HDR Photography: Image Layers
When using Photoshop Elements 8 to work with HDR images, remember that you can store image elements — such as copies of the image with different effects and adjustments applied independently — on layers. It’s like stacking different images on top of each other. Each layer is unique, but in the same file. Layers combine to create what you see.
Image layers are separate and can be selected, moved, deleted, and edited independently. They don’t congeal or otherwise stick together. There is, however, a pecking order to what you can and can’t see. A layer on the bottom is normally invisible — covered up by layers above it. There are exceptions to this, which spices things up — and makes blending possible.
The exceptions are
Transparency: Pixels and layers can be deleted (or not painted). When this happens, what’s there is a transparent pixel or layer. It still exists, but it has no content. It’s like a transparent sheet that allows everything beneath it to show through.
Transparent areas allow you to blend the contents of different layers together as if they were one solid image. This is very helpful if you want to sharpen one area of an image but not the rest. You would put the sharpened layer above one without extra sharpening and delete pixels from the sharper layer that you don’t want to appear sharpened.
This figure illustrates replacement clouds in an image. The rest of the layer has been deleted. You can see the transparent areas as a checkerboard, which allows whatever is underneath this layer to be seen. The bottom layer, which has the building and the rest of the scene, is hidden at the moment. When it is turned on, the clouds blend right in.
Semi-transparency (also known as opacity): Pixels and layers can be partially transparent. This means they haven’t been completely deleted. This allows content below to show through, but only partially.
Opacity is a great way to blend parts of layers or entire layers together. Overlaying a realistic semi-transparent layer on top of a less realistic version of the image increases the apparent realism of the image.
This figure shows the effects of a partially transparent layer on top of the background. The nontransparent pixels in the clouds layer have been selected so you can see their extent. The layer’s visibility has been reduced to 49% to blend.
Blending modes: This is when you get to change the rules. Normally, opacity is king, which means that layers are blended — or blocked — based on what layers on top are opaque. You can change this behavior to include blending based on lightness, color, differences, and so forth.
Masking: Masking is a form of transparency and semi-transparency, but it’s not available in Photoshop Elements (except when you’re working with panoramas). When you mask something, you’re telling Photoshop to hide it, which allows content below to shine through.
If you have Photoshop Elements, you can duplicate the effect of masks by erasing parts of layers. You won’t be able to alter the area after it’s deleted (which is one of the best things about a mask), but the blending works the same way.