Creating A Crop Area Mask

Cropping is the removing of edges so the remaining image contains just what is desired.  There are two reasons why an image would be cropped.  First, to remove undesired or distracting edges.  Second, to change the shape of the image to fit a predefined aspect ratio, such as 8x10.

Resizing an image means to change the physical dimensions of the image so when displayed or printed, it is of the desired size.

Cropping and resizing are often done together.  There are four basic steps to crop and resize an image.

1. Define a crop area mask in the master file.  A crop area mask is a type of selection or mask that shows the area to be cropped.
2. Make a copy of the master file.  This copy is often referred to as the output file.
3. Physically crop the output file.
4. Resize the output file to its desired dimensions.

Step 1 above, define a crop area mask, is discussed on this page.  Physically cropping and resizing the output file is discussed on the Cropping and Resizing page.

 

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Creating the crop area Mask

One should never physically crop the master file.  Instead, we create a visual crop, called the crop area mask, in the master file.  This allows you to see what the cropped image will look like before pixels are deleted.  The crop area mask will also be used later to physically remove the pixels in the output file.  This page describes three non-destructive methods to create a crop area mask.

 

Using a layer mask - Good for single Layer image files

This technique uses a layer mask to set the crop area.  It is good for image files with just one layer.  This technique is safe to use on a master file.

1. Show/Hide the steps
   1. Activate the layer containing the image.  It is assumed this layer does not have a layer mask.
   2. Click Select > All.  This will create a selection around the entire image.
   3. Click Select > Transform Selection.  This changes the selection outline into a selection grid.
   4. Grab the middle handle of the edge(s) to be cropped and drag the handle into the image until the unwanted pixels are outside the transformation box.
   5. For precise cropping, use the rulers (View > Rulers) or the width and height values in the Info panel to see exactly where the selection borders are.
   6. Press Enter to make the resized selection active.  Or, press Esc to cancel the transformation selection.
   7. Click the Add layer mask icon at the bottom of the Layers panel.  This will create a layer mask on the image layer based on the selection.
   8. Your image should now be visually cropped with the undesired pixels hidden by the layer mask.
   9. If pixels can still be seen in the cropped area, then there are other pixel-containing layers in the document.  In this case, use one of the techniques described below.

 

Using an alpha channel - Good for Multi-Layered image files

The alpha channel technique can be used on single or multiple layer images.  The disadvantage of this method is it does not show you the area to be cropped unless the alpha channel is loaded as a selection.  This technique is safe to use on a master file.

1. Show/Hide the steps
   1. Activate the layer containing the image.
   2. Click Select > All.  This will create a selection around the entire image.
   3. Click Select > Transform Selection.  This changes the selection outline into a selection grid.
   4. Grab the middle handle of the edge(s) to be cropped and drag the handle into the image until the unwanted pixels are outside the transformation box.
   5. For precise cropping, use the rulers (View > Rulers) or the width and height values in the Info panel to see exactly where the selection borders are.
   6. Press Enter to make the resized selection active.  Or, press Esc to cancel the transformation selection.
   7. Make the Channels panel the active panel.
   8. Alt - click (Option - click) the Save selection as channel icon at the bottom of the Channels panel.
   9. Type a meaningful name, such as Crop Area Mask, in the Name text box of the New Channel dialog box.
   10. Make sure Masked Areas is checked, and click OK.
   11. Make the Layers panel the active panel.
   12. Click Select > Deselect to remove the selection grid.

 

Using a normal layer - Good for multi-Layered image files

This technique is the web authors favorite because it easily shows both the cropped image and the entire image simply by clicking the crop area mask layer's eye icon on and off.  This technique is safe to use on a master file.

1. Show/Hide the steps
   1. Create a new layer by clicking Layer > New > Layer.  Or, by clicking the Create a new layer icon on the bottom of the Layers panel.  Name it something meaningful, such as Crop Area Mask.
   2. If the new layer is not the top most layer, drag it to the top.
   3. Click Select > All.  This will create a selection around the entire image.
   4. Click Select > Transform Selection.  This changes the selection outline into a selection grid.
   5. Grab the middle handle of the edge(s) to be cropped and drag the handle into the image until the unwanted pixels are outside the transformation box.
   6. For precise cropping, use the rulers (View > Rulers) or the width and height values in the Info panel to see exactly where the selection borders are.
   7. Press Enter to make the resized selection active.  Or, press Esc to cancel the transformation selection.
   8. Click Select > Inverse.
   9. Type the letter d to set the default foreground/background colors and type the letter x until black is the foreground color .
   10. Using the paint bucket tool , fill the selected area with black, or any other color of your choice.  The web author uses 30% gray because this matches the gray surrounding the document when the document is in full screen mode.
   11. Click Select > Deselect to remove the selection outline.
   12. Click the eye icon of the crop area mask layer on and off to see the effect of the crop.

 
Remember, these techniques do not physically crop the image.  Physically cropping an image is described on the Cropping and Resizing page.