Short Introduction to Working with Layer Masks

To proceed from here, you must first consider what you want to take from which layer. Starting with the topmost layer, details-highlights, you will need the sky and the branches of the trees. You can delete or, better, hide the rest of the layer, using a layer mask. Next, you will need the temple and the tree trunks from the second layer. Since the sky remains in the topmost layer, you won't need to worry about the sky on the median layer. It will be covered by the sky of the topmost layer. You want to see the hill in the foreground as well as the bushes from the details-shadows layer, so you will have to hide these areas in the details-highlights and in the median layer.

You could paint a mask for the areas in the image that should remain. First select Quick Mask and then switch to selection mode to delete the remaining contents of the layer. Principally, this is the procedure that was used in section 3.14.4. Permanently modifying or otherwise deleting visual content is called destructive editing. There is nothing objectionable about working this way; just remember that you should only work with a copy of the image. The original should always be kept as a backup. This way, you can fall back on it in case you make an irreparable mistake or you want to do something else with the same image.

The possibilities of nondestructive editing in GIMP are to be increased with the further implementation of the GEGL library. The intention is to have a function similar to the adjustment layers in Adobe Photoshop, where tonality correction (Colors > Levels) is not applied directly to the pixels. Rather, some sort of mask is placed over the layer and subsequent changes are applied later.

Image contents don't have to be irrevocably deleted. Layer masks are available to avoid permanently changing or even damaging your image. Actually, the initial approach is the same. Select the desired image or layer that is to remain visible with a selection method of your choice. Edit the selection with any of the previously learned masking and selection techniques. However, instead of inverting the selection and deleting remaining image content, apply the selection to the layer that needs editing with a layer mask. The layer mask hides the unselected image contents. This illustrates an essential nondestructive editing method.

Create the selection on the layer that appears to be best for selecting the preferred image areas. Then select the layer in the Layers dialog that you want to edit. Right-click and choose Add Layer Mask. Then in the dialog box that opens, choose Selection from the Initialize Layer Mask to menu. The layer mask will be applied and the selection will be masked.

The result is that you will see a second image that shows the layer mask next to the preview image of the layer. Initially, it displays a white border, which means that the layer mask is active.

0

Edit Layer Attributes...

New Layer...

New from Visible

%

Duplicate Layer

Anchor Layer

mi

Merge Down

9

Delete Layer

P±i

Layer Boundary Size...

Layer to Image Size

*

Scale Layer...

m

Add Layer Mask...

Apply Layer Mask

9

Delete Layer Mask

Show Layer Mask

Edit Layer Mask

Disable Layer Mask

m

Mask to Selection

Add Alpha Channel

Remove Alpha Channel

Alpha to Selection

Merge Visible Layers...

Flatten Image

Figure 3.113

The layer's context menu in the Layers dialog with an active layer mask

Figure 3.113

The layer's context menu in the Layers dialog with an active layer mask

O CHAPTER 3 USING MASKS AND L AY E R S — PA I N T I N G, FILLING, AND COLOR TOOLS

The individual layers are named after the level of brightness that is to be depicted on an area in the image. The topmost layer is called details-highlights even though it is the darkest image. However, the brightest areas are depicted here with the most contrast. Inversely, the bottommost layer is called details-shadows even though it is the brightest image. The dark image sections are the best exposed and feature the contrast you want.

Begin with creating a selection of the area that is to be left as it is. In the topmost layer, details-highlights, it is the sky. You can easily select the sky by using the Select By Color tool. Do this work on the bottommost layer details-shadows where the depiction of the sky is the brightest. Here the tool will find the area of the sky most easily.

Then switch to the Quick Mask mode (Select > Toggle Quick Mask or click the button at the bottom left of the image window). Edit the selection in this mode with paint tools until only the areas that are to be hidden are masked. You don't have to work precisely at pixel level. Your image in the mode should look like figure 3.114.

Selections are independent from the layer on which they are created. In the example image, you select the sky on the details-shadows layer because you can easily select it with the Fuzzy Select tool. The selection is then applied on the details-highlights layer.

The following entries in the context menu of the dock are also active:

Apply Layer Mask: Deletes the masked image content and subsequently deletes the layer mask.

Delete Layer Mask: Deletes only the layer mask. The previously masked image area will be displayed again.

