Merge image comparisons make it easy to compare various types of image file and see precisely which pixels have been modified. Merge for Mac OS X supports all the image types supported by the operating system, including multi-page formats such as PDF.
You can open a new image comparison window by selecting the New Image Comparison command in the File menu.
Two empty areas where the compared images will be displayed take up the majority of the window. Above each file pane is an entry field that you can use to enter the path to a file that you want to compare.
To compare two images, you can type the paths of two files into the entry fields or use the buttons on the right-hand end of the entry fields to choose files to compare.
The 
button opens a file-browsing sheet and the 
button displays a list of files that you have recently compared. If there are other versions of the file named in entry field
available for comparison (e.g. in Time Machine), the 
button displays a list of these other versions.
Click the 
button in the toolbar or press the Enter key when you have chosen two files to compare.
You can also drag and drop files from Finder onto the filename entry fields or onto the file panes.
The image comparison display
Two sample image files (test1.png and test2.png) are provided in the Merge installation directory. The following screenshot shows the results of a comparison between these two image files.
The control strip at the bottom of the image comparison window enables you to customize the appearance of the comparison results. The screenshot below shows the above image comparison, but with only the changed pixels visible.
Show unchanged pixels
Toggles the display of unchanged pixels. When changed pixels are not displayed, the chequered transparent background is shown in their place.
Show changed pixels
Toggles the display of changed pixels. When changed pixels are not displayed, the chequered transparent background is shown in their place.
Highlight changed pixels
Highlights changed pixels by altering their colour.
Swap
This slider control can be used to blend the compared files together - on top of one another. If you drag the slider all the way to the right, the two files will be completely swapped.
Scale
This slider magnifies the image so that you can see pixel-level changes more easily.
View controls
You can use the n-Way Comparison (Side-By-Side) and n-Way Comparison (Stacked) commands in the Window menu to change the layout of the files within the file comparison window so that the compared files are displayed one above the other (stacked) or side-by-side (the default).
There are many options that you can set to customize how Merge compares and displays files. There are help topics that will enable you learn how to customize Merge so that it produces the best results for you.
Navigation
As you scroll one file up and down using its scroll bar, the other file is scrolled so that the two files remain aligned. You can temporarily prevent Merge from keeping both files aligned by holding down the Ctrl+Cmd keys while scrolling. When you release Ctrl+Cmd, the files will snap back into alignment.
Mouse navigation
Merge supports mice with wheels. If you have a three-button mouse, you can click within a file panel with the middle mouse button to enter a panning mode. The file will scroll in the direction you move the mouse. To stop panning, click the middle mouse button again. If your mouse has a wheel, you can use it to scroll up and down within the active file. If your mouse or keyboard has backwards and forwards buttons, you can use them to navigate to the previous or next change.
Switching between multiple file comparisons
Merge allows you to work with multiple file comparisons open at the same time. A tab strip along the bottom of the main file comparison window allows you to switch between file comparisons with a single mouse click.
Three-way file comparisons
In addition to two-way file comparison, the Professional Edition of Merge enables you compare (and, for text files, merge) three files. Please see the Three-Way File Comparisons topic for more information.