This bug report was migrated from our old Bugzilla tracker.

These attachments are available in the static archive:

• testcase (test_bug_3162_lines.c, text/x-csrc, 2017-10-18 14:53:57 +0000, 3404 bytes)
• patch (test_bug_3162_lines.c, text/x-csrc, 2017-10-18 14:59:17 +0000, 3404 bytes)
• patch (patch_bug_3162_lines.c, text/x-csrc, 2017-10-18 15:11:15 +0000, 3786 bytes)

Reported in version: 2.0.3
Reported for operating system, platform: Windows 7, x86_64

Comments on the original bug report:

On 2015-11-02 17:26:00 +0000, Marcel Bakker wrote:

windows 7 + opengl is ok
windows 7 + directx is ok
windows 7 + software does not draw the line
debian(VM) + opengl does not draw the line
debian(VM) + software does not draw the line

I think this is a Software renderer bug
, and suspect the debian + opengl is another problem, maybe https://bugzilla.libsdl.org/show_bug.cgi?id=2796
(i have other missing pixels with debian + opengl)

Reproduce :
...
SDL_SetHint( SDL_HINT_RENDER_DRIVER, "software");
...
SDL_Point line_1pixel[2] = {{ 10, 10}, {10, 10}};
SDL_RenderDrawLines( render, line_1pixel, 2);

On 2017-10-18 14:52:30 +0000, Sylvain wrote:

*** Bug 3600 has been marked as a duplicate of this bug. ***

On 2017-10-18 14:53:57 +0000, Sylvain wrote:

Created attachment 3005
testcase

I have checked with a testcase:

opengl :

• does not draw lines when there are 1 pixel.
• can miss corner points. more than 2, it can miss N points.

opengles2 :

• can miss corner points. more than 2, it can miss N points.

software renderer :

• does not draw lines when there are 1 pixel.

you can also build a line of 1 point by using :
SDL_Point points[] = {{20, 20}, {20, 20}, {20, 20}, {20, 20}};
and then, opengles2 also misses to draw the pixel !

I Suggest the following solution:

• SDL_DrawLines (SDL_DrawLine and SDL_DrawRect) should redirect to SDL_DrawPoints when line are 1 pixel.

• Remove all corner-case code that try to add points at the end of lines, because it's just wrong. so let the driver decides to draw the points or not.

eventually,
in addition to SDL_DrawLines(points), proceed to a SDL_DrawPoints(points) to make sure the point are drawn.

or we can also modify each renderer to always draws vertices, in addition to the lines.

On 2017-10-18 14:59:17 +0000, Sylvain wrote:

Created attachment 3006
patch

here's a patch with previous things.
but it can be also written so that drawing all vertices is done in each renderer code.

On 2017-10-18 15:11:15 +0000, Sylvain wrote:

Created attachment 3007
patch

the patch, drawing the vertices can be optional

On 2017-10-18 15:16:09 +0000, Sam Lantinga wrote:

The problem is that some renderers will draw the vertices, depending on the driver, so we'll get double draw of those points, which isn't what we want in the case where there are blend modes applied.

The right solution is to switch line drawing to use textures, which is incredibly fast on modern graphics hardware, and can be pixel perfect.

On 2017-10-18 21:15:56 +0000, Sam Lantinga wrote:

*** Bug 3600 has been marked as a duplicate of this bug. ***

On 2017-10-18 21:23:22 +0000, Sylvain wrote:

Can you precise what you mean ?

I quickly tried to use a 1D texture with glTexImage1D(), and it still have the same behavior:

• doesn't draw 1 pixel lines / misses sometimes vertices and extremities (if I remove the code that explicitly does it).

I also switched the GL_LINE_STRIP to GL_TRIANGLE_STRIP. Drawing is incorrect of course, but it shows also that vertices are still not drawn.