Perspective projected stroked path with variable width

[Mathieu Monney-3]

Hi,

I'm pretty new to AGG and I'm trying to use to render lines with  
variable width in order to produce a "perspective effect" where the  
line width gets lower as it goes far away. What I have in the input is  
only vertices defining the line. So my goal would be to transform  
these vertices into a polygon representing the contour of the stroked  
line with a variable width. I tried to use a pipeline like path ->  
conv_stroke -> conv_stroke (in order to get the border of stroke)  
However it doesn't give me a polygon on the output so applying a  
perspective transform to it will not change the stroke width  
accordingly. What solution could I use?

Thanks a lot in advance.

Best regards,

Mathieu Monney

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[Stephan Assmus]

Hi,

Mathieu Monney schrieb:

> I'm pretty new to AGG and I'm trying to use to render lines with  
> variable width in order to produce a "perspective effect" where the  
> line width gets lower as it goes far away. What I have in the input is  
> only vertices defining the line. So my goal would be to transform  
> these vertices into a polygon representing the contour of the stroked  
> line with a variable width. I tried to use a pipeline like path ->  
> conv_stroke -> conv_stroke (in order to get the border of stroke)  
> However it doesn't give me a polygon on the output so applying a  
> perspective transform to it will not change the stroke width  
> accordingly. What solution could I use?

Hm, but when you apply a perspective transform to the last step of your
pipeline, your entire shape should be transformed. I don't know what you
mean by "it doesn't give me a polygon". The stroke output is also a
polygon in the sense of AGG, but the perspective transformation does not
care about that. It simply transforms any coordinates. How do you
configure the perspective transformation? In any case, I can confirm you
this is possible with AGG, we just need to figure out where you are
doing something wrong or where you have a misunderstanding. :-)

Best regards,
-Stephan

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[Petr Kobalíček]

Hi Mathieu,

you need to do this:
- convert curves to polygons
- apply transform
- render

hope that helps
- Petr

2009/9/16 Mathieu Monney <mathieu.monney@...>:

> Hi,
>
> I'm pretty new to AGG and I'm trying to use to render lines with
> variable width in order to produce a "perspective effect" where the
> line width gets lower as it goes far away. What I have in the input is
> only vertices defining the line. So my goal would be to transform
> these vertices into a polygon representing the contour of the stroked
> line with a variable width. I tried to use a pipeline like path ->
> conv_stroke -> conv_stroke (in order to get the border of stroke)
> However it doesn't give me a polygon on the output so applying a
> perspective transform to it will not change the stroke width
> accordingly. What solution could I use?
>
> Thanks a lot in advance.
>
> Best regards,
>
> Mathieu Monney
>
> ------------------------------------------------------------------------------
> Come build with us! The BlackBerry® Developer Conference in SF, CA
> is the only developer event you need to attend this year. Jumpstart your
> developing skills, take BlackBerry mobile applications to market and stay
> ahead of the curve. Join us from November 9-12, 2009. Register now!
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[Mathieu Monney-3]

Hi,

Thanks for your answer.

Actually I didn't really how to set the perspective transformation.  
What does the matrix terms mean? In order to set up my perspective  
matrix to test, I used the image_perspective demo and set up the quad  
of the image in order to have the perspective I want. Then I used the  
produced matrix to put it in my program. Dirty but I thought it should  
work :-).

So can you explain a little more what the perspective matrix means? I  
should have the sufficient knowledge to understand the affine matrix  
but this perspective matrix sounds awkward for me. Is there any  
documentation somewhere about that?

Actually I just found out that I mixed up my pipeline and I applied  
the perspective transformation directly to the path and then stroke  
it. So I reverse that and now it works :-). However I still don't know  
how to set up this perspective matrix :-).
So I have the following pipeline: transform < stroke< stroke<path> > ,  
perspective >. Is that optimal or is there any more optimal way to  
convert the thick lines to polygons?

Thanks a lot.

