Rmagick - How to Find Out the Pixel Dimension of an Image

RMagick - how do I find out the pixel dimension of an image

require 'rmagick'
img = Magick::Image.ping( 'demo.png' ).first
width = img.columns
height = img.rows

Note .ping method imported from comments. If you need to read the image to process it, then use Magick::Image.read( 'demo.png' ).first - the use of ping works in this stand-alone code and speeds processing up for some image types where IM can just read a header block. This is useful for cases where you don't need to load the image itself to do work on it.

Is there a simple way to get image dimensions in Ruby?

libimage-size is a Ruby library for calculating image sizes for a wide variety of graphical formats. A gem is available, or you can download the source tarball and extract the image_size.rb file.

resizing image with rmagick/imagemagick to be less than a specified product of its width and height

You need to properly calculate the ratio, which is a square root from your desired dimension divided by (row multiplied by col):

row, col = [2793, 1970]
ratio = Math.sqrt(4_000_000.0 / (row * col))
[row, col].map &ratio.method(:*)
#⇒ [
#  [0] 2381.400006266842,
#  [1] 1679.6842149465374
#]
[row, col].map(&ratio.method(:*)).reduce(:*)
#∞ 3999999.9999999995

RMagick: How to check if all pixels in a particular area of the image are transparent?

I would take your existing image and shrink it to that 3x3 size:

play = image.resize(3, 3, CubicFilter, 0.5)

Then you can create the code by checking each remaining pixel using:

code = ""
(0..2).each do |ix|
  (0..2).each do |iy|
    code += play.pixel_color(ix,iy).opacity == 65535 ? "0" : "1"
    code += iy == 2 ? "-" : "" unless ix == 2 && iy == 2
  end
end

I compare opacity to 65535 because when I inspected a pixel I knew was transparent this is what returned:

=> red=65535, green=65535, blue=65535, opacity=65535 

Get the colour of a pixel in rMagick

This is done by:

logo.pixel_color(0,0)

Can ImageMagick return the image size?

You could use an extra call to identify:

convert -size 320x240 image.jpg; identify -format "%[fx:w]x%[fx:h]" image.jpg

rmagick pixel color value

They are stored in a 'quantum depth' of 16-bits. You can rebuild the library to change this. Or you can simply divide each value by 257.

There's a function called MagickExportImagePixels which can get you the 8-bit pixel data that you want. Whenever you perform a transformation etc on an image it will get converted back to 16-bit pixels.

Rmagick each_pixel, how does it work?

The thing is, the array you get back from each_pixel is a new dataset. The data needs to be stored back to the image.

Use get_pixels and store_pixels instead:

img = Magick::ImageList.new('img.jpg').first
pixels = img.get_pixels(0,0,img.columns,img.rows)

for pixel in pixels
    avg = (pixel.red + pixel.green + pixel.blue) / 3
    pixel.red = avg
    pixel.blue = avg
    pixel.green = avg
end

img.store_pixels(0,0, img.columns, img.rows, pixels)
img.display

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