[cairo] Aliased corner with rounded rectangle

[Lorenzo]

You can see the image with black  pixel at this url:

https://ibb.co/gbuXrU


This is done using glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

Instead, using GL_ONE the image is showing perfectly!


Thank you very much for your help.

Lorenzo


On 08/09/2018 05:55 PM, Bill Spitzak wrote:
> Though I would not expect it to look as bad as described, you want the 
> first argument to be GL_ONE, not GL_SRC_ALPHA.
>
>
> On Thu, Aug 9, 2018 at 6:07 AM, Lorenzo <gatti88 at libero.it 
> <mailto:gatti88 at libero.it>> wrote:
>
>     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.
>>
>>
>>
>>
>>
>>             -- 
>>             cairo mailing list
>>             cairo at cairographics.org <mailto:cairo at cairographics.org>
>>             https://lists.cairographics.org/mailman/listinfo/cairo
>>             <https://lists.cairographics.org/mailman/listinfo/cairo>
>>
>>
>>
>
>

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