Best regards,

Mathieu



Le 16 sept. 2009 à 17:05, Stephan Aßmus a écrit :

> Hi,
>
> Mathieu Monney schrieb:
>> I'm pretty new to AGG and I'm trying to use to render lines with
>> variable width in order to produce a "perspective effect" where the
>> line width gets lower as it goes far away. What I have in the input  
>> is
>> only vertices defining the line. So my goal would be to transform
>> these vertices into a polygon representing the contour of the stroked
>> line with a variable width. I tried to use a pipeline like path ->
>> conv_stroke -> conv_stroke (in order to get the border of stroke)
>> However it doesn't give me a polygon on the output so applying a
>> perspective transform to it will not change the stroke width
>> accordingly. What solution could I use?
>
> Hm, but when you apply a perspective transform to the last step of  
> your
> pipeline, your entire shape should be transformed. I don't know what  
> you
> mean by "it doesn't give me a polygon". The stroke output is also a
> polygon in the sense of AGG, but the perspective transformation does  
> not
> care about that. It simply transforms any coordinates. How do you
> configure the perspective transformation? In any case, I can confirm  
> you
> this is possible with AGG, we just need to figure out where you are
> doing something wrong or where you have a misunderstanding. :-)
>
> Best regards,
> -Stephan
>
> ------------------------------------------------------------------------------
> Come build with us! The BlackBerry® Developer Conference in SF, CA
> is the only developer event you need to attend this year. Jumpstart  
> your
> developing skills, take BlackBerry mobile applications to market and  
> stay
> ahead of the curve. Join us from November 9-12, 2009. Register  
> now!
> http://p.sf.net/sfu/devconf
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[Petr Kobalíček]

Hi Mathieu,

I think that you must first understand how rasterization in antigrain
works. The pipeline in antigrain is something like middleware between
your final image and your vector data. Antigrain rasterizer can only
rasterize polygons, so here the pipeline is to create set (or one) of
closed polygons from your vector data.

For example, you have some vector data:
1. you need to convert curves to lines (curve pipeline)
2. you need to stroke (stroke pipeline)
3. you need to apply transform (here your perspective matrix is used -
transform pipeline)
4. you are ready to rasterize

if you swap step 2 and 3 it's logical that stroke width will be
constant in are places. Antigrain gives you ability to swap various
steps in the pipeline.

About perspective matrix:
- it's like affine matrix but contains another fields. It's sometimes
called as transform matrix. For detailed description look at
QTransform class in qt: http://doc.trolltech.com/4.5/qtransform.html .

Regards
- Petr

2009/9/16 Mathieu Monney <mathieu.monney@...>:

> Hi,
>
> Thanks for your answer.
>
> Actually I didn't really how to set the perspective transformation.
> What does the matrix terms mean? In order to set up my perspective
> matrix to test, I used the image_perspective demo and set up the quad
> of the image in order to have the perspective I want. Then I used the
> produced matrix to put it in my program. Dirty but I thought it should
> work :-).
>
> So can you explain a little more what the perspective matrix means? I
> should have the sufficient knowledge to understand the affine matrix
> but this perspective matrix sounds awkward for me. Is there any
> documentation somewhere about that?
>
> Actually I just found out that I mixed up my pipeline and I applied
> the perspective transformation directly to the path and then stroke
> it. So I reverse that and now it works :-). However I still don't know
> how to set up this perspective matrix :-).
> So I have the following pipeline: transform < stroke< stroke<path> > ,
> perspective >. Is that optimal or is there any more optimal way to
> convert the thick lines to polygons?
>
> Thanks a lot.
>
> Best regards,
>
> Mathieu
>
>
>
> Le 16 sept. 2009 à 17:05, Stephan Aßmus a écrit :
>
>> Hi,
>>
>> Mathieu Monney schrieb:
>>> I'm pretty new to AGG and I'm trying to use to render lines with
>>> variable width in order to produce a "perspective effect" where the
>>> line width gets lower as it goes far away. What I have in the input
>>> is
>>> only vertices defining the line. So my goal would be to transform
>>> these vertices into a polygon representing the contour of the stroked
>>> line with a variable width. I tried to use a pipeline like path ->
>>> conv_stroke -> conv_stroke (in order to get the border of stroke)
>>> However it doesn't give me a polygon on the output so applying a
>>> perspective transform to it will not change the stroke width
>>> accordingly. What solution could I use?
>>
>> Hm, but when you apply a perspective transform to the last step of
>> your
>> pipeline, your entire shape should be transformed. I don't know what
>> you
>> mean by "it doesn't give me a polygon". The stroke output is also a
>> polygon in the sense of AGG, but the perspective transformation does
>> not
>> care about that. It simply transforms any coordinates. How do you
>> configure the perspective transformation? In any case, I can confirm
>> you
>> this is possible with AGG, we just need to figure out where you are
>> doing something wrong or where you have a misunderstanding. :-)
>>
>> Best regards,
>> -Stephan
>>
>> ------------------------------------------------------------------------------
>> Come build with us! The BlackBerry® Developer Conference in SF, CA
>> is the only developer event you need to attend this year. Jumpstart
>> your
>> developing skills, take BlackBerry mobile applications to market and
>> stay
>> ahead of the curve. Join us from November 9-12, 2009. Register
>> now!
>> http://p.sf.net/sfu/devconf
>> _______________________________________________
>> Vector-agg-general mailing list
>> Vector-agg-general@...
>> https://lists.sourceforge.net/lists/listinfo/vector-agg-general
>
>
> ------------------------------------------------------------------------------
> Come build with us! The BlackBerry® Developer Conference in SF, CA
> is the only developer event you need to attend this year. Jumpstart your
> developing skills, take BlackBerry mobile applications to market and stay
> ahead of the curve. Join us from November 9-12, 2009. Register now!
> http://p.sf.net/sfu/devconf
> _______________________________________________
> Vector-agg-general mailing list
> Vector-agg-general@...
> https://lists.sourceforge.net/lists/listinfo/vector-agg-general
>

