I am learning the ropes of Darktable (expert in Aperture and Capture One). I am trying to do an export. The manual (ver 4.4, verified again after updating to 4.6) describes options dpi and max size. I don’t see any such options in the export module in lighttable. Under “global options” I see set size, allow upscaling, high quality resampling and all the others listed in the manual, save for dpi and max size. Hence I think I found the right module. Do I need to do something to enable them, anything else I am missing? I am trying to reproduce the export behavior I used in Capture one, which is to set the longest edge to 2048. Thanks!
Just type in your desired size, that’s all.
The only real export sizing is by pixels. Everything else is just metadata.
If you want to limit the longest side to 2048, simply enter 2048 x 2048:
Setting the (max) size in pixels inserts a resolution metadata tag of 300 DPI.
If you want darktable to calculate the pixel count (e.g. you want to print a 15x10 image at 300 dpi), then switch to cm and enter the size + resolution:
(15 cm x 10 cm is ~5.91" x 3.94", at 300 dpi you get the values shown after which is equal to.) Your chosen DPI value will be stored in the metadata.
There is no way to export at a specified max. pixel size and set a custom DPI value in the metadata.
This is described in the manual at darktable 4.6 user manual - export
global options
set size
Choose how to measure the maximum size of your exported imagein pixels (for file): Enter the maximum width and height in pixels.
in cm (for print): Enter the maximum width and height in cm and define the image’s dpi. The equivalent size in pixels will be automatically calculated.
in inch (for print): Enter the maximum width and height in inches and define the image’s dpi. The equivalent size in pixels will be automatically calculated.
by scale (for file): Enter a multiplier to specify by how much the exported image should be scaled compared to the input image. For example, entering a value of 0.5 will result in an output image with half the width and height (in pixels) of the original image.
dpi
If units of cm or inches are chosen, set the dpi of the output image. The dpi will also be stored in the Exif data of the exported image. It will be automatically set to 300 if “in pixels” or “by scale” is chosen.max size
Set the maximum width and height of the exported image(s) in pixels, cm or inches (depending on the selected unit) – zero means that no constraint will be set on that dimension. Exported images will be constrained so as not to exceed either of these values, while retaining the correct aspect ratio. Set both to zero to export with the original dimensions (after cropping). If the entered values exceed the original dimensions darktable will either export with the original dimensions or upscale the image, depending on the “allow upscaling” parameter.
2 Likes
Got it thanks. dpi comes and goes depending on other options (reasonably, but I had an expectation of all fields being visible and becoming unavailable or greyed out if inactive – just my personal expectation from using other sw, not the lay of the land). For max size, I thought the size fields were described under “set size”, intending “exact size” as opposed to “max size” So I would rename the section “set size” to “set units” or some such. “meters” are units, not a distance. Thanks again.
“meter” is a unit, 10 cm is a size / distance, so in the export dialog you do set a size, expressed with a unit (to give the numbers meaning).
And you set indeed a maximum size for each dimension, not an exact size. Two reasons for that:
- you want to maintain the aspect ratio of your image, so if you give two sizes which have a different ratio, the image would hve to be stretched or compressed in one direction to get exact sizes.
- you can not allow upscaling, in which case it’s possible neither of the size requirements can be fulfilled (when giving a DPI)
you don‘t export units, you export pixels - the number of pixels in each dimension defines the size of the export.
Unfortunately this isn’t properly understood as those endless discussions on dpi numbers of image file metadata indicates.
It doesn’t help to support that confusion by using confusing terms…
Compulsory reading: The Myth of DPI.
1 Like
Setting the DPI in an image file is only interesting if you work with that image in professional layout software because it will determine how the image is placed there.
In a reasonable graphics design workflow the designer and not the photographer will set the dpi of all incoming images to either a standard value or what they require for that particular design. This helps in placing images to know how well they are suited for that design.
Outside such a workflow the dpi settings in files are meaningless.
2 Likes
I would like to thank everyone for pitching in but I feel like the conversation is veering off topic. I would like to make clear that I consider the issue resolved. I now I understand how to use this module.
1 Like
If you add solved in the title it might cut down further explanation as you seem to have it well in hand now… 
1 Like
Done.
2 Likes