Apply stroke to caption: using MagickWand

Question

ImageMagick lets you apply a caption to an image. The caption text is automatically sized and wrapped to fit into the area that you define.

Using ImageMagick via the command line, I am able to define a stroke width and colour for this caption like so:

convert -size 300x300 -stroke black -strokewidth 1 -fill white \
    -background transparent -gravity center \
    caption:"This is a test of the caption feature in ImageMagick" ~/out.png

I cannot find anywhere online how to apply these attributes using the MagickWand C bindings. I can create a caption and change its font and font colour, but I cannot figure out how to add a stroke.

I want to know this information in order to add support for this to the Wand bindings for Python. I would be open to an alternative way to accomplish automatically sized text with gravity and a stroke, but preferably not requiring an inelegant workaround or external software.

As further information, I am using ImageMagick 6.9.10-10 on macOS 10.13.6 installed via Homebrew.

Answer 1

Technically you would be responsible for building a drawing-context, and calculate word wrapping. Usually by invoking the MagickQueryMultilineFontMetrics.

However, the caption: protocol was provided as a short-cut. You can review the source code to see how to achieve such calculations, but if your not interested, one can quickly hack a solution with MagickSetOption before invoking the Image Read methods.

With C

#include <wand/MagickWand.h>

int main(int argc, const char * argv[]) {
    MagickWandGenesis();
    MagickWand * wand;
    wand = NewMagickWand();
    // -size 300x300
    MagickSetSize(wand, 300, 300);
    // -stroke black
    MagickSetOption(wand, "stroke", "BLACK");
    // -strokewidth 1
    MagickSetOption(wand, "strokewidth", "1");
    // -fill white
    MagickSetOption(wand, "fill", "WHITE");
    // -background transparent
    MagickSetOption(wand, "background", "TRANSPARENT");
    // -gravity center
    MagickSetGravity(wand, CenterGravity);
    // caption:"This is a test of the caption feature in ImageMagick"
    MagickReadImage(wand, "caption:This is a test of the caption feature in ImageMagick");
    // ~/out.png
    MagickWriteImage(wand, "~/out.png");
    wand = DestroyMagickWand(wand);
    MagickWandTerminus();
    return 0;
}

With wand

from wand.image import Image
from wand.api import library

with Image() as img:
    # -size 300x300
    library.MagickSetSize(img.wand, 300, 300)
    # -stroke black
    library.MagickSetOption(img.wand, b"stroke", b"BLACK")
    # -strokewidth 1
    library.MagickSetOption(img.wand, b"strokewidth", b"1")
    # -fill white
    library.MagickSetOption(img.wand, b"fill", b"WHITE")
    # -background transparent
    library.MagickSetOption(img.wand, b"background", b"TRANSPARENT")
    # -gravity center
    img.gravity = "center"
    # caption:"This is a test of the caption feature in ImageMagick"
    img.read(filename="caption:This is a test of the caption feature in ImageMagick")
    # ~/out.png
    img.save(filename="~/out.png")