Automatically set resolution of vector images based on density

[lookfirst]

I'd like to be able to set the output DPI to 300 for an EPS -> PNG conversion. We are successfully doing the output as 300 with some Java code:

https://github.com/depsypher/pngtastic/blob/master/src/main/java/com/googlecode/pngtastic/core/PngChunkInserter.java#L17

I've found all the previous issues that talk about doing this: #181 and #96. In #96, I saw that a change was made to output as 72 dpi, but in my simple tests, I'm still seeing the output as 25 dpi. screenshot: http://take.ms/NeZoO

osx 10.11 sharp v0.12.1 node 5.4.0 libvips 8.1.1 (brew) libpng 1.6.20 (brew)

I'd love to hear your thoughts on this. Thanks.

[lovell]

Hi Jon, I see you've since discovered #110. Its density option should provide what you're looking for to solve this problem. I'll close this question in preference to it.

[lookfirst]

@lovell I think this issue should be reopened. How can I get the output png image to be 300dpi? This is relatively easy with graphicsmagick by specifying the density option. http://aheckmann.github.io/gm/docs.html#density

[lookfirst]

            gm(response)
                .resize(width, height)
                .density(density, density)
                .units('PixelsPerInch')
                .stream(toFormat)
                .pipe(res);

[jcupitt]

I suppose there are times when you might want to explicitly set the DPI tag on an output image.

In libvips you do this with a combination of vips_copy(), to set the resolution fields in the header in pixels/mm, and vips_image_set_string() to set the "resolution-unit" tag to in or cm. Something like (untested):

// xres/yres in pixels/mm
// unit can be "cm" or "in", meaning xres and yres will be converted to that
// unit on write ... so set_res(11.81, 11.81, "in") means 300 dpi
VImage set_res(VImage in, double xres, double yres, char *unit)
{
  return in.
    copy(VImage::option()->set("xres", xres)->set("yes", yres)).
    set("resolution-unit", unit);
}

[lovell]

Hi Jon, when outputting bitmap formats that support "resolution", libvips will faithfully set this based on its Xres and Yres internal values. Here's the code for PNG output - https://github.com/jcupitt/libvips/blob/master/libvips/foreign/vipspng.c#L868-L872

It looks like the real problem is that *magick often provides no value for the input resolution - https://github.com/jcupitt/libvips/blob/master/libvips/foreign/magick2vips.c#L384-L386 - and libvips will assume a default resolution of 1 pixel/mm, =25 pixels/inch.

The *magick API accepts a string for density rather than numeric value(s), which increases the complexity of fixing this in libvips at input time. However the sharp API accepts a single numeric value, so the least complex solution here is to set libvips Xres and Yres values (via the method that John posted above whilst I was writing this comment).

[lookfirst]

Thanks @lovell and @jcupitt

The images I'm generating are being digitally printed on tshirts and the printer needs to have the correct DPI specified. =)

[lovell]

Commit bb37dc1 on the needle branch should provide everything needed for this. It also exposes the density attribute via metadata() so all the tests can use it too.

[lookfirst]

@lovell :heart: :+1: :heart: :heart_eyes:

[lookfirst]

@lovell Can I get a release with this in it please? =)

[lovell]

@lookfirst This will be in v0.14.0. We're waiting for libvips v8.2.3+ for the hist_entropy fix.

[lookfirst]

@lovell Released! =)

[lovell]

@lookfirst needle merged to master, now awaiting https://github.com/Homebrew/homebrew-science/pull/3459 for OS X users.

In the meantime, if you're able to share details publicly, it's always great to learn more about the problems people are solving with this module in the survey at #35

[lovell]

You'll be pleased to hear that v0.14.0 is now available via npm.

[lookfirst]

@lovell I just tested this and I'm still seeing the PNG being output with a DPI of 25.

    let transform = (width, height, density, toFormat) => {
        return sharp(null, {density: density})
            .resize(width, height)
            .max()
            .withoutEnlargement()
            .toFormat(toFormat);
    };

Screenshot of Preview.app info window for the generated image: http://take.ms/ZbJwG

[lovell]

@lookfirst Is this on OS X? What flags did you use when installing vips? IIRC support for this varies between differing versions of ImageMagick and GraphicsMagick.