[Bug 93280] New: [SNA] Inconsistency in line visibility when using different line-widths

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=93280

            Bug ID: 93280
           Summary: [SNA] Inconsistency in line visibility when using
                    different line-widths
           Product: xorg
           Version: git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/intel
          Assignee: chris at chris-wilson.co.uk
          Reporter: romain.vavassori at hotmail.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org

Created attachment 120376
  --> https://bugs.freedesktop.org/attachment.cgi?id=120376&action=edit
archive containing program, patch and screenshots of the issue

There is an inconsistency in line visibility for lines of zero length
depending on the line-width attribute. This happens when using SNA instead of
UXA with xf86-video-intel driver.

I have done a program to show this issue (see test.c and screenshots in
attachment). The program draw a grid of sample lines with varying parameters:
- following X axis: increasing line length (0 to 2)
- following Y axis: increasing line-width (1 to 3)

Normally we expect to have the same behavior on each row of the grid (either
visible lines or not) but in the case of zero length lines, we obtain a visible
line only if line-width == 1.

>From the docs [1] it is said that for a line-width >= 1, zero length lines
should not be visible when drawn with CapButt attribute (as in our case).

I have put a patch in attachment solving the issue (patch.diff). Bug and patch
have been tested with Debian Jessie official version (2.99.917) and also with
git latest commit (da9ad388).

[1] http://tinf2.vub.ac.be/~dvermeir/manual/xlib/GC/manipulating.html

environment
-----------

uname -a:
Linux crunchbang 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt11-1 (2015-05-24)
x86_64 GNU/Linux

lspci -nn:
00:02.0 VGA compatible controller [0300]: Intel Corporation 4th Gen Core
Processor Integrated Graphics Controller [8086:0416] (rev 06)

versions:
- xorg: Debian Jessie official releases
- xf86-video-intel: bug present in either Debian Jessie release (2.99.917) or
latest git commit (da9ad388)

The driver used is i915 with SNA.

I can provide other logs if needed.

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