How to compare two screenshots on a Mac
On a Mac, Quick Look lets you flip between two screenshots, and window tiling puts them side by side, so you can spot the changes by eye. To have every changed pixel marked, use Compare Pro (Shotbolt Pro) in Shotbolt.
What macOS can do
macOS gives you two ways to look at two screenshots together. Both leave the comparing to your eyes.
- Flip between them. Select both screenshots in Finder and press the Space bar to open them in Quick Look. Press the Left Arrow or Right Arrow key to go from one to the other, or press Command-Return to see them together in an index sheet.
- Tile them. Open each screenshot in its own window. Make one window active, choose Window → Move & Resize and pick a layout under Halves, then do the same for the other. You can also tile a window by dragging it to the left or right edge of the screen.
Neither marks what changed. A small shift or a slightly different color is easy to miss.
Steps in Shotbolt
Compare Pro (Shotbolt Pro) works on screenshots in Shotbolt's History. Every download from this site starts with 7 days of Pro.
- Take both screenshots with Shotbolt, for example before and after a change.
- Start the comparison in one of three ways:
- Right-click the newer screenshot's Quick Access card, or the screenshot in History, and choose Compare with Previous Pro (Shotbolt Pro). It compares the screenshot with the one you took before it.
- In History, select two screenshots, then right-click and choose Compare Pro (Shotbolt Pro), or press ⌥⌘C.
- Right-click one screenshot and choose Compare with… Pro (Shotbolt Pro). Click a recent screenshot, drop any picture on the window, or click Choose File….
- The Compare window opens. The older screenshot is Before and the newer one is After; click Swap to flip them.
- Read the count in the bar at the bottom, such as 3 changes, or Identical. Press ] for the next change and [ for the previous one, and the view zooms to it.
- To keep the result, click Export and choose Difference Image or Annotated Side by Side, with every change boxed and numbered. It arrives as a new screenshot in Quick Access and History.
The four views
Pick a view at the top of the window. Zoom and position stay the same when you switch.
- Side by Side shows Before and After next to each other. Zooming or dragging one moves the other with it.
- Swipe wipes from Before to After as you drag the slider at the bottom.
- Onion Skin lays After over Before, and the slider fades between them.
- Difference fades Before and marks every changed pixel in magenta.
Two settings at the top decide what counts. Auto-align, on by default, finds a small shift between the pictures so the whole screen does not light up as changed. Tolerance sets how big a color difference has to be before it counts, which hides the soft edges of smoothed text.
Limits
- Small shifts only. Auto-align corrects a shift of up to 32 pixels in each direction. A larger move shows as a change.
- No scaling. Pictures are compared pixel for pixel. If one is a different size, the area only the bigger one covers is marked as changed. A screenshot taken at a different zoom, or on a display with a different scale, is not resized to match, so the two do not line up.
- Tolerance cuts both ways. Raise it and small real changes, such as a slightly different shade, stop counting too.
- Screenshots only. Recordings and step guides cannot be compared, and a comparison always starts from a screenshot in History.
Questions
Can I compare a picture I did not take with Shotbolt?
Yes, as the second picture. Right-click a screenshot in History, choose Compare with… Pro (Shotbolt Pro), and drop the other picture on the window or click Choose File…. The first picture always comes from History.
What does the magenta mean?
In the Difference view, magenta marks the pixels that changed in After. When the two are different sizes, the area only one of them covers is marked in a fainter magenta.