CMYK colors not as expected

CMYK colors not as expected

[Hans van der Meer]

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I would be pleased to get some advice from "color-knowledgeable people" on the following matter.

In TV-broadcasts there are usually two kinds of subtitles present: Teletext as characters and DVB as pixels. These can be extracted and rendered with Metapost. I have done this, but I am not very satisfied with the colors produced. On the television screen they look bright, when rendered in Metapost they are dull and muddy. The text is barely readable.

As I am fairly sure I did extract the correct bits from the CLUT contained in the stream, I wonder if I am doing something not right at the Metapost side. In the datastream the values given are the most significant 6, 4, 4 and 2 bits of respectively Y, Cr, Cb, T. These are scaled to the values given to Metapost by dividing them by 255.

In the zip one finds the program (ConTeXt MKIV) used for typesetting, an example pixel map with clut, and the output. I would be obliged to get some comments.


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Hans van der Meer


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Re: CMYK colors not as expected

[luigi scarso]

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2011/6/28 Hans van der Meer <h.vandermeer@uva.nl>

> I would be pleased to get some advice from "color-knowledgeable people" on
> the following matter.
>
> In TV-broadcasts there are usually two kinds of subtitles present: Teletext
> as characters and DVB as pixels. These can be extracted and rendered with
> Metapost. I have done this, but I am not very satisfied with the colors
> produced. On the television screen they look bright, when rendered in
> Metapost they are dull and muddy. The text is barely readable.
>
> As I am fairly sure I did extract the correct bits from the CLUT contained
> in the stream, I wonder if I am doing something not right at the Metapost
> side. In the datastream the values given are the most significant 6, 4, 4
> and 2 bits of respectively Y, Cr, Cb, T. These are scaled to the values
> given to Metapost by dividing them by 255.
>
> In the zip one finds the program (ConTeXt MKIV) used for typesetting, an
> example pixel map with clut, and the output. I would be obliged to get some
> comments.
>
>
> have you already seen
 http://en.wikipedia.org/wiki/YCbCr
?
-- 
luigi

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Re: CMYK colors not as expected

[Taco Hoekwater]

On 06/29/11 08:08, luigi scarso wrote:
> 
> 
> have you already seen 
>  http://en.wikipedia.org/wiki/YCbCr

As luigi says, the Y/Cb/Cr (with or without T) is not
CMYK, it is a different kind of color space altogether,
and is conceptually much closer to RGB than to CMYK.

The wiki page has equations to convert YCbCr to RGB.
I have no idea what the 'K' component in your CLUT
is supposed to be.

Best wishes,
Taco

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Re: CMYK colors not as expected

[Taco Hoekwater]

On 06/29/11 15:59, Taco Hoekwater wrote:
> 
> The wiki page has equations to convert YCbCr to RGB.
> I have no idea what the 'K' component in your CLUT
> is supposed to be.

From some patent(?) doc, it seems this is a multiplier
for the Cb and Cr values...
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Re: CMYK colors not as expected

[Hans Hagen]

On 28-6-2011 2:06, Hans van der Meer wrote:
> I would be pleased to get some advice from "color-knowledgeable people" on the following matter.
>
> In TV-broadcasts there are usually two kinds of subtitles present: Teletext as characters and DVB as pixels. These can be extracted and rendered with Metapost. I have done this, but I am not very satisfied with the colors produced. On the television screen they look bright, when rendered in Metapost they are dull and muddy. The text is barely readable.
>
> As I am fairly sure I did extract the correct bits from the CLUT contained in the stream, I wonder if I am doing something not right at the Metapost side. In the datastream the values given are the most significant 6, 4, 4 and 2 bits of respectively Y, Cr, Cb, T. These are scaled to the values given to Metapost by dividing them by 255.
>
> In the zip one finds the program (ConTeXt MKIV) used for typesetting, an example pixel map with clut, and the output. I would be obliged to get some comments.

As you're doing a lot of calculations, you can consider using bitmaps

\starttext

\startMPpage
     draw
         bitmapimage(2,2,"334455 667788 99aabb ccddee")
         scaled 3cm
         rotated 15 ;
     draw
         bitmapimage(2,2,"33 55 77 99")
         scaled 2cm
         rotated 30 ;
     draw
         bitmapimage(2,2,"0000ff00 ff00ff00 00ff0000 ffff0000")
         scaled 1cm
         rotated 45 ;
\stopMPpage

\stoptext


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maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
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