(again:) Magick++: how to "-sampling-factor XxY"

[jbarth_ubhd]

Dear Forum,

why does this not work?:

im.read(width, height, "RGB", CharPixel, im_rgb);
im.magick("JPEG");
im.quality(75);
im.defineValue("JPEG", "sampling-factor", "1x1,1x1,1x1");
im.write(varMap["of"].as<string>());

J. Barth

[glennrp]

Dear Forum,

why does this not work?:

im.read(width, height, "RGB", CharPixel, im_rgb);
im.magick("JPEG");
im.quality(75);
im.defineValue("JPEG", "sampling-factor", "1x1,1x1,1x1");
im.write(varMap["of"].as<string>());

J. Barth

There seems to be a bug in coders/jpeg.c. At line 2300

Code: Select all

value=GetImageProperty(image,"jpeg:sampling-factor");

should probably be

Code: Select all

  value=GetImageOption(image_info,"jpeg:sampling-factor");
  if (value == (char *) NULL)
    value=GetImageProperty(image,"jpeg:sampling-factor");

We have to check both Options and Properties so we handle
commandline input and also image data inherited from the
input image. With this change it works from the commandline

Code: Select all

convert logo: -define jpeg:sampling-factor="1x1,1x1,1x1" logo.jpg
convert logo.jpg logo2.jpg

logo.jpg and logo2.jpg both have the desired sampling-factors.
I assume that "im.defineValue" works the same as "-define" on the commandline.

[anthony]

We have to check both Options and Properties so we handle
commandline input and also image data inherited from the
input image. With this change it works from the commandline

Code: Select all

convert logo: -define jpeg:sampling-factor="1x1,1x1,1x1" logo.jpg
convert logo.jpg logo2.jpg

logo.jpg and logo2.jpg both have the desired sampling-factors.
I assume that "im.defineValue" works the same as "-define" on the commandline.

-define sets an artifact, which is what is looked up by various coders and operators as needed.


in IMv7 the properity (or escape) lookup will look for "attributes" (non-string per-image values, like width), "properties" (saved image meta-data), "artefacts" (non-saved per-image settings), and options (global user settings). In IMv6 "options" are coped (repeatly) into "artefacts", IMv7 have a link to it.

Note: Artefact lookup IMv7 also does a global "option" lookup, following the link back to the containing "wand" that holds image sequences.

Many API's do not have a 'wand' of global "options", so you need to set artifacts, on every image directly.