Convert DNG to WebP

Inspection and survey documents are not read at a desk as often as the people writing them assume. They are opened on a phone in a car park, on a roof, in a plant room or at a site gate, on whatever signal is available, by somebody who wants to check one photograph against what is in front of them. Forty full-size images in that context is a document that does not open.

That is the whole case for this conversion. A drone frame arrives at 5,472 by 3,648 or more and weighs tens of megabytes as a raw file; the same picture at the width a report actually displays, encoded as WebP, is a couple of hundred kilobytes. Nothing about the usefulness of the photograph changes, and the document goes from unusable on a mobile connection to instant.

DNG began as Adobe’s open archival raw format and it is still used that way, but the files most people now hold did not come from a camera with a shutter. Drones write DNG for their raw stills, phones write it from computational pipelines, and both produce it in volume for work that ends up in documents rather than galleries.

That matters for what comes out of this conversion, because the picture extracted is the rendering the device itself produced. On a drone that rendering already includes lens distortion correction, noise reduction tuned to a small sensor, and tone mapping designed for high-contrast aerial scenes. It is the picture the operator saw and approved on the controller, which for evidential work is a defensible thing to publish.

Decide the display width first. A photograph placed full-width in a report is typically shown at 900 to 1,200 pixels; one in a two-column layout at half that; a thumbnail grid at 300. A 5,472-pixel drone frame is between four and eighteen times more than any of those, and every pixel above the display width is downloaded and discarded.

Set that number as the maximum width during the conversion. Reducing 5,472 to 1,200 removes about ninety-five per cent of the pixels, which saves an order of magnitude more than moving the quality slider ever will and costs nothing a reader can see. Only after that is it worth thinking about quality, and the default of 82 is a good place to leave it.

For the same visible quality WebP typically comes in a quarter to a third smaller than JPEG. On a single image that is unremarkable; on a report carrying forty of them it is several megabytes, and on a mobile connection several megabytes is several seconds of somebody standing in the rain looking at a spinner.

The format has been a published standard since 2010 and is decoded by every current browser, so there is no compatibility argument left to have about displaying it. Where it still bites is upload: some client portals, document management systems and PDF pipelines accept only JPEG or PNG. That is a question about the destination rather than about the format, and it is worth answering with one test file rather than with a converted job.

Aerial and site photographs are among the most heavily geotagged files anybody produces. A drone records latitude, longitude, altitude, heading and camera angle; a phone records where the person stood. The conversion here decodes to pixels and encodes again, and none of that reaches the WebP.

For a published document that is the right default. A report shared with a client, uploaded to a portal or attached to a planning submission should disclose location deliberately in its text, not accidentally in the files, and there are real cases where the position of a camera on a private site is not something to broadcast. The corollary is that the coordinates now live only in the DNG, so read them out and record them in the report before the originals are archived — an inspection photograph with no position is not evidence of much.

This is the one raw format where extraction can legitimately fail, because a DNG’s preview is written by software rather than fixed by a camera. Adobe’s converter and Lightroom offer Embedded, Medium Size, Full Size and None, and a file written with a small preview or none has nothing above the 640 by 480 threshold this converter will accept.

The failure is explicit rather than silent, and it does not mean the file is damaged — the sensor data is intact. Re-save the DNG with a full-size preview, or develop it in the raw software you have. If a batch from one source fails consistently, it is a property of how those files were produced and worth fixing once at the source rather than file by file.

The picture was compressed once when the device wrote the preview into the DNG, and again here when it is encoded as WebP. At quality 82 and report dimensions that second pass is not visible on the kind of subject matter a survey contains — brickwork, roof tiles, panels, drains, vegetation.

A third pass appears if the destination re-encodes uploads, which many portals and content systems do. That is the argument for converting from the DNG every time rather than from an image exported earlier in the week, and for leaving the quality setting alone rather than shaving it down: the passes you do not control will take their share regardless, and starting them from a clean file is the only part of the chain you own.

WebP supports an alpha channel and a lossless mode as well as the lossy one used here, which is why it turns up in web design contexts where a cut-out is needed. A drone or site photograph has nothing transparent in it and nothing that benefits from lossless encoding, so neither capability is doing any work.

What is worth knowing is that annotations added afterwards — arrows, outlines, a redaction over a number plate or a face — are usually drawn in an editor and saved from there. If those overlays need to sit on a transparent layer, save that layer separately as PNG; the WebP produced here is a flattened photograph and is the right thing to place underneath it.

Drop the whole set at once, apply one maximum width across it, and take the result as a ZIP. Consistency is worth more in a report than in a gallery: images converted at different widths sit unevenly in a template and make a document look assembled rather than produced.

Input files are accepted up to 100 MB on the free tier, which covers drone and phone DNGs comfortably — a 20-megapixel drone raw is usually well under 30 MB. The work runs on your own processor, so a hundred-frame job is limited by your machine rather than by a queue behind other people’s uploads.

Survey, insurance, security and infrastructure photographs are routinely covered by a contract that says where they may be stored. Running them through a free web converter puts copies on a server whose location, retention policy and ownership you have not established, which is a compliance problem before it is a technical one.

Here the DNG is read from disk by the page, scanned for its preview, decoded and re-encoded, all locally. No request carries the image, which the network tab will show and a disconnected machine will prove — and being able to demonstrate that is occasionally as useful as the fact itself.