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opengl: Build out full GL_LINES and respect the diamond-exit rule.
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Likewise for the GLES1 and GLES2 renderers.

This solves the missing pixel at the end of a line and removes all the
heuristics for various platforms/drivers. It's possible we could still use
GL_LINE_STRIP with this and save some vertex buffer space, assuming this
doesn't upset some driver somewhere, but this seems to be a clean fix that
makes the GL renderers match the software renderer output.

Diamond-exit rule explanation:
http://graphics-software-engineer.blogspot.com/2012/04/rasterization-rules.html

Fixes Bugzilla #3182.
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@icculus
icculus committed Feb 10, 2020
1 parent 13155b3 commit ed10d94
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Showing 3 changed files with 175 additions and 71 deletions.
113 changes: 61 additions & 52 deletions src/render/opengl/SDL_render_gl.c
View file
Expand Up @@ -865,6 +865,60 @@ GL_QueueDrawPoints(SDL_Renderer * renderer, SDL_RenderCommand *cmd, const SDL_FP
return 0;
}

static int
GL_QueueDrawLines(SDL_Renderer * renderer, SDL_RenderCommand *cmd, const SDL_FPoint * points, int count)
{
GLfloat *verts;
int i;

SDL_assert(count >= 2); /* should have been checked at the higher level. */

verts = (GLfloat *) SDL_AllocateRenderVertices(renderer, (count-1) * 4 * sizeof (GLfloat), 0, &cmd->data.draw.first);
if (!verts) {
return -1;
}

cmd->data.draw.count = count;

/* GL_LINE_STRIP seems to be unreliable on various drivers, so try
to build out our own GL_LINES. :(
If the line segment is completely horizontal or vertical,
make it one pixel longer, to satisfy the diamond-exit rule.
We should probably do this for diagonal lines too, but we'd have to
do some trigonometry to figure out the correct pixel and generally
when we have problems with pixel perfection, it's for straight lines
that are missing a pixel that frames something and not arbitrary
angles. Maybe !!! FIXME for later, though. */

for (i = 0; i < count-1; i++, points++) {
GLfloat xstart = 0.5f + points[0].x; /* 0.5f to get to the center of the pixel. */
GLfloat ystart = 0.5f + points[0].y;
GLfloat xend = 0.5f + points[1].x;
GLfloat yend = 0.5f + points[1].y;

if (xstart == xend) { /* vertical line */
if (yend > ystart) {
yend += 1.0f;
} else {
ystart += 1.0f;
}
} else if (ystart == yend) { /* horizontal line */
if (xend > xstart) {
xend += 1.0f;
} else {
xstart += 1.0f;
}
}

*(verts++) = xstart;
*(verts++) = ystart;
*(verts++) = xend;
*(verts++) = yend;
}

return 0;
}

static int
GL_QueueFillRects(SDL_Renderer * renderer, SDL_RenderCommand *cmd, const SDL_FRect * rects, int count)
{
Expand Down Expand Up @@ -1228,59 +1282,14 @@ GL_RunCommandQueue(SDL_Renderer * renderer, SDL_RenderCommand *cmd, void *vertic
case SDL_RENDERCMD_DRAW_LINES: {
const GLfloat *verts = (GLfloat *) (((Uint8 *) vertices) + cmd->data.draw.first);
const size_t count = cmd->data.draw.count;
SDL_assert(count >= 2);
SetDrawState(data, cmd, SHADER_SOLID);
if (count > 2 && (verts[0] == verts[(count-1)*2]) && (verts[1] == verts[(count*2)-1])) {
data->glBegin(GL_LINE_LOOP);
/* GL_LINE_LOOP takes care of the final segment */
for (i = 1; i < count; ++i, verts += 2) {
data->glVertex2f(verts[0], verts[1]);
}
data->glEnd();
} else {
#if defined(__MACOSX__) || defined(__WIN32__)
#else
int x1, y1, x2, y2;
#endif

data->glBegin(GL_LINE_STRIP);
for (i = 0; i < count; ++i, verts += 2) {
data->glVertex2f(verts[0], verts[1]);
}
data->glEnd();
verts -= 2 * count;

/* The line is half open, so we need one more point to complete it.
* http://www.opengl.org/documentation/specs/version1.1/glspec1.1/node47.html
* If we have to, we can use vertical line and horizontal line textures
* for vertical and horizontal lines, and then create custom textures
* for diagonal lines and software render those. It's terrible, but at
* least it would be pixel perfect.
*/

data->glBegin(GL_POINTS);
#if defined(__MACOSX__) || defined(__WIN32__)
/* Mac OS X and Windows seem to always leave the last point open */
data->glVertex2f(verts[(count-1)*2], verts[(count*2)-1]);
#else
/* Linux seems to leave the right-most or bottom-most point open */
x1 = verts[0];
y1 = verts[1];
x2 = verts[(count-1)*2];
y2 = verts[(count*2)-1];

if (x1 > x2) {
data->glVertex2f(x1, y1);
} else if (x2 > x1) {
data->glVertex2f(x2, y2);
}
if (y1 > y2) {
data->glVertex2f(x1, y1);
} else if (y2 > y1) {
data->glVertex2f(x2, y2);
}
#endif
data->glEnd();
data->glBegin(GL_LINES);
for (i = 0; i < count-1; ++i, verts += 4) {
data->glVertex2f(verts[0], verts[1]);
data->glVertex2f(verts[2], verts[3]);
}
data->glEnd();
break;
}

