Answer
GIMP 2.10 decides whether to show a 16‑bit depth option in the PNG or TIFF export dialog by looking at two things:
- The image’s current precision (set via
Image → Precision).
- Whether the selected file‑format plug‑in reports that it can write 16‑bit data.
If both conditions are true, the dialog presents a “16 bit” checkbox; if either is false, the option is omitted or greyed out and the export falls back to 8‑bit.
Confirmed facts
- Internal precision can be 8‑bit integer, 16‑bit integer, or 32‑bit floating point (GEGL).
- The PNG and TIFF plug‑ins expose a 16‑bit option only when the image precision is 16‑bit integer (or higher) and the plug‑in was compiled with 16‑bit support.
- The JPEG plug‑in never offers a 16‑bit option; any higher‑precision data is converted to 8‑bit during export.
- When the export dialog is opened via
File → Export As with “Select file type (by extension)” disabled, or when using a legacy export preset that forces 8‑bit, the 16‑bit selector is hidden regardless of precision.
Likely explanation
The export dialog queries the image’s precision metadata and the plug‑in’s capability flags. If the plug‑in flags indicate 16‑bit support but the image precision is 8‑bit or 32‑bit float, the UI hides the 16‑bit choice because the plug‑in treats floating‑point data as incompatible with integer bit‑depth selection. Conversely, when the precision is set to 16‑bit integer, the plug‑in’s 16‑bit flag is honoured and the checkbox appears.
Steps to verify (for your case)
- Open or create an image.
- Choose
Image → Precision and set it to 16‑bit integer.
- Choose
File → Export As, select PNG (or TIFF) as the file type.
- In the export options dialog, look for a 16 bit checkbox; it should be enabled.
- Repeat with precision set to 8‑bit or 32‑bit float; the checkbox should disappear or be greyed out.
Missing diagnostic detail
To give a definitive recommendation, please confirm which precision mode you currently have set (Image → Precision): 8‑bit integer, 16‑bit integer, or 32‑bit floating point.
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