[1.96] Wrong display of DPI in "Thumbnails&Details"

[johannes.kastl]

Good Morning,

sorry if this one has been reported, I found nothing in the board.

When displaying pictures in "Thumbnails&Details"-Mode, I get some image properties in the second column. Included is something like "72 x 72 DPI". Which seems to appear with all pictures I looked at, no matter what DPI setting they had.

In the "Properties"-Window (ALT+ENTER) I see

Pixel per Inch: ??? x ???

The EXIF data for the image say:

X-Resolution 300
Y-Resolution 300

When setting the DPI via "Image"->"Set DPI", the wrong value is still displayed in the Properties-Column. But at least the "Properties"-Window (ALT+ENTER) displays the right value.

Is this a known bug? Or maybe a feature?

I know DPI is a not the most important thing ever, but when set in the picture, Xnview should display it right.

Regards,
Johannes

[xnview]

Do you have enabled 'use exif dpi info'?

[johannes.kastl]

No, I did not. Now it displays the correct values.

But why does Xnview display wrong values when no DPI is "set" and this option is not checked, and why the strange value in the properties (Pixel per Inch: ??? x ???)?

[xnview]

But why does Xnview display wrong values when no DPI is "set" and this option is not checked, and why the strange value in the properties (Pixel per Inch: ??? x ???)?

JPEG header has DPI info, and EXIF can have DPI too. Sometimes, DPI is only written in EXIF

[johannes.kastl]

Then why does Xnview display DPI Info, if there is none in the JPEG header (and the option mentioned above is not set)? If there is none, display none. Do not display the wrong value.

[mlu]

how do I enable "use exif dpi info"?

[frobert]

More exactly: It is the optional APP0/JFIF segment that has DPI info, or "pixel densities". A JPEG file may very well contain no resolution information at all and still be compliant with the JPEG standard.
As already said, a second place where resolutions can be found is the optional APP1/EXIF segment. Strictly speaking, JFIF and EXIF are supposed to be mutually exclusive, because both standards wants their segment to be the first one. But in practice, you often find both segments, one of them necessarily in the "wrong" place.
Thirdly, the optional APP13/Photoshop IRB (Image Resource Block) segment has provision for storing resolutions.

Of course, life becomes interesting when two or more of those sources are present and disagree...

Or when a rational values found in IRB (or EXIF) segment has a null denominator... Or a null numerator...

Or when some software try to out-smart the standards : Photoshop used to (still does ?) authoritatively declares 72 dpi for JPEG devoid of any resolution information. Ditto when it deemed the resolution to be too low (<10 dpi IIRC). It also forces the X and Y resolutions to be the same...