Re: Alpha channel transparency value consistency

[Ethan A M. <sf...@us...>]

On Thursday, 07 July, 2016 12:01:27 Daniel J Sebald wrote:
> I'm working on something with alpha channel and noticed that the alpha 
> channel 0 to 1 range behaves differently for color specification and for 
> input data.  For color spec (help colorspec):
> 
>   "#AARRGGBB" represents an RGB color with an alpha channel (transparency)
>   value in the high bits. An alpha value of 0 represents a fully opaque 
> color;
>   i.e., "#00RRGGBB" is the same as "#RRGGBB".
> 
> and for rgbalpha (help alpha):
> 
>   The `rgbalpha` plotting style assumes that each pixel of input data 
> contains
>   an alpha value in the range [0:255].  A pixel with alpha = 0 is purely
>   transparent and does not alter the underlying contents of the plot. A 
> pixel
>   with alpha = 255 is purely opaque.
> 
> Was there a reason for the opposite correlation of the opacity/transparency?

Removing the inconsistency between line colors and image pixel colors 
was on the list of possible major changes in gnuplot version 5.
There was a bit of discussion at the time, but no one argued strongly
enough for unification to outweigh the clear disadvantages:
- breaking backward compatibility
- inherent conflict of obvious defaults:
  line color #00RRGGBB "obviously" should be the same as #RRGGBB
    but
  image processing has historically used alpha=0 as fully transparent

This same inconsistency of whether "0" is fully transparent or fully
opaque is present in real-world image formats and in other program 
implementations.

	Ethan