most cases it is the highlights that are the most troublesome in that they have lost detail. Fortunately we can apply a simple extension to the above technique to protect the highlights.

Step 5

Duplicate the image (Ctrl+D).
Flatten the duplicate ( Layers -> Flatten Image).

Step 6

In the duplicate, run a threshold filter ( Image -> Colors -> Threshold).

In the threshold histogram, click and drag to the right to select all the pixels at the upper end of the scale. Retry or adjust the selection using the number controls in the dialog box until the display shows most of the pixels you want to preserve as white and all the rest black.

You only need to approximate this, since we're going to clean up the mask anyway.

Step 7

To clean up the mask, I switched to the paintbrush (), hit "x" in the mask image to switch the foreground and background colors (Black/White to White/Black), selected a nice opaque brush in the Brushes dialog and painted the few pixels of the sky white that hadn't been turned white (the darkest parts of the clouds).

Now the black parts. I switched the fg/bg colors back to (Black/White). I could have painted black all over the lake, but I had a faster idea in mind. I used the marquee selection tool () to select the whole area and then using the fill tool () I just clicked in the selection to fill it black in one fell swoop.

I then "feathered" the mask so that it will blend the layers without a harsh transition by applying a 6 pixel gaussian blur to the mask.

Step 8

Invert the mask ( Image -> Colors -> Invert), so that the white parts correspond to the parts of the combined layers that you want to keep and the black parts correspond to the parts that should only reflect the original image (the highlights).

Step 9

Go to the Layers dialog. Select the overlay blur image in the "Image" drop down box (if it is not selected already).

Right-click on the Blur Overlay layer and select Add Layer Mask. In the Add Mask Options dialog, select White (Full Opacity) and click OK.

Now go back to the blurred threshold image, select all and copy (Ctrl+A then Ctrl+C). Go back to the overlay blur image and paste (Ctrl+V). Go to the Layers dialog and click the anchor button () to anchor the mask.

Tip: Ctrl-click on the layer mask icon in the Layers dialog to toggle the effect of the layer mask to compare the image with and without the highlight mask.

Step 10

Here's the result. Compare to the original image (right image). I would flatten and apply a smart sharpening (edges) to finish it out.

Tips

• Try other layer blending modes. "Multiply" often works well, but creates an even darker image than "Overlay"; you may have to really crank up the levels on the upper layer. One nice thing about Multiply is that you probably won't need the highlights mask.
  "Screen" has a milder effect than either "Overlay" or "Multiply" and will generally just intensify the colors/saturation. You may not want to lighten the upper layer at all, or perhaps even darken it. Experiment to see what effects you can get.

Other Examples

Using "Multiply" for the blending mode. (Final image on the right)

Using "Overlay" for the blending mode, plus a highlight mask for the sky and waterfall. (Final image on the right)

Intention

In this tutorial I'll show you how to sepia tone a color or black and white image. This technique is modeled on the traditional darkroom method of sepia toning in that the sepia color is applied "unevenly" to areas of different tonality. It works much better than most of the simple methods I've seen for image editors and provides great control!
The basic technique is to create a "Sepia Mask" that will apply a sepia color layer according to a layer mask that is based on the tonality of the image.
Giving credit where credit is due: I did not come up with this method. I adapted it for GIMP from a Photoshop tutorial on the RetouchPRO web site.

The Procedure

Here is the original image, loaded into GIMP. It has a bit of an old-time look, so I thought it might work well as a sepia-toned image.

Step 1

You can compare the result we will get below to GIMP's built-in Script-Fu for sepia toning ( Script-Fu -> Decor -> Old Photo) as shown at right.
Come back and compare to this when you get to the end of the tutorial.

Step 2

Duplicate the original image ( Image -> Duplicate or Ctrl+D). You may want to minimize the original now (or close it) so you don't choose it by mistake.

Using whatever technique you like best, convert the duplicate to B&W. I recommend the channel mixer for best results, but you can also convert to grayscale ( Image -> Mode -> Grayscale) or desaturate ( Image -> Colors -> Desaturate).

Whatever technique you use, convert the B&W image back into RGB mode when you're done ( Image -> Mode -> RGB).
In this case I simply did a mode change to grayscale, then back to RGB mode.

Step 3

Double-click on the foreground color swatch to bring up the Color Selection dialog. Dial in the color RED=162, GREEN=138 and BLUE=101 (you can experiment with this color too; this is a good starting point though.)

Step 4

Bring up the Layers dialog (Ctrl+L) and click on the button for a new layer. Give it the name "Sepia Mask" and choose the option to fill it with the foreground color. Click OK.
You should see nothing but the color now in the image window, since it obscures the image in the layer below. We are going to selectively apply this color to the image.

Step 5

Right-click on the Sepia Mask layer and select Add Layer Mask. In the Add Mask Options dialog, choose White (Full Opacity).

Step 6

In the Layers dialog, click on (select) the Background layer. Go up to the image window, select all and copy (Ctrl+A then Ctrl+C). In the Layers dialog, click on the layer mask icon in the Sepia Mask layer (the little white square). Then go back up to the image window and paste (Ctrl+V)

Step 7

In the Layers dialog, click the Anchor button to anchor the pasted image into the layer mask.
In the image window, invert the color ( Image -> Colors -> Invert). This layer mask insures that the shadow parts of the image receive most of the color, the mid-tones a little less, and the highlights little to none, much the way a print sepia-toned the traditional way would be.

Step 8

In the Layers dialog, change the Mode (blending mode) of the layer to "Color". This applies the color from the Sepia Mask layer according to the layer mask to the image.
You now have your base result. It might be a good idea to save this under a new name at this point.

Step 9

Once you've got the base image, you can duplicate it (Ctrl+D), flatten the duplicate ( Layers -> Flatten Image) and then experiment with:

• Hue and Saturation ( Image -> Colors -> Hue..Saturation)
• Color Balance ( Image -> Colors -> Color Balance)
• Levels or Curves ( Image -> Colors -> Levels|Curves)
• etc. etc!

You can always compare the result to the base image. If you want to start over, just duplicate the base image again and off you go.

Hint: if GIMP had adjustment layers, like Photoshop, we'd just create one of those to experiment with further adjustments. Since GIMP does not have those, we have to flatten the image to apply some of the standard controls and filters. Ergo, work on a duplicate or save the base image..

Here I decided that the image was a little flat and so I punched up the contrast using curves.

Step 10

As a final touch, I ran the "Old Photo" Script-Fu without the "sepia" and "mottle" options. Here is the final image.

Tips and Tweaks

• You can reduce the effect of the sepia toning by adjusting the opacity of the Sepia Mask layer.
• Try adding some grain or noise to the image (probably before you desaturate).
• We used a single, flat color for the sepia color. Experiment with different colors, multiple colors or a color gradient for the Sepia Mask layer.
• You can apply levels, curves or other adjustments to the contrast mask to increase or decrease "absorbency" of the sepia color into different areas of the image.
• The method by which you get a black and white image makes a big difference in how the sepia toning comes out.

The one on the left was from a grayscale conversion ( Image -> Mode -> Grayscale); the one on the right started as a desaturate ( Image -> Colors -> Desaturate). Notice that there is a lot more blue-channel noise in the right-hand one. This can add a nice "grain" effect (see tip above also) if that is what you are looking for. For this particular image, I prefer the smoother tonality and darker contrast of the left-hand image.