Show Layer Mask: Displays the layer mask as a black-and-white image in the image window.

Edit Layer Mask: Allows the editing of the layer mask with paint tools. The masks are painted in black; white deletes the mask or adds areas that shoud be visible.

Disable Layer Mask: Allows you to disable the layer mask without deleting it. The masked section can be seen again.

Mask to Selection: Converts the layer mask into a selection.

The Procedure

After the mask has been completed, switch back into the selection mode. Apply a soft edge of 4 pixels radius (Select > Feather). Now set the top layer, details-highlights, to active. As previously described, insert a layer mask by right-clicking and choosing Add Layer Mask, and then choose Selection from the Initialize Layer Mask to menu.

Depending on the accuracy of the mask, the areas of the hill and the temple will be hidden on this layer. The underlying layer will shine through. If you are not satisfied with the mask, you can edit it. Right-click in the active layer and choose Show Layer Mask. Again, you can edit the mask with the paint tools: Black enlarges the mask, and white can be used to delete areas from it. Then you can delete the selection. Once you have created a layer mask from it, you can recover the selection from the layer mask (Layer > Mask > Mask to Selection). You can also leave the selection as it is to apply it to the second layer, median.

Make the details-highlights layer invisible by clicking the eye symbol in the Layers dialog. Set the median layer to active. Switch to the Quick Mask mode in the image window again. Now you are going to "erase" the areas out of the mask that should stay visible in the median layer. Essentially, this will be the trees and the temple. To erase here means to paint it white. Your image should look like the example in figure 3.115.

The next step is to switch back into selection mode. You won't have to hide the selection this time. It still has a soft edge from your previous work. Now right-click and choose Add Layer Mask from the context menu of the Layers dialog. Basically, that's it. You just need to delete the selection. Don't forget to save the image.

If you see any need to make any corrections, you can create a selection on one of the layer masks (in the Layers dialog, right-click and choose Mask to Selection). Then you must delete the old layer mask. Edit the mask with

3.15 GIMP AND HDR

paint tools in the image, changing to Quick Mask mode. Thereafter, create a selection out of the mask and from there a new layer mask.

Figure 3.114

The mask in the image marks the area that is to be hidden. Paint tools can be used to edit the mask. In the Layers dialog on the right, you can see a second preview image next to the detailshighlights layer. This indicates the layer mask, that has already been inserted into this layer.

J Figure 3.115

The mask for the second layer, median

Figure 3.114

The mask in the image marks the area that is to be hidden. Paint tools can be used to edit the mask. In the Layers dialog on the right, you can see a second preview image next to the detailshighlights layer. This indicates the layer mask, that has already been inserted into this layer.

J Figure 3.115

The mask for the second layer, median

Figure 3.116

The reference JPEG image from the camera

Figure 3.116

The reference JPEG image from the camera

Figure 3.117

The HDR image created with FDRTools

Figure 3.117

The HDR image created with FDRTools

This was necessary when I was editing the image; the trees came out better when they were depicted as whole on the topmost layer. Therefore, I had to subsequently erase the mask covering the trees in the details-highlights layer.

This way of working is very labor intensive, but the finished image is rather satisfactory. The method to create a "real" HDR image with the appropriate program is somewhat easier. Basically, it depends on the right choice of program settings. You will have to experiment a little. This can take some time as the processing of the finished HDR image takes time, depending on the file size.

Compare the three images (figure 3.116, figure 3.117, and figure 3.118): the reference image that hasn't been edited since it was taken, the LDR image that was created with GIMP by blending the images, and the "real" HDR image that was created with FDRTools Basic. We will take a closer look at creating HDR images in the following section.

I have listed here links to interesting tutorials and examples suggesting similar methods of editing bracketed images in GIMP:

A good tutorial:

http://www.gimp.org/tutorials/Blending_Exposures/

And two further tutorials:

http://www.luminous-landscape.com/tutorials/digital-blending.shtml

http://en.wikibooks.org/wiki/The_GIMP/Blending_ Exposures

On the topic of HDR formats:

http://www.linux.com/articles/50413

And an article on HDRI in Wikipedia:

http://en.wikipedia.org/wiki/High_Dynamic_Range_ Image

Figure 3.118

The pseudo HDR image created with GIMP

Figure 3.118

The pseudo HDR image created with GIMP

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