Convert TIFF to GIF

GIF’s famous limitation is 256 colours, and the material that dominates this page does not care. A fax, a Group 4 scan, a filled-in form, a line drawing or a black-on-white document uses two of those 256 slots, so the palette imposes no loss whatsoever — every pixel comes through exactly, and the file is small because GIF’s LZW compression is very good at long runs of identical pixels.

That is the case worth being on this page for, and it is common: bitonal is the default mode of nearly every document scanner ever sold. A greyscale scan is almost as good, since 256 greys is precisely what an 8-bit greyscale image has. The trouble starts with colour and with halftone, and it starts abruptly rather than gradually.

Something has to decide which colours survive, and here it is gifenc, which builds a palette by pairwise nearest-neighbour clustering — a port of PnnQuant.js — rather than by how often a colour appears. The palette is chosen from a reduced colour space rather than the full sixteen million, which costs a little accuracy and saves several seconds on a large scan; for a format that ends at 256 entries that is the right way round.

The result on continuous tone is nobody’s idea of good and it is not obviously mangled either. A sky becomes a series of visible steps, skin tones go blotchy, and a halftone-printed original picks up a moiré that was not in the scan. On this site’s samples the photograph is 37,227 bytes as GIF against 14,700 as WebP — bigger and worse at the same time, which is the clearest possible sign that the destination’s requirement rather than the picture is what brought you here. No quality control is offered on this pair, and there is nothing for one to do — a GIF encoder’s only real lever is which 256 colours to keep, and that choice is made by the quantiser rather than by a number you set.

GIF supports transparency in a limited way: one palette entry can be nominated as invisible. The encoder used here never nominates one, so nothing in the output is transparent. What the transparent pixels become is the background colour on the options panel, applied before the palette is built, and it starts at white.

Set it to whatever the destination shows behind the image before you convert a batch. A scanned logo saved with a cut-out, or a TIFF exported from a design tool with an alpha channel, comes back on a solid rectangle of that colour, and the antialiased edge is blended into it — so a cut-out flattened onto white and then placed on a dark page carries a pale fringe. If the transparency itself is part of the image, TIFF to PNG and TIFF to WebP both keep it, and TIFF to ICO does too if the destination is a favicon.

Two container facts collide on this page and neither is obvious. A TIFF can hold many images and the reader here takes the first; a GIF can hold many frames and the writer here emits one. So a six-page scanned fax becomes a single still picture of page one, silently.

This also disposes of a hope that brings some people to the pair: a multi-page TIFF does not become an animated GIF. Nothing here assembles frames from pages, and even if it did, an animation is the wrong container for a document. Split the TIFF in ImageMagick or IrfanView and convert each page, or use TIFF to PDF, which is the only conversion in this family that keeps every page in one file.

By a lot, because the starting point is uncompressed. This site’s samples are 615,400 bytes as TIFF at 480 by 320 in every case, and as GIF they are 6,326 bytes for the screenshot-like scene, 7,688 for the flat graphic and 37,227 for the photograph.

The ordering of those three is the useful part. GIF is at its best on flat, hard-edged material — which is what a scanned document is — and at its worst on continuous tone, which is the exact opposite of how JPEG behaves. That is why an old system that accepts GIF and JPEG will handle your scanned form better as a GIF and your scanned photograph better as a JPG, and why sending both through the same route is a mistake worth avoiding.

Everything except the pixels of the first page, effectively. Sixteen bits per channel are reduced to eight before quantisation, which then reduces them to 256 colours in total. CMYK is converted to screen colour by formula rather than through the embedded ICC profile, so a print scan shifts. Layers are flattened. EXIF, XMP, IPTC, the ICC profile and any GPS block are dropped, with no setting that keeps them: the conversion decodes to pixels and encodes from pixels, so by the time the GIF is written there is nothing left to carry.

For the job this page exists for, none of that is a loss that matters: the destination is a system that wanted an 8-bit picture of a document. It matters if the TIFF is your only copy, which is the standing advice on every page in this family — keep the TIFF, upload the GIF. The original is never modified by converting it.

GIF stores its dimensions in sixteen bits, so an image can be up to 65,535 pixels on an edge. That is four times WebP’s ceiling of 16,383 and comfortably above anything a document scanner produces, so unlike the WebP route this conversion does not fail on a large scan.

What does bite on a large scan is memory and patience rather than the format. Quantising a 20,000-pixel-wide image means examining several hundred million pixels in the tab, and the resulting GIF, while much smaller than the TIFF, will still be a substantial file that the destination system may itself refuse on size grounds. Reducing the width first — the option is on the same panel — is usually the sensible move, and a document being viewed on a screen rarely needs more than 2,000 pixels across.

PNG, in almost every case. TIFF to PNG keeps every pixel with no palette at all, is read by everything written since the 1990s, and on flat scanned content is frequently smaller than the GIF as well — this site’s screenshot-like sample is 2,608 bytes as PNG against 6,326 as GIF. If the accepted list includes PNG, there is no argument for GIF.

If the destination is a web page you control, TIFF to WebP is smaller again and universally supported. If the scan is a document that will be filed or forwarded, TIFF to PDF keeps every page and is what the recipient expects. GIF is the right answer only when a list somewhere says so, and this page is written on the assumption that a list does.

The whole conversion runs in the tab: utif2 decodes the TIFF, gifenc builds the palette and writes the GIF, and both are fetched only when they are needed. Nothing about the file is uploaded, which is the point when the scans are records, correspondence or anything with a name on it.

The free limit is 100 MB per file, which is roughly a 600-dpi A4 colour scan stored uncompressed. Batches work the same way — drop a folder, watch the rows, take a ZIP — and the quantiser is the slow step, so a run of large colour scans takes noticeably longer than a run of bitonal ones. Converting a few files first and checking them in the destination system is worth the two minutes before committing a whole folder to a format nobody would choose freely.