ImageMagick help -- need to change DPI

Question

I have a 1000x1000 300dpi image that I need to convert to a 100x100 96dpi thumbnail. How do I do this in ImageMagick? I'm after the smallest possible file size at the highest possible quality.

Doing something like this:

convert myimage.png -quality 100 -resize 100 PNG8:mynewimage.png

.... does change the dimension, but still maintains the DPI. If I can get this to change to 96dpi, I should get a smaller file size.

I've tried -density, etc., but can't seem to make them work for me. Maybe I put the commands in the wrong order or passed the wrong parameters. Any assistance is greatly appreciated. Thanks.

Answer 1

The short version is, if you want a 100x100 image in PNG format, the line you have will already give you best quality at smallest file size. You can't do any better than that without a) coding to a lossy format (JPEG) or b) reducing the color depth of your image.

For a slightly longer explanation, straight from Wikipedia: "Dots per inch (DPI) is a measure of spatial printing or video dot density, in particular the number of individual dots that can be placed in a line within the span of 1 inch (2.54 cm). The DPI value tends to correlate with image resolution, but is related only indirectly."

DPI has nothing to do with getting a smaller file size; your 100x100 image measures 100x100 pixels, no matter whether you see it on a 300dpi screen or a 96dpi one (it will just look smaller on the 300dpi screen). The amount of information is the same either way.

"-density" won't help either, as it only works when "encoding a raster image while rendering (reading) vector formats such as Postscript, PDF, WMF, and SVG into a raster image". Those formats are resolution-independant, so it makes sense to tell ImageMagick the DPI to which you want the image rasterized. However, your DPI should be a function of the output device you plan to use. In your case, since you're starting with an already-rasterized image, this has no use.

PNG is a lossless format, so the -quality parameter only controls the zlib compression level; any gains in image size will be minimal, but it's worth using.