RMagick: scale and resize image for thumbnail

问题

I want to resize/scale an image. The originals have not the same dimensions like 300x200 or 512x600. I want to resize the image to 100x100 but DONT crop anything from the image or change ratio. Ideally the image will be first scale the long edge to 100 (aspect ratio) and then fill up the smaller edge with white.

 .---------.
 |- - - - -|
 |  IMAGE  |
 |- - - - -|
 '---------'

I dont use Paperclip or Rails, just RMagick.

回答1:

I've done it with merging the resized image with a new 100x100 image. Thats for sure not the best way but it works:

img = Magick::Image.read("file.png").first
target = Magick::Image.new(100, 100) do
  self.background_color = 'white'
end
img.resize_to_fit!(100, 100)
target.composite(img, Magick::CenterGravity, Magick::CopyCompositeOp).write("file-small.png)

回答2:

After playing with it for a while I got Fu86's composite trick to work like so:

img = Image.read("some_file").first().resize_to_fit!(width, height)
target = Image.new(width, height) do
    self.background_color = 'white'
end
target.composite(img, CenterGravity, AtopCompositeOp).write("some_new_file")

AtopCompositeOp seems to work better than CopyCompositeOp, which turned part of my background black for some reason.

回答3:

image = Magick::Image.read("filename").first
resized = image.resize_to_fit(width, height)     # will maintain aspect ratio, so one of the resized dimensions may be less than the specified dimensions
resized.background_color = "#FFFFFF"             # without a default, background color will vary based on the border of your original image
x = (resized.columns - width) / 2                # calculate necessary translation to center image on background
y = (resized.rows - height) / 2
resized = resized.extent(width, height, x, y)    # 'extent' fills out the resized image if necessary, with the background color, to match the full requested dimensions. the x and y parameters calculated in the previous step center the image on the background.
resized.write("new_filename")

Note: on heroku, which as of this posting uses imagemagick 6.5.7-8, I needed to multiply the x and y translations by -1 (and send positive numbers). Version 6.8.0-10 expects negative numbers.

回答4:

It seems you want to use change_geometry...

来源：https://stackoverflow.com/questions/4681192/rmagick-scale-and-resize-image-for-thumbnail

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image

rmagick