Re: [AGG] Gaps in the lightweight rasterizer

[Maxim S. <mcs...@ya...>]

> Hi, I'm using the lightweight rasterizer component of AGG, and I'm 
> experiencing "gaps" between shapes. Below are a couple pictures that 
> illustrate what I mean.
> 
> First, you can see the gaps easily here:
> http://www.ottoproject.com/agg_rasterizer_gaps/solid_black.jpg
> 
> This second picture shows how the crosses are made - a central square 
> surrounded by four rectangles:
> http://www.ottoproject.com/agg_rasterizer_gaps/false_colors.jpg
> 
> Of course, the solution for a shape like this cross would be to make it 
> one single shape, instead of multiple shapes lined up. However, my 
> concern is not this cross shape specifically, but the general case of 
> two polygons that share an edge. Is there any way to prevent such gaps 
> from appearing between them?

It's not that easy to achieve the flawless stitching with the used method of
anti-aliasing. The problem is that if you blend a pixel with opacity=0.5 over 
another one with opacity=0.5 too, you will have the resulting opacity=0.75. It
works as if you paint over a glass with translucent ink; theoretically you will
never achieve full opacity. You will have the very same problem is Adobe SVG
viewer, for example.

The rectangles you draw "don't know" anything about each other (you draw them
separately, right?), so that, we can only rely on the blending formula. 

You can achieve the flawless result if you use comp_op_plus instead of
alpha-blend, like this:
http://www.antigrain.com/stuff/gouraud_stitching.zip

But you will need to use a separate RGBA buffer, initially set to zero (all
channels). Then you draw into it and finally alpha-blend the whole buffer to
the parent (main) one. And the shapes you draw into this intermediate buffer
must not overlap! That's the principal restriction.

I'm working on another rasterization algorithm (actually, pretty similar to the
existing one) that supports the Flash data model. There you don't have closed
contours, but just a set of edges with attributes: left-fill and right-fill. -1
means "no fill". So that, you can render a multicolor scene in one pass, with
perfect stitching. 
http://www.antigrain.com/stuff/gouraud_mesh.zip
The main point is that you don't have tro use a separate frame buffer; instead
there's only one scanline is used internally. But the price is that your
initial data must be prepared in a Flash-like way (no overlaps and no
self-intersections).

I just tried to check if the Flash rasterization algorithm can be used for 
the traditional triangle rendering. It perfectly handles it!

The main point is the whole mesh is rasterized in one pass; and the rasterizer
calculates the blending coefficients for each pixel in the scanline. Each
scanline consists of a number of spans associated with the attribute (style).
The style is not obligatory a solid color, it can be an image, Gouraud/Phong
shader, or any pixel shader in general. The spans can overlap with the
respective blending coefficients; and it's how anti-aliasing works here. Say,
if a pixel is covered by two adjacent triangles (0.5/0.5), the shader function
is called twice, and then the resulting color values are just summed up. In
general, it's called as many times as many triangles cover the pixel. This
method produces perfect stitching with perfect, 256-level anti-aliasing. 

McSeem



		
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