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Why can a small HEIC file take time to decode on a phone?

A small HEIC file can hide a huge picture, and your phone has to unpack its pixels. If you're making a copy, writing the new file can take even longer.

FluffyBench · 4 min read · Measured in an earlier phone study

How can a small HEIC become such a big job?

Think of a HEIC as a folded map. It's a compressed image file: small enough to tuck away, with a much bigger picture inside. Your phone has to unfold it into a grid of pixels before it can show or change the picture. That unpacking step is called decoding.

A 200-megapixel phone photo was about 2.4 MB saved. Unpacking its 12,240 × 16,320 pixels into red, green, blue and transparency (RGBA) takes about 800 MB for one pixel grid.

Bar lengths share a scale: a 2.4 MB HEIC file and an unpacked 12,240 × 16,320 pixels RGBA grid of about 800 MB.
File size counts the packed bytes; dimensions tell you how big a pixel grid needs rebuilding. How we measured

Which step takes longest when a phone converts a HEIC?

Making a copy in another format adds work after decoding. The phone has to encode the picture: pack the pixels into a new file.

In FluffyBench HEIC Converter, unpacking one large HEIC took about 12 seconds; writing its WebP copy took about 100. Writing WebP also took longest in FluffyBench Image Space Saver.

Shared-scale stage durations. FluffyBench HEIC Converter: decode 12.1 s, color 2.6 s, encode 99.9 s. FluffyBench Image Space Saver: decode 37.5 s, color 10.1 s, encode 99.7 s; additional PNG work 13.8 s.
Unpacking was the smaller part of the conversion wait. The chart follows the same HEIC through each tool on an older phone.

Doing it in FluffyBench

Need a copy for an app or site that asks for JPG? FluffyBench HEIC Converter makes that copy in your browser.

Here's how to make a JPG with the original pixel dimensions.

  1. Current FluffyBench HEIC Converter: Choose images, To JPG and Settings.
    Current FluffyBench HEIC Converter: Choose images, To JPG and Settings.
    Step 1. Add your HEIC, leave To at JPG for this walkthrough, and open Settings with the gear button.
    1. Choose images adds your HEIC photo.
    2. Leave To at JPG to make a JPG copy.
    3. The gear button opens Settings.
  2. Open JPG Settings with Quality 92 and Size Original selected.
    Open JPG Settings with Quality 92 and Size Original selected.
    Step 2. Quality balances fine detail against file size. Size set to Original keeps the photo's width and height.
    1. Leave Quality at 92.
    2. Leave Size on Original.
  3. Settings closed with Convert 1 to JPG ready above the selected photo.
    Settings closed with Convert 1 to JPG ready above the selected photo.
    Step 3. Close Settings with the same button, then press Convert to make your copy. Photo: NOAA, public domain.
    1. The gear button also closes Settings.
    2. Convert 1 to JPG starts the conversion.
  4. Completed JPG with its output size, dimensions and download arrow.
    Completed JPG with its output size, dimensions and download arrow.
    Step 4. Check the new size and dimensions, then download your JPG. Keep the HEIC until you've checked the saved copy in the app or site that needs it.
    1. Read the JPG's size and dimensions.
    2. The download arrow saves your JPG.

Working with HEIC photos mixed with other image formats?

FluffyBench Image Space Saver

FluffyBench Image Space Saver converts a mixed batch of image formats in one workspace.

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