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Full, or fuller, Transparency?

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This work is for authoring time, Windows 10, Firefox; I want to use an existing image to guide my coded drawing on a CANVAS, without it becoming a permanent part of the drawing.

I have image files - in the present case, PNGs - and I have HTML pages containing CANVAS elements.  They all have white backgrounds (easily alterable for the CANVAS, including to "transparent").

What I would really like to do is to make the background of the CANVAS, but not the drawing on it, super-transparent, so that I can see in that region right through the window to the underlying window[s] (or desktop) underneath, which would show one of those PNGs underneath.  Then I could use the PNG as a guide while composing on the CANVAS.  As expected, and tested, I can make the background transparent, but not super-transparent; and I cannot make the page in which the CANVAS lies transparent itself.  N.B. "I cannot" != "It is impossible to".

I suppose I could display a PNG in the page as usual, make the background of the CANVAS transparent, and make the CANVAS draggable over the PNG - perhaps putting the drag X & Y distances into INPUT TYPE=TEXT elements so that they would be remembered by Firefox on page reload.


The drawing on the CANVAS is predominantly lines in the X & Y directions, and fills of those shapes.  One of the current PNGs is like that; the other is likewise but rotated by about 14 degrees.  It appears that I can un-rotate that PNG by simultaneously skewing it by +14 deg horizontal and -14 deg vertical (the loss of quality is unimportant).  Or I could temporarily use CTX.rotate(Math.PI*14/180) or similar in the CANVAS (and tilt my head to compensate).


Any recommendations, better suggestions, or other comments?


-- 
  (c) John Stockton, near London, UK.  Using Google Groups.           |
 Mail: J.R.""""""""@physics.org - or as Reply-To, if any.             |

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Full, or fuller, Transparency? dr.j.r.stockton@gmail.com - 2019-10-26 11:17 -0700
  Re: Full, or fuller, Transparency? "Michael Haufe (TNO)" <tno@thenewobjective.com> - 2019-10-26 16:39 -0700
    Re: Full, or fuller, Transparency? dr.j.r.stockton@gmail.com - 2019-10-27 08:25 -0700

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