Expand Down Expand Up @@ -1619,7 +1628,7 @@ GL_CreateRenderer(SDL_Window * window, Uint32 flags)
renderer->QueueSetViewport = GL_QueueSetViewport;
renderer->QueueSetDrawColor = GL_QueueSetViewport; /* SetViewport and SetDrawColor are (currently) no-ops. */
renderer->QueueDrawPoints = GL_QueueDrawPoints;
renderer->QueueDrawLines = GL_QueueDrawPoints; /* lines and points queue vertices the same way. */
renderer->QueueDrawLines = GL_QueueDrawLines;
renderer->QueueFillRects = GL_QueueFillRects;
renderer->QueueCopy = GL_QueueCopy;
renderer->QueueCopyEx = GL_QueueCopyEx;
Expand Down
66 changes: 57 additions & 9 deletions src/render/opengles/SDL_render_gles.c
View file
Expand Up @@ -560,6 +560,60 @@ GLES_QueueDrawPoints(SDL_Renderer * renderer, SDL_RenderCommand *cmd, const SDL_
return 0;
}

static int
GLES_QueueDrawLines(SDL_Renderer * renderer, SDL_RenderCommand *cmd, const SDL_FPoint * points, int count)
{
GLfloat *verts;
int i;

SDL_assert(count >= 2); /* should have been checked at the higher level. */

verts = (GLfloat *) SDL_AllocateRenderVertices(renderer, (count-1) * 4 * sizeof (GLfloat), 0, &cmd->data.draw.first);
if (!verts) {
return -1;
}

cmd->data.draw.count = count;

/* GL_LINE_STRIP seems to be unreliable on various drivers, so try
to build out our own GL_LINES. :(
If the line segment is completely horizontal or vertical,
make it one pixel longer, to satisfy the diamond-exit rule.
We should probably do this for diagonal lines too, but we'd have to
do some trigonometry to figure out the correct pixel and generally
when we have problems with pixel perfection, it's for straight lines
that are missing a pixel that frames something and not arbitrary
angles. Maybe !!! FIXME for later, though. */

for (i = 0; i < count-1; i++, points++) {
GLfloat xstart = 0.5f + points[0].x; /* 0.5f to get to the center of the pixel. */
GLfloat ystart = 0.5f + points[0].y;
GLfloat xend = 0.5f + points[1].x;
GLfloat yend = 0.5f + points[1].y;

if (xstart == xend) { /* vertical line */
if (yend > ystart) {
yend += 1.0f;
} else {
ystart += 1.0f;
}
} else if (ystart == yend) { /* horizontal line */
if (xend > xstart) {
xend += 1.0f;
} else {
xstart += 1.0f;
}
}

*(verts++) = xstart;
*(verts++) = ystart;
*(verts++) = xend;
*(verts++) = yend;
}

return 0;
}

static int
GLES_QueueFillRects(SDL_Renderer * renderer, SDL_RenderCommand *cmd, const SDL_FRect * rects, int count)
{
Expand Down Expand Up @@ -900,16 +954,10 @@ GLES_RunCommandQueue(SDL_Renderer * renderer, SDL_RenderCommand *cmd, void *vert
case SDL_RENDERCMD_DRAW_LINES: {
const GLfloat *verts = (GLfloat *) (((Uint8 *) vertices) + cmd->data.draw.first);
const size_t count = cmd->data.draw.count;
SDL_assert(count >= 2);
SetDrawState(data, cmd);
data->glVertexPointer(2, GL_FLOAT, 0, verts);
if (count > 2 && (verts[0] == verts[(count-1)*2]) && (verts[1] == verts[(count*2)-1])) {
/* GL_LINE_LOOP takes care of the final segment */
data->glDrawArrays(GL_LINE_LOOP, 0, (GLsizei) (count - 1));
} else {
data->glDrawArrays(GL_LINE_STRIP, 0, (GLsizei) count);
/* We need to close the endpoint of the line */
data->glDrawArrays(GL_POINTS, (GLsizei) (count - 1), 1);
}
data->glDrawArrays(GL_LINES, 0, (GLsizei) ((count-1) * 2));
break;
}

