About

Animated GIFs store frames as indexed-color images using the GIF89a specification with LZW-compressed pixel data. Adding text to a GIF requires decoding every frame, rendering the overlay via a rasterization pass, then re-encoding the modified frames back into a valid binary stream with correct Graphics Control Extension blocks preserving original delay values (in hundredths of a second) and disposal methods. Getting this wrong produces garbled frames, broken transparency, or corrupted files that won't play. This tool performs full binary-level GIF parsing and re-encoding entirely in your browser. No server upload occurs. Frame count, color table depth (up to 256 colors per frame), and inter-frame timing are preserved. Note: re-quantization to 256 colors may introduce minor dithering artifacts on photographic-quality GIFs.

Formulas

Each frame in a GIF is stored as an indexed-color image. The pixel buffer size is computed from the frame dimensions:

bufferSize = width × height

where width and height are the frame dimensions in pixels. Each pixel maps to a color table index (0 - 255).

LZW compression uses a dictionary that grows from an initial size of 2^{minCodeSize + 1} entries. The clear code and end-of-information code are defined as:

clearCode = 2^minCodeSize

eoiCode = clearCode + 1

where minCodeSize is the LZW minimum code size stored in the image sub-block (typically 2 to 8).

Frame delay in milliseconds is derived from the stored value:

delay_ms = delay_raw × 10

Text overlay position is stored as normalized coordinates to allow repositioning:

x_pixel = x_norm × width

y_pixel = y_norm × height

where x_norm and y_norm are values between 0 and 1.

Reference Data

Property	GIF89a Spec Value	Notes
Max Colors per Frame	256	Indexed palette constraint
LZW Min Code Size	2 - 8 bits	Defined per image sub-block
Frame Delay Unit	10 ms	Stored as hundredths of a second
Default Delay (if 0)	100 ms	Browsers typically use ~100ms
Max Image Size	65535 × 65535 px	16-bit unsigned per dimension
Transparency	1-bit flag	One palette index = transparent
Disposal Method 0	Unspecified	No disposal action
Disposal Method 1	Do Not Dispose	Frame persists under next
Disposal Method 2	Restore to Background	Clear to background color
Disposal Method 3	Restore to Previous	Restore canvas to prior state
Header Signature	GIF89a	6-byte ASCII identifier
Trailer Byte	0x3B	Marks end of GIF stream
Extension Introducer	0x21	Precedes all extension blocks
Image Separator	0x2C	Precedes each image descriptor
Graphic Control Ext.	0xF9	Label for frame timing/disposal
Comment Extension	0xFE	Embeds text metadata in stream
Application Extension	0xFF	Used for NETSCAPE looping block
NETSCAPE Loop Count	0 = infinite	Sub-block of Application Ext.
Interlace Pattern	4-pass	Rows: 0,8,4,2 then fill
Typical Web GIF Size	< 5 MB	Larger files cause slow decode

Frequently Asked Questions

Re-encoding requires re-quantizing each modified frame to a 256-color palette and re-compressing with LZW. The new color distribution (including text colors) and different dictionary patterns during compression produce a different byte stream. Files with many unique colors from anti-aliased text may be slightly larger. Conversely, frames with large flat-color regions may compress smaller.

GIF frames can be smaller than the logical screen and placed at arbitrary offsets. This tool composites each frame onto a full-size canvas (matching the logical screen dimensions) before applying text, so the overlay position is consistent across all frames regardless of individual frame offsets or disposal methods.

The original transparent color index is preserved where possible. However, pixels under the text overlay are replaced with opaque text colors. If your GIF uses transparency for compositing between frames (disposal method 1 or 3), the tool reconstructs full composite frames to avoid visual artifacts, which means the output may use disposal method 1 throughout.

The Canvas 2D text rendering API supports any character your browser's font stack can render, including Unicode symbols and many emoji. However, since GIF is limited to 256 colors per frame, multi-color emoji will be rasterized to the nearest palette colors and may lose detail. Monochrome or simple symbols work best.

Processing happens entirely in the browser using JavaScript. A 500-frame GIF at 500×500 pixels requires approximately 500 × 500 × 500 × 4 = 500 MB of raw pixel memory. Practical limits depend on your device RAM. For most devices, GIFs under 10 MB / 200 frames process reliably. Larger files may cause tab crashes on mobile devices.

The preview uses requestAnimationFrame which targets 60 fps and approximates frame delays. Very fast GIF delays (under 20 ms) are browser-clamped to ~20 ms minimum. The exported file preserves the exact original delay values, so playback in image viewers or browsers will match the source timing.