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render: GL/GLES now draw lines almost perfectly matching software renderer.

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One place known to differ in a significant way is a single line segment that
starts and ends on the same point; the GL renderers will light up a single
pixel here, whereas the software renderer will not. My current belief is this
is a bug in the software renderer, based on the wording of the docs:

"SDL_RenderDrawLine() draws the line to include both end points."

You can see an example program that triggers that difference in Bug #2006.

As it stands, the GL renderers might _also_ render diagonal lines differently,
as the the Bresenham step might vary between implementations (one does three
pixels and then two, the other does two and then three, etc). But this patch
causes those lines to start and end on the correct pixel, and that's the best
we can do, and all anyone really needs here.

Not closing any bugs with this patch (yet!), but here are several that it
appears to fix. If no other corner cases pop up, we'll call this done.

Reference Bug #2006.
Reference Bug #1626.
Reference Bug #4001.
...and probably others...
This commit is contained in:
Ryan C. Gordon 2021-09-19 15:47:24 -04:00
parent 857cc7c0c9
commit ca9a321715
No known key found for this signature in database
GPG key ID: FA148B892AB48044
3 changed files with 74 additions and 78 deletions
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43
src/render/opengl/SDL_render_gl.c
View file

@ -890,39 +890,38 @@ static int
GL_QueueDrawLines(SDL_Renderer * renderer, SDL_RenderCommand *cmd, const SDL_FPoint * points, int count)
{
int i;
GLfloat prevx, prevy;
const size_t vertlen = (sizeof (GLfloat) * 2) * count;
GLfloat *verts = (GLfloat *) SDL_AllocateRenderVertices(renderer, vertlen, 0, &cmd->data.draw.first);
if (!verts) {
return -1;
}
cmd->data.draw.count = count;
/* Offset to hit the center of the pixel. */
for (i = 0; i < count; i++) {
*(verts++) = 0.5f + points[i].x;
*(verts++) = 0.5f + points[i].y;
}
/* 0.5f offset to hit the center of the pixel. */
prevx = 0.5f + points->x;
prevy = 0.5f + points->y;
*(verts++) = prevx;
*(verts++) = prevy;
/* Make the last line segment one pixel longer, to satisfy the
diamond-exit rule. */
verts -= 4;
{
const GLfloat xstart = verts[0];
const GLfloat ystart = verts[1];
const GLfloat xend = verts[2];
const GLfloat yend = verts[3];
if (ystart == yend) { /* horizontal line */
verts[(xend > xstart) ? 2 : 0] += 1.0f;
} else if (xstart == xend) { /* vertical line */
verts[(yend > ystart) ? 3 : 1] += 1.0f;
} else { /* bump a pixel in the direction we are moving in. */
/* bump the end of each line segment out a quarter of a pixel, to provoke
the diamond-exit rule. Without this, you won't just drop the last
pixel of the last line segment, but you might also drop pixels at the
edge of any given line segment along the way too. */
for (i = 1; i < count; i++) {
const GLfloat xstart = prevx;
const GLfloat ystart = prevy;
const GLfloat xend = points[i].x + 0.5f; /* 0.5f to hit pixel center. */
const GLfloat yend = points[i].y + 0.5f;
/* bump a little in the direction we are moving in. */
const GLfloat deltax = xend - xstart;
const GLfloat deltay = yend - ystart;
const GLfloat angle = SDL_atan2f(deltay, deltax);
verts[2] += SDL_cosf(angle);
verts[3] += SDL_sinf(angle);
}
prevx = xend + (SDL_cosf(angle) * 0.25f);
prevy = yend + (SDL_sinf(angle) * 0.25f);
*(verts++) = prevx;
*(verts++) = prevy;
}
return 0;

