There are times when, in my line of work, I have a number of images of a certain size that I require to be a different size for whatever reason. What I need to a way to batch process the images.
Batch Processing
Batch Processing is the execution of given task or tasks, using a computer, without requiring manual intervention from the computer operator. Batch Processing gets the computer to do the repetitive work so that you, or I, don't have to.
The Task
In my particular example, I sized a web browser to a specific size that happened to be 1196x907 pixels. It was that size for no reason other than that is how large it needed to be in order to fit all of the required page content into the browser window.
For the purposes of another tutorial, I took a series of screenshots of the web browser showing different web pages that I wanted to use in the tutorial.
Why Taking a Photo of a Screen Sucks and How to Take Screenshots
How to Become an OS X Screenshot Wizard
The screenshots were at the browser window size of 1196x907 pixels. For Envato tutorials, we use a screenshot width of 850 pixels.
For your own purposes, you might be wanting to resize a number of photos to a consistent size, for example. The principle is the same.
1. Organise the Files
In order to make this work, you'll need to be organised about it.
I created a new folder on the Desktop and placed all of the screenshot images into that folder. It needn't be there, you can place it in Documents if you prefer.
2. Select All of the Files in Finder
Open Finder and select all of the files in the folder by clicking, holding the click and dragging the mouse cursor from top left to bottom right across all of the files. Or from any corner to the diagonally opposite corner.
For the best results, I recommend that the only files within the folder are the ones that you wish to batch process.
Alternatively, you may press Command-A to select all of the files within the folder.
3. Open All of the Files in Preview
With all of the files selected, secondary-click the mouse on any one of the selected files to bring up a menu. Navigate to Open With > Preview (default) to open all of the files in Preview.
This has the effect of opening all of the files into one Open all of the files in Preview window.
4. Select All of the Images in Preview
This step is important and it is easy to miss, so pay attention.
Click into the pane on the left of the Preview window, the area where the thumbnail images are shown.
If you do not see this area, click on the View Menu icon, the icon that is furthest to the left and immediately below the Red and Amber circles on the top left of the Preview window. Click Thumbnails, from the drop-down menu, to reveal the thumbnails pane.
Click with the mouse in the left-hand pane area and press Command-A to select all of the thumbnail images.
5. Resize All of the Images in Preview
With all of the thumbnails selected, navigate to the menu bar at the top of the screen and click Tools > Adjust Size...
From the resulting dialogue box, enter the dimensions that you require of the resized images.
In my particular example, I wish to resize to 850 pixels wide. Since the option Scale proportionally is ticked, the height of the images will be calculated for me. I can leave that box blank.
Click the blue OK button.
6. Export the Resized Images From Preview
With all of the images resized within Preview, it is necessary to save them.
Navigate to the menu bar at the top of the screen and click File > Save. Alternatively, you can click Command-S.
The resized files will be saved to the same location with the same filenames, just resized.
Limitations
Whilst this is a quick and handy technique, you may also which to convert file formats. In my example, I wish to convert from .png to .jpg.
Unfortunately, you'll need to be aware that this operation appears to be unreliable in macOS Sierra. For some reason, Preview only successfully converts the first third of the selected files. Select 30 files and ten are converted. Select ten files and only three have their file format changed.
With this limitation in mind, it may be that you look to Automator, Alfred or a specific software application to conduct batch processing of images, ...especially if you wish to perform more actions than just resizing the images.
Conclusion
In this tutorial, I have shown you how to organise and resize a number of images using Preview in macOS Sierra.
This is a handy and quick way of achieving a result, but the method does come with limitations.
