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I am not asking how AffineTransform works, but how to use its translate method.

I read the API many times, but still do not understand how it works.

public void translate(double tx,double ty)

Concatenates this transform with a translation transformation. This is equivalent to calling concatenate(T), where T is an AffineTransform represented by the following matrix: 

      [   1    0    tx  ]
      [   0    1    ty  ]
      [   0    0    1   ]

Questions:

  1. affineTransform.translate(100, 0) somehow moves the image +100 pixel to the right and +100 pixel to the bottom. Why is this so?

  2. Do I use translate() to move my images if I need to move my images very often? If not, what is the recommended method/ways to move my rotating/rotated images in a frame?

user3437460
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    1- Generally yes, it should move 100x and 0y, why its not would require a runnable example to determine. 2- this depends. Normally, I'd create a copy of the Graphics context before applying it or keeping a copy of the original, otherwise, you'll need to reverse it. Remember, any translations you apply are accumulative – MadProgrammer Feb 22 '15 at 20:25

1 Answers1

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affineTransform.translate(100, 0) somehow moves the image +100 pixel to the right and +100 pixel to the bottom. Why is this so?

Generally speaking, yes, it should move the drawing offset to 100x and 0y, meaning that the 0x0 position of the Graphics context will now be 100x0 from the origin point of the original Graphics context.

Remember, translations are accumulative, this means that if you had previously translated the Graphics context in some way, this will now add to it (first translation 0x100, second translation 100x0, you are now 100x100 from the original Graphics contexts origin point...

Do I use translate() to move my images if I need to move my images very often? If not, what is the recommended method/ways to move my rotating/rotated images in a frame?

Generally, I use Graphics#create to create a copy of the Graphics context (this copies the current state/properties, but will still generate output back to the original output of the Graphics context), apply the translations to the copy, paint whatever I want and then dispose of the copy. This leaves the original context unchanged (in regards to the properties that I have changed)

This means you can do multiple translations in isolation which won't affect other translations performed later.

Another method would be to reverse the translations, but frankly, calling dispose on a copy is just simpler and easier...

MadProgrammer
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  • Really appreciate your solution. However are you able to explain why is it shifting down and shifting right when I only supply 100 to `tx` ? https://www.mathsisfun.com/definitions/translation.html shows that by giving `tx` 100, the image should only shift right by 100 pixel. But why Java's translate is shifting it down as well? – user3437460 Feb 23 '15 at 09:01
  • Not unless you can give a runnable example. You might consider having a look at [this question](http://stackoverflow.com/questions/23507422/graphics-context-misaligned-on-first-paint) where I can issues with an initial transformation – MadProgrammer Feb 23 '15 at 09:11