Diff Overlay Techniques: Slider, Onion Skin, and Blend Modes

Diff Overlay Techniques

Different overlay techniques reveal different types of changes between images. Choosing the right visualization method depends on what kind of differences you are looking for.

Slider Overlay

The slider overlay divides the viewport into left and right halves, showing one image on each side. Dragging the slider reveals more or less of each image.

Best for:

• Before/after comparisons where you want to see the transition
• Comparing overall layout and composition
• Presenting changes to non-technical stakeholders

Technique: A vertical line splits the viewport. The "before" image is clipped to the left of the line, and the "after" image to the right.

Onion Skinning

Onion skinning overlays one image on top of the other with adjustable opacity. Varying the opacity fades between the two images.

Best for:

• Detecting subtle position shifts and alignment changes
• Comparing animation frames
• Verifying element spacing and proportions

Technique: Both images are rendered at the same position. The top image's opacity is controlled by a slider (0% = fully "before", 100% = fully "after").

Difference Blend Mode

The difference blend mode computes the absolute difference between each pixel of the two images. Identical areas appear black, while different areas appear as bright colors.

Best for:

• Identifying exactly which pixels changed
• Detecting anti-aliasing and subpixel differences
• Finding subtle color shifts

Technique: Math.abs(pixelA - pixelB) for each channel. Unchanged areas produce (0,0,0) = black. Changed areas produce non-zero values.

Highlight Overlay

Changed pixels are highlighted in a distinct color (typically magenta or red) over a grayscale or dimmed version of one of the images.

Best for:

• Clearly identifying changed regions in context
• Communicating changes in reports and documentation
• Automated visual testing output

Technique: Compute pixel diff, then overlay changed pixels in magenta on a desaturated version of the original image.

Side-by-Side

Both images are displayed next to each other at the same zoom level with synchronized scrolling and panning.

Best for:

• Comparing images with very different content
• Reviewing before/after with full context for each image
• Wide screens with sufficient horizontal space