This is terrible, but the OpenGL standard says that lines are half open, which means that one endpoint is not covered so adjoining lines don't overlap. It also doesn't define which end is open, and indeed Mac OS X and Linux differ. Mac OS X seems to leave the second endpoint open, but Linux uses the right-most endpoint for x major lines and the bottom-most endpoint for y major lines.
authorSam Lantinga <slouken@libsdl.org>
Sat, 21 Nov 2009 07:14:21 +0000
changeset 3473 7bdc10624cba
parent 3472 fdd2f2e9cd97
child 3474 1edb86163d62
This is terrible, but the OpenGL standard says that lines are half open, which means that one endpoint is not covered so adjoining lines don't overlap. It also doesn't define which end is open, and indeed Mac OS X and Linux differ. Mac OS X seems to leave the second endpoint open, but Linux uses the right-most endpoint for x major lines and the bottom-most endpoint for y major lines.
src/video/SDL_renderer_gl.c
--- a/src/video/SDL_renderer_gl.c	Sat Nov 21 06:34:43 2009 +0000
+++ b/src/video/SDL_renderer_gl.c	Sat Nov 21 07:14:21 2009 +0000
@@ -1150,22 +1150,20 @@
                     (GLfloat) renderer->b * inv255f,
                     (GLfloat) renderer->a * inv255f);
 
+    /* The line is half open, so we need tiny segments at the endpoints
+     * so that we guarantee coverage of the beginning and final pixels.
+     * http://www.opengl.org/documentation/specs/version1.1/glspec1.1/node47.html
+     */
     data->glBegin(GL_LINES);
+    /* Ensure coverage of the first point */
+    data->glVertex2f(0.1f + x1, 0.1f + y1);
+    data->glVertex2f(0.5f + x1, 0.5f + y1);
+    /* Draw the requested line */
     data->glVertex2f(0.5f + x1, 0.5f + y1);
     data->glVertex2f(0.5f + x2, 0.5f + y2);
-    data->glEnd();
-
-    /* For some reason the rightmost or lowest endpoint is skipped */
-    data->glBegin(GL_POINTS);
-    if (x1 > x2) {
-        data->glVertex2f(0.5f + x1, 0.5f + y1);
-    } else if (x2 > x1) {
-        data->glVertex2f(0.5f + x2, 0.5f + y2);
-    } else if (y1 > y2) {
-        data->glVertex2f(0.5f + x1, 0.5f + y1);
-    } else if (y2 > y1) {
-        data->glVertex2f(0.5f + x2, 0.5f + y2);
-    }
+    /* Ensure coverage of the second point */
+    data->glVertex2f(0.5f + x2, 0.5f + y2);
+    data->glVertex2f(0.9f + x2, 0.9f + y2);
     data->glEnd();
 
     return 0;