[cairo] Aliased corner with rounded rectangle

[Lorenzo]

Sorry, by i already have the BlendFunc method.

I initialize my opengl system with this code:

glViewport(0, 0, drm->rect()->w, drm->rect()->h);
glDisable(GL_CULL_FACE);
glEnable(GL_LESS);
glEnable(GL_BLEND);
glEnable(GL_TEXTURE_2D);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);


Lorenzo.


On 08/08/2018 08:00 PM, Bill Spitzak wrote:
> glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
>
> Sorry, I managed to cut & paste the one example I had that did not say 
> "ALPHA"
>
> On Wed, Aug 8, 2018 at 10:59 AM, Bill Spitzak <spitzak at gmail.com 
> <mailto:spitzak at gmail.com>> wrote:
>
>     You have to change the OpenGL compositing to over the
>     premultiplied image. I think glBlendFunc(GL_ONE,
>     GL_ONE_MINUS_SRC_COLOR) is what you need.
>
>     On Wed, Aug 8, 2018 at 8:16 AM, Lorenzo <gatti88 at libero.it
>     <mailto:gatti88 at libero.it>> wrote:
>
>         Hi,
>
>         i have a problem with the creation of a rounded rectangle with
>         cairo.
>
>         Environment info:
>
>         cairo version: 1.14.8-1
>
>         OS: linux
>
>
>         A linux client creates a rounded rectangle with the following
>         code:
>
>                 let x = 0.0
>                 let y = 0.0
>                 let w = Double(size.w)
>                 let h = Double(size.h)
>                 let r = 20.0
>
>                 logger.verbose("rounded rectangle
>         x=\(x),y=\(y),w=\(w),h=\(h),r=\(r)")
>
>                 cairo_move_to(context, x, y + r)
>                 cairo_arc(context, x + r, y + r, r, Double.pi,
>         -Double.pi / 2.0)
>                 cairo_line_to(context, x + w - r, y)
>                 cairo_arc(context, x + w - r, y + r, r, -Double.pi /
>         2.0, 0.0)
>                 cairo_line_to(context, x + w, y + h - r)
>                 cairo_arc(context, x + w - r, y + h - r, r, 0.0,
>         Double.pi / 2.0)
>                 cairo_line_to(context, x + r, y + h)
>                 cairo_arc(context, x + r, y + h - r, r, Double.pi /
>         2.0, Double.pi)
>                 cairo_close_path(context)
>
>
>                 cairo_set_source_rgba(context, cl.red, cl.green,
>         cl.blue, cl.alpha);
>                 cairo_fill(context)
>
>
>         After the surface fill  i have 2 different operations to be
>         performed:
>
>             - write the png on the disk using
>         "cairo_surface_write_to_png(surface, "testcairo.png")"
>
>             - display the png with opengl es 3.0 as a texture (done by
>         copy the surface raw bytes with
>         cairo_image_surface_get_data(surface)
>
>               in a shared memory used by the opengl server program)
>
>
>         Save the image on disk is ok, the image is good.
>
>         Display the image as a texture with opengl results in an ugly
>         image: corners have some black pixels around them.
>
>         Why this difference?
>
>
>         I have done some checks:
>
>         - data obtained with cairo_image_surface_get_data are the same
>         that the graphics server loads from the shared memory (checked
>         PixelxPixel) so the data transfer don't is the problem
>         (checked also any signed/unsigned byte conversion)
>
>         - loading the disk png as a texture instead of loading data
>         from the shared memory results in a good image with no problems.
>
>
>         My opinion is that the cairo_surface_write_to_png performs an
>         antialiasing algorithm and accessing raw data bypass it taking
>         the original image.
>
>
>         Is my opinion correct? If yes, any advice to solve the problem?
>
>
>         Regards,
>
>         Lorenzo.
>
>
>
>
>
>         -- 
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>         cairo at cairographics.org <mailto:cairo at cairographics.org>
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>
>
>

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