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[Stephan Assmus]

Hi,

Mathieu Monney schrieb:
> Actually I just found out that I mixed up my pipeline and I applied  
> the perspective transformation directly to the path and then stroke  
> it. So I reverse that and now it works :-). However I still don't know  
> how to set up this perspective matrix :-).
> So I have the following pipeline: transform < stroke< stroke<path> > ,  
> perspective >. Is that optimal or is there any more optimal way to  
> convert the thick lines to polygons?

Good to know you figured it out. Sorry, but I cannot help explain the
perspective matrix, but this is probably a subject that you can google.
When I used the persepective transformation, I've always used the quad
to quad utility methods to produce the matrix I wanted.

Best regards,
-Stephan

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[Mathieu Monney-3]

Hi Petr,

Thanks for your complete response. It really helps.

However I still have two questions:

- What do you mean by converting curves to lines ? In the input I have  
already points defining the segments of my lines. So it is already ok,  
isn't it ?
- Is there any performance speed up which can be made in order to draw  
the same lines under a different perspective. I mean I want to draw my  
lines using one perspective matrix and then later draw the same lines  
again with another perspective matrix. What kind of things should I be  
aware of in order AGG not to recompute all the vectors data again?

Thanks a lot.

Best,

Mathieu

Le 17 sept. 2009 à 06:36, Petr Kobalíček a écrit :