Expand Down Expand Up @@ -1158,7 +1206,7 @@ GLES_CreateRenderer(SDL_Window * window, Uint32 flags)
renderer->QueueSetViewport = GLES_QueueSetViewport;
renderer->QueueSetDrawColor = GLES_QueueSetViewport; /* SetViewport and SetDrawColor are (currently) no-ops. */
renderer->QueueDrawPoints = GLES_QueueDrawPoints;
renderer->QueueDrawLines = GLES_QueueDrawPoints; /* lines and points queue vertices the same way. */
renderer->QueueDrawLines = GLES_QueueDrawLines;
renderer->QueueFillRects = GLES_QueueFillRects;
renderer->QueueCopy = GLES_QueueCopy;
renderer->QueueCopyEx = GLES_QueueCopyEx;
Expand Down
67 changes: 57 additions & 10 deletions src/render/opengles2/SDL_render_gles2.c
View file
Expand Up @@ -783,6 +783,60 @@ GLES2_QueueDrawPoints(SDL_Renderer * renderer, SDL_RenderCommand *cmd, const SDL
return 0;
}

static int
GLES2_QueueDrawLines(SDL_Renderer * renderer, SDL_RenderCommand *cmd, const SDL_FPoint * points, int count)
{
GLfloat *verts;
int i;

SDL_assert(count >= 2); /* should have been checked at the higher level. */

verts = (GLfloat *) SDL_AllocateRenderVertices(renderer, (count-1) * 4 * sizeof (GLfloat), 0, &cmd->data.draw.first);
if (!verts) {
return -1;
}

cmd->data.draw.count = count;

/* GL_LINE_STRIP seems to be unreliable on various drivers, so try
to build out our own GL_LINES. :(
If the line segment is completely horizontal or vertical,
make it one pixel longer, to satisfy the diamond-exit rule.
We should probably do this for diagonal lines too, but we'd have to
do some trigonometry to figure out the correct pixel and generally
when we have problems with pixel perfection, it's for straight lines
that are missing a pixel that frames something and not arbitrary
angles. Maybe !!! FIXME for later, though. */

for (i = 0; i < count-1; i++, points++) {
GLfloat xstart = 0.5f + points[0].x; /* 0.5f to get to the center of the pixel. */
GLfloat ystart = 0.5f + points[0].y;
GLfloat xend = 0.5f + points[1].x;
GLfloat yend = 0.5f + points[1].y;

if (xstart == xend) { /* vertical line */
if (yend > ystart) {
yend += 1.0f;
} else {
ystart += 1.0f;
}
} else if (ystart == yend) { /* horizontal line */
if (xend > xstart) {
xend += 1.0f;
} else {
xstart += 1.0f;
}
}

*(verts++) = xstart;
*(verts++) = ystart;
*(verts++) = xend;
*(verts++) = yend;
}

return 0;
}

static int
GLES2_QueueFillRects(SDL_Renderer * renderer, SDL_RenderCommand *cmd, const SDL_FRect * rects, int count)
{
Expand Down Expand Up @@ -1294,17 +1348,10 @@ GLES2_RunCommandQueue(SDL_Renderer * renderer, SDL_RenderCommand *cmd, void *ver
}

case SDL_RENDERCMD_DRAW_LINES: {
const GLfloat *verts = (GLfloat *) (((Uint8 *) vertices) + cmd->data.draw.first);
const size_t count = cmd->data.draw.count;
SDL_assert(count >= 2);
if (SetDrawState(data, cmd, GLES2_IMAGESOURCE_SOLID) == 0) {
if (count > 2 && (verts[0] == verts[(count-1)*2]) && (verts[1] == verts[(count*2)-1])) {
/* GL_LINE_LOOP takes care of the final segment */
data->glDrawArrays(GL_LINE_LOOP, 0, (GLsizei) (count - 1));
} else {
data->glDrawArrays(GL_LINE_STRIP, 0, (GLsizei) count);
/* We need to close the endpoint of the line */
data->glDrawArrays(GL_POINTS, (GLsizei) (count - 1), 1);
}
data->glDrawArrays(GL_LINES, 0, (GLsizei) ((count-1) * 2));
}
break;
}
Expand Down Expand Up @@ -2099,7 +2146,7 @@ GLES2_CreateRenderer(SDL_Window *window, Uint32 flags)
renderer->QueueSetViewport = GLES2_QueueSetViewport;
renderer->QueueSetDrawColor = GLES2_QueueSetViewport; /* SetViewport and SetDrawColor are (currently) no-ops. */
renderer->QueueDrawPoints = GLES2_QueueDrawPoints;
renderer->QueueDrawLines = GLES2_QueueDrawPoints; /* lines and points queue vertices the same way. */
renderer->QueueDrawLines = GLES2_QueueDrawLines;
renderer->QueueFillRects = GLES2_QueueFillRects;
renderer->QueueCopy = GLES2_QueueCopy;
renderer->QueueCopyEx = GLES2_QueueCopyEx;
Expand Down

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