[mapserver-dev] transparent color

[Stephen Woodbridge]

Frank Warmerdam wrote:
> Folks,
> 
> In bug 1806 there is a report that COLOR -1 -1 -1 is not being treated as
> transparent when classifying raster data. Investigating I find that the
> code is checking for transparent as -255 -255 -255 using:
> 
> #define MS_TRANSPARENT_COLOR(color) (((color).red==-255 || 
> (color).green==-255 || (color).blue==-255)?MS_TRUE:MS_FALSE)
> 
> In fact, -1 -1 -1 is the default value to which colors are initialized
> and it is considered "invalid" according to:
> 
> #define MS_VALID_COLOR(color) (((color).red==-1 || (color).green==-1 || 
> (color).blue==-1)?MS_FALSE:MS_TRUE)
> 
> I have found no material discussing such color values in the reference
> documentation, though the mapserver tutorial does suggest using -1 -1 -1
> as a color for classified data to make it transparent.
> 
> The vector code likely does effectively treat this as transparent, but
> the raster code treats invalid, but not transparent as a special case 
> meaning
> use the underlying color of the raster data instead of applying a color
> based on classification.  I hesitate to break this capability by treating
> invalid as transparent.

Frank,

It is my understanding the -1 -1 -1 in the vector code just uses the 
underlying color, ie: it does not set a new color for the pixel. This is 
how we draw a "transparent" polygon where OUTLINECOLOR is set to a valid 
color and you only get the outline and the polygon does not blot out the 
underlying image.

I may have propagated the notion that this was "transparent" but maybe 
there is a better name for this.

Behaviorally what is the difference between INVALID and TRANSPARENT. I 
would assume that a true transparent color would "tint" the area with 
its color while allowing the underlying colors to show through. This 
would be the behavior of using the Alpha channel.

I believe the alpha channel stuff was added as part of the AGG code.

-Steve W

> Also,  I find in the mapsymbol.h include file that colorObj now has an
> alpha field.  I'm not clear on how widely this is being used, or whether it
> is intended to be the "future" of transparency.
> 
> Should we document -255 -255 -255 as transparent? Promote use of alpha?
> 
> Feedback is welcome!
> 
> Best regards,