Eliminating virtual pixels for initiation unidirectional motion blur

[josephs-pixel]

ImageMagick version

7.1.0-62-Q16

Operating system, version and so on

Windows 10

Description

Here is what I have and works well:

-virtual-pixel edge -morphology convolve comet:0x500+90

. . . but the problem lies in the initiation of the motion blur, specifically at the top edge. Currently, IM is utilizing pixel data from above the image's top edge to begin the motion blur at the top edge. While I grasp this concept, it doesn't align with what I require. I need the motion blur to begin precisely at the top edge, without any calculations referencing data above it. Think of it as placing a paintbrush at the top edge and smoothly dragging it downwards, ensuring that all color information originates from the top edge and not from above it.

Any help is appreciated!

[snibgo]

The pixels in the output image near the top edge have to come from somewhere. We can make that "somewhere" transparent, then composite the output over the input, like this:

magick toes.png ( +clone -virtual-pixel none -morphology convolve comet:0x50+90 ) -composite x.png

Does that do what you want?

[josephs-pixel]

Thank you for that elegant solution. Yes, that actually works well enough. I was thinking something like that. I can't imagine any other way to go about it because as you mentioned the pixels have to come from somewhere.

[josephs-pixel]

I've experimented further with the suggestion provided, and while it does function, I'm still in search of a more refined outcome. Interestingly, during my exploration, I stumbled upon a technique referred to as "graduated blur" on another website, which seems to align more closely with my desired effect. Specifically, I noticed that the motion blur initiates from the top edge of the image and excludes pixel information located above the image. Do you have any insights on how this effect can be achieved in ImageMagick? Any guidance, even if it's just pointing me in the right direction, would be immensely appreciated!

https://gmic.eu/oldtutorial/dyidiffusion_graduated-blurs.shtml

[snibgo]

As that G'MIC page says, ImageMagick has -compose Blur -composite, which uses an image to vary the amount of blur. See my page Selective blur for examples.

For your case:

magick ^
  toes.png ^
  -set option:MYSIZE %wx%h ^
  ( -size %[MYSIZE] gradient:Black-White ) ^
  -compose Blur -set option:compose:args 0x50 -composite ^
  x2.png

This is Windows CMD syntax. For Windows BAT syntax, double each % to %%.

This makes a gradient image that is black at the top and white at the bottom, the same size as the input image. This is used as a mask for a blur that has sigma=0 in the x-direction (ie no blur), and sigma=50 in the y-direction.

This is fast enough for web-sized images. For 35m pixel images, it takes a long time, eg 30 minutes. If that is not acceptable, we can resize the input down, do the graduated blur, and resize up.

[josephs-pixel]

Yes! Thank you. This is very nice. Its does take a long time to process (without resize) as you mentioned. But I am okay with that because I only need to apply it to one image.