> Hi Mathieu,
>
> I think that you must first understand how rasterization in antigrain
> works. The pipeline in antigrain is something like middleware between
> your final image and your vector data. Antigrain rasterizer can only
> rasterize polygons, so here the pipeline is to create set (or one) of
> closed polygons from your vector data.
>
> For example, you have some vector data:
> 1. you need to convert curves to lines (curve pipeline)
> 2. you need to stroke (stroke pipeline)
> 3. you need to apply transform (here your perspective matrix is used -
> transform pipeline)
> 4. you are ready to rasterize
>
> if you swap step 2 and 3 it's logical that stroke width will be
> constant in are places. Antigrain gives you ability to swap various
> steps in the pipeline.
>
> About perspective matrix:
> - it's like affine matrix but contains another fields. It's sometimes
> called as transform matrix. For detailed description look at
> QTransform class in qt: http://doc.trolltech.com/4.5/qtransform.html .
>
> Regards
> - Petr
>
> 2009/9/16 Mathieu Monney <mathieu.monney@...>:
>> Hi,
>>
>> Thanks for your answer.
>>
>> Actually I didn't really how to set the perspective transformation.
>> What does the matrix terms mean? In order to set up my perspective
>> matrix to test, I used the image_perspective demo and set up the quad
>> of the image in order to have the perspective I want. Then I used the
>> produced matrix to put it in my program. Dirty but I thought it  
>> should
>> work :-).
>>
>> So can you explain a little more what the perspective matrix means? I
>> should have the sufficient knowledge to understand the affine matrix
>> but this perspective matrix sounds awkward for me. Is there any
>> documentation somewhere about that?
>>
>> Actually I just found out that I mixed up my pipeline and I applied
>> the perspective transformation directly to the path and then stroke
>> it. So I reverse that and now it works :-). However I still don't  
>> know
>> how to set up this perspective matrix :-).
>> So I have the following pipeline: transform < stroke< stroke<path>  
>> > ,
>> perspective >. Is that optimal or is there any more optimal way to
>> convert the thick lines to polygons?
>>
>> Thanks a lot.
>>
>> Best regards,
>>
>> Mathieu
>>
>>
>>
>> Le 16 sept. 2009 à 17:05, Stephan Aßmus a écrit :
>>
>>> Hi,
>>>
>>> Mathieu Monney schrieb:
>>>> I'm pretty new to AGG and I'm trying to use to render lines with
>>>> variable width in order to produce a "perspective effect" where the
>>>> line width gets lower as it goes far away. What I have in the input
>>>> is
>>>> only vertices defining the line. So my goal would be to transform
>>>> these vertices into a polygon representing the contour of the  
>>>> stroked
>>>> line with a variable width. I tried to use a pipeline like path ->
>>>> conv_stroke -> conv_stroke (in order to get the border of stroke)
>>>> However it doesn't give me a polygon on the output so applying a
>>>> perspective transform to it will not change the stroke width
>>>> accordingly. What solution could I use?
>>>
>>> Hm, but when you apply a perspective transform to the last step of
>>> your
>>> pipeline, your entire shape should be transformed. I don't know what
>>> you
>>> mean by "it doesn't give me a polygon". The stroke output is also a
>>> polygon in the sense of AGG, but the perspective transformation does
>>> not
>>> care about that. It simply transforms any coordinates. How do you
>>> configure the perspective transformation? In any case, I can confirm
>>> you
>>> this is possible with AGG, we just need to figure out where you are
>>> doing something wrong or where you have a misunderstanding. :-)
>>>
>>> Best regards,
>>> -Stephan
>>>
>>> ------------------------------------------------------------------------------
>>> Come build with us! The BlackBerry® Developer Conference in  
>>> SF, CA
>>> is the only developer event you need to attend this year. Jumpstart
>>> your
>>> developing skills, take BlackBerry mobile applications to market and
>>> stay
>>> ahead of the curve. Join us from November 9-12, 2009. Register
>>> now!
>>> http://p.sf.net/sfu/devconf
>>> _______________________________________________
>>> Vector-agg-general mailing list
>>> Vector-agg-general@...
>>> https://lists.sourceforge.net/lists/listinfo/vector-agg-general
>>
>>
>> ------------------------------------------------------------------------------
>> Come build with us! The BlackBerry® Developer Conference in SF,  
>> CA
>> is the only developer event you need to attend this year. Jumpstart  
>> your
>> developing skills, take BlackBerry mobile applications to market  
>> and stay
>> ahead of the curve. Join us from November 9-12, 2009. Register  
>> now!
>> http://p.sf.net/sfu/devconf
>> _______________________________________________
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>> Vector-agg-general@...
>> https://lists.sourceforge.net/lists/listinfo/vector-agg-general
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>
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> developing skills, take BlackBerry mobile applications to market and  
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> ahead of the curve. Join us from November 9-12, 2009. Register  
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[Stephan Assmus]

Mathieu Monney schrieb:

> Hi Petr,
>
> Thanks for your complete response. It really helps.
>
> However I still have two questions:
>
> - What do you mean by converting curves to lines ? In the input I have  
> already points defining the segments of my lines. So it is already ok,  
> isn't it ?
> - Is there any performance speed up which can be made in order to draw  
> the same lines under a different perspective. I mean I want to draw my  
> lines using one perspective matrix and then later draw the same lines  
> again with another perspective matrix. What kind of things should I be  
> aware of in order AGG not to recompute all the vectors data again?

The stroker works on the fly. It does not cache the computed coordinates
if I am not mistaken. If you needed to use a curve converter (doesn't
seem to be the case), it would also generate points on the fly. So you
cannot do anything, except record the output in another
agg::path_storage and use that as a cache. But once you use more
sophisticated fills, you will quickly notice that the vector
calculations are not usually any performance problem. That is my own
experience, though, it depends on the complexity of your vector data.

Best regards,
-Stephan

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