43
src/render/opengles/SDL_render_gles.c
View file

@ -564,39 +564,38 @@ static int
GLES_QueueDrawLines(SDL_Renderer * renderer, SDL_RenderCommand *cmd, const SDL_FPoint * points, int count)
{
int i;
GLfloat prevx, prevy;
const size_t vertlen = (sizeof (GLfloat) * 2) * count;
GLfloat *verts = (GLfloat *) SDL_AllocateRenderVertices(renderer, vertlen, 0, &cmd->data.draw.first);
if (!verts) {
return -1;
}
cmd->data.draw.count = count;
/* Offset to hit the center of the pixel. */
for (i = 0; i < count; i++) {
*(verts++) = 0.5f + points[i].x;
*(verts++) = 0.5f + points[i].y;
}
/* 0.5f offset to hit the center of the pixel. */
prevx = 0.5f + points->x;
prevy = 0.5f + points->y;
*(verts++) = prevx;
*(verts++) = prevy;
/* Make the last line segment one pixel longer, to satisfy the
diamond-exit rule. */
verts -= 4;
{
const GLfloat xstart = verts[0];
const GLfloat ystart = verts[1];
const GLfloat xend = verts[2];
const GLfloat yend = verts[3];
if (ystart == yend) { /* horizontal line */
verts[(xend > xstart) ? 2 : 0] += 1.0f;
} else if (xstart == xend) { /* vertical line */
verts[(yend > ystart) ? 3 : 1] += 1.0f;
} else { /* bump a pixel in the direction we are moving in. */
/* bump the end of each line segment out a quarter of a pixel, to provoke
the diamond-exit rule. Without this, you won't just drop the last
pixel of the last line segment, but you might also drop pixels at the
edge of any given line segment along the way too. */
for (i = 1; i < count; i++) {
const GLfloat xstart = prevx;
const GLfloat ystart = prevy;
const GLfloat xend = points[i].x + 0.5f; /* 0.5f to hit pixel center. */
const GLfloat yend = points[i].y + 0.5f;
/* bump a little in the direction we are moving in. */
const GLfloat deltax = xend - xstart;
const GLfloat deltay = yend - ystart;
const GLfloat angle = SDL_atan2f(deltay, deltax);
verts[2] += SDL_cosf(angle);
verts[3] += SDL_sinf(angle);
}
prevx = xend + (SDL_cosf(angle) * 0.25f);
prevy = yend + (SDL_sinf(angle) * 0.25f);
*(verts++) = prevx;
*(verts++) = prevy;
}
return 0;

46
src/render/opengles2/SDL_render_gles2.c
View file

@ -691,9 +691,9 @@ GLES2_QueueDrawLines(SDL_Renderer * renderer, SDL_RenderCommand *cmd, const SDL_
const SDL_bool colorswap = (renderer->target && (renderer->target->format == SDL_PIXELFORMAT_ARGB8888 || renderer->target->format == SDL_PIXELFORMAT_RGB888));
int color;
int i;
const size_t vertlen = (2 * sizeof (float) + sizeof (int)) * count;
GLfloat prevx, prevy;
const size_t vertlen = ((2 * sizeof (float)) + sizeof (int)) * count;
GLfloat *verts = (GLfloat *) SDL_AllocateRenderVertices(renderer, vertlen, 0, &cmd->data.draw.first);
if (!verts) {
return -1;
}
@ -706,33 +706,31 @@ GLES2_QueueDrawLines(SDL_Renderer * renderer, SDL_RenderCommand *cmd, const SDL_
cmd->data.draw.count = count;
/* Offset to hit the center of the pixel. */
for (i = 0; i < count; i++) {
*(verts++) = 0.5f + points[i].x;
*(verts++) = 0.5f + points[i].y;
/* 0.5f offset to hit the center of the pixel. */
prevx = 0.5f + points->x;
prevy = 0.5f + points->y;
*(verts++) = prevx;
*(verts++) = prevy;
*((int *)verts++) = color;
}
/* Make the last line segment one pixel longer, to satisfy the
diamond-exit rule. */
verts -= 3 + 3;
{
const GLfloat xstart = verts[0];
const GLfloat ystart = verts[1];
const GLfloat xend = verts[3 + 0];
const GLfloat yend = verts[3 + 1];
if (ystart == yend) { /* horizontal line */
verts[(xend > xstart) ? 3 + 0: 0] += 1.0f;
} else if (xstart == xend) { /* vertical line */
verts[(yend > ystart) ? 3 + 1: 1] += 1.0f;
} else { /* bump a pixel in the direction we are moving in. */
/* bump the end of each line segment out a quarter of a pixel, to provoke
the diamond-exit rule. Without this, you won't just drop the last
pixel of the last line segment, but you might also drop pixels at the
edge of any given line segment along the way too. */
for (i = 1; i < count; i++) {
const GLfloat xstart = prevx;
const GLfloat ystart = prevy;
const GLfloat xend = points[i].x + 0.5f; /* 0.5f to hit pixel center. */
const GLfloat yend = points[i].y + 0.5f;
/* bump a little in the direction we are moving in. */
const GLfloat deltax = xend - xstart;
const GLfloat deltay = yend - ystart;
const GLfloat angle = SDL_atan2f(deltay, deltax);
verts[3 + 0] += SDL_cosf(angle);
verts[3 + 1] += SDL_sinf(angle);
}
prevx = xend + (SDL_cosf(angle) * 0.25f);
prevy = yend + (SDL_sinf(angle) * 0.25f);
*(verts++) = prevx;
*(verts++) = prevy;
*((int *)verts++) = color;
}
return 0;