Getting RMagick/ImageMagick gravity with text - ruby

Question

Here is Ruby code:
require 'rmagick'
include Magick
img = Image.new(300, 300)
draw = Draw.new
draw.line(0, 150, 300, 150)
draw.line(150, 0, 150, 300)
# for each of known gravity constants...
%w[NorthWestGravity NorthGravity NorthEastGravity WestGravity CenterGravity EastGravity SouthWestGravity SouthGravity SouthEastGravity].
each{|g|
# set gravity to this value...
draw.gravity Magick.const_get(g)
# ...and draw text with this constant name
draw.text 150, 150, g
}
draw.draw(img)
img.write('tmp/gravity.png')
Here is image which it produces:
For SouthEast/NorthWest and similar gravities result is as expected (text is near 150,150, moved in desired direction). But for South, North and others result is really pretty weird.
As far as I can understand from code, RMagick just translates gravity and text commands into corresponding ImageMagick drawing primitives, so, I suppose its something in ImageMagick's gravity concept that I can't get.
What is it?..

Answer 1

I suppose its something in ImageMagick's gravity concept that I can't get.
What is it?..
The key to understanding what's going on is to locate the CenterGravity text.
Shifted left by 150px, and down by 150px.
Now compare compare NorthWestGravity position.
Also translated left & down by 150px respectively. Seeing a trend?
Your issue is with this line...
draw.text 150, 150, g
The Magick::Draw API maps to MVG spec. Use Magick::Draw.push & Magick::Draw.pop to control drawing context.
Edit from comments...
For setting the origin of text to be drawing, you'll need to calculate the position after evaluation the text/type metrics.
Example.
require 'rmagick'
include Magick
img = Image.new(300, 300) {
self.background_color = "palegreen"
}
draw = Draw.new
dotes = Draw.new # Dotes added for point of origin
dotes.fill = "red"
cursor = 1
# for each of known gravity constants...
%w[NorthWestGravity NorthGravity NorthEastGravity WestGravity CenterGravity EastGravity SouthWestGravity SouthGravity SouthEastGravity].
each{|g|
offsetX = 150
offsetY = cursor * 25
dotes.circle offsetX, offsetY, offsetX+2, offsetY+2
# Get metrics of text
metrics = draw.get_type_metrics(img, g)
# Full width
if %w[NorthWestGravity WestGravity SouthWestGravity].include? g then
offsetX -= metrics[:width]
end
# Full height
if %w[SouthWestGravity SouthGravity SouthEastGravity].include? g then
offsetY += metrics[:ascent]
end
# Half width
if %w[NorthGravity SouthGravity CenterGravity].include? g then
offsetX -= metrics[:width] / 2
end
# Half height
if %w[WestGravity CenterGravity EastGravity].include? g then
offsetY += metrics[:ascent] / 2
end
draw.text offsetX, offsetY, g
cursor += 1
}
dotes.draw(img)
draw.draw(img)
img.write('output.png')