About

Merging two GIF animations with a smooth cross-fade requires frame-level pixel interpolation. Each frame from GIF A in the transition zone is blended with the corresponding frame from GIF B using a linear alpha factor t that progresses from 0 to 1. A miscalculated blend produces visible banding, frame-rate mismatches, or color table corruption. This tool decodes both GIF files at the binary level (GIF89a spec with LZW decompression), performs per-pixel interpolation across the transition region, and re-encodes the result as a valid GIF. Output dimensions match the larger of the two inputs. Frame delays are preserved from source animations and averaged during the crossfade zone.

Limitations: transparency in source GIFs is flattened to a white matte before blending. Very large GIFs (exceeding 500 frames total or 20MB file size) may strain browser memory. The crossfade duration is specified in number of frames, not milliseconds, giving precise control over the transition length. Pro tip: for the smoothest result, use source GIFs with similar frame rates and canvas dimensions.

Formulas

Each pixel in the crossfade zone is computed by linear interpolation between the corresponding pixels of GIF A and GIF B:

c_out = c_A ⋅ (1 − t) + c_B ⋅ t

where the blend factor t for frame index i within the crossfade region of n total blend frames is:

t = in − 1

where c_out = output pixel channel value (R, G, or B), c_A = pixel channel from GIF A frame, c_B = pixel channel from GIF B frame, t = interpolation factor (0 = fully A, 1 = fully B), i = current frame index within the crossfade zone, n = total number of crossfade frames. The operation is applied independently to each of the three color channels (Red, Green, Blue) and clamped to the integer range [0, 255].

Reference Data

Parameter	Description	Typical Range	Default
Crossfade Frames	Number of frames in the blend zone	2 - 60	10
Frame Delay	Time per frame in the GIF	20 - 200ms	Source value
GIF89a Header	Magic bytes: 47 49 46 38 39 61	-	-
LZW Min Code Size	Initial dictionary bit width	2 - 8 bits	Derived from color depth
Max Color Table	Global Color Table entries	2 - 256	256
Disposal Method 0	No disposal specified	-	Leave as-is
Disposal Method 1	Do not dispose	-	Keep frame
Disposal Method 2	Restore to background	-	Clear canvas
Disposal Method 3	Restore to previous	-	Revert canvas
Max Output Width	Largest width of both inputs	1 - 2048px	Auto
Max Output Height	Largest height of both inputs	1 - 2048px	Auto
Loop Count	Netscape extension loop value	0 (infinite) - 65535	0
Blend Formula	Per-channel linear interpolation	-	c = a(1−t) + b·t
Quantization	Median-cut color reduction to 256	-	Applied per frame

Frequently Asked Questions

The output canvas size is set to the maximum width and maximum height of both inputs. The smaller GIF is centered on the canvas, and any exposed area is filled with white. This prevents cropping of either source animation.

The crossfade region uses the last N frames of GIF A and the first N frames of GIF B, where N is the number of crossfade frames you specify. If a source GIF has fewer frames than N, all its frames are used and the blend factor t is distributed across those available frames. Frames outside the crossfade zone play at their original timing.

Crossfade frames contain gradients of blended colors that are difficult for LZW compression to encode efficiently. Each blended frame typically has more unique colors than a single-source frame. The tool applies median-cut quantization to reduce each frame to 256 colors, but the entropy of blended content remains higher than typical GIF frames.

No. Transparent pixels in source GIFs are composited against a white background before blending. This avoids undefined behavior in the interpolation math where one pixel is transparent and the other is opaque. If both source GIFs use transparency for identical regions, the output could be modified to retain it, but this edge case is not handled.

Each input file is limited to 20 MB. The practical constraint is browser memory. A 500×500 pixel GIF with 200 frames consumes approximately 200 MB of raw pixel data (500 × 500 × 4 bytes × 200 frames). Two such GIFs plus the output would require roughly 600 MB. Modern browsers can handle this, but older devices with limited RAM may experience slowdowns.

Frame delays in the crossfade region are averaged between the corresponding frames of GIF A and GIF B. For example, if GIF A's frame has a delay of 50 ms and GIF B's corresponding frame has 100 ms, the crossfade frame delay is set to 75 ms. Outside the crossfade zone, original delays are preserved exactly. GIF delays have a minimum resolution of 10 ms per the specification.