How to do a gradient in Postscript Re: [Open-graphics] Linux Tag 2007 [Poster-Preview]

[James Richard Tyrer]

James Richard Tyrer wrote:
> Dieter wrote:
>>> But does postscript support the same types of gradients that SVG 
>>> does?=20
>>> SVG lets you make pretty complex gradients, and while the PS version 
>>> of=20
>>> the poster does have some kind of gradient, it's not the same as the=20
>>> SVG version. So either there's a bug in the conversion, or PS just=20
>>> doesn't support complex gradients.
>>
>> I don't know what you mean by "complex gradients".  My copy of the red 
>> book
>> has wandered off, but PS is a full blown programming language, so I 
>> imagine
>> it can do what you want.
>>
>> Taking a quick glance at the wikipedia SVG page, looks like someone 
>> decided
>> to create yet another graphics language, a couple of browsers have 
>> partial
>> support, Adobe will drop support in less than a year, yadda yadda 
>> yadda...
>>
>> The final output is supposed to be a large piece of paper, right?  What
>> language(s) does the printer speak?  Probably not SVG.
> 
> It appears that a ShadingType 2 has built in support for extending 
> beyond the end points like InkScape does.

What InkScape calls "None" and SVG calls "pad" is not the default for 
PS.  it requires this to be stated:

/Extend [true true]

> It appears (page 266 & 267 PLRM) that a ShadingType 2 dictionary can 
> include a function definition.  If none is defined, 

Actually, you always have to define a function -- see below.

> then a linear 
> shading is used, however, you should be able to define a f(t) do do 
> whatever you want.
> 
So, for the Green G quoting from IS's PS output:

<<
/ShadingType 2
/ColorSpace /DeviceRGB
/Coords [2338.1521 277.10916 2357.165 266.13211]
/Extend [true true]
/Domain [0 1]
/Function <<
/FunctionType 3
/Functions
[
<<
/FunctionType 2
/Domain [0 1]
/C0 [0 0.67843139 0.14509805]
/C1 [1 1 1]
/N 1
 >>
]
/Domain [0 1]
/Bounds [ ]
/Encode [ 0 1 ]
 >>
 >>

PS will not simply repeat or reflect a gradient (it will only "Extend". 
  The gradient must be defined over the full extent of the shading.  So, 
in this case, you would need to change:

/Coords [2319.1392 288.08621 2376.1779 255.15506]

which is the simple part -- this does work.

The hard part is to then define a function for the color that will 
provide the needed color over this new interval.

Does anyone actually know PostScript, this would be faster. :-D

IAC, the function needs to go from White to Green to White to Green 
rather than from Green to White as it is now.

-- 
JRT