weston/src/vertex-clipping.c

/*
 * Copyright © 2012 Intel Corporation
 *
 * Permission to use, copy, modify, distribute, and sell this software and
 * its documentation for any purpose is hereby granted without fee, provided
 * that the above copyright notice appear in all copies and that both that
 * copyright notice and this permission notice appear in supporting
 * documentation, and that the name of the copyright holders not be used in
 * advertising or publicity pertaining to distribution of the software
 * without specific, written prior permission.  The copyright holders make
 * no representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 *
 * THE COPYRIGHT HOLDERS DISCLAIM ALL WARRANTIES WITH REGARD TO THIS
 * SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
 * FITNESS, IN NO EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE FOR ANY
 * SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER
 * RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF
 * CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */
#include <assert.h>
#include <float.h>
#include <math.h>

#include <GLES2/gl2.h>

#include "vertex-clipping.h"

GLfloat
float_difference(GLfloat a, GLfloat b)
{
	/* http://www.altdevblogaday.com/2012/02/22/comparing-floating-point-numbers-2012-edition/ */
	static const GLfloat max_diff = 4.0f * FLT_MIN;
	static const GLfloat max_rel_diff = 4.0e-5;
	GLfloat diff = a - b;
	GLfloat adiff = fabsf(diff);

	if (adiff <= max_diff)
		return 0.0f;

	a = fabsf(a);
	b = fabsf(b);
	if (adiff <= (a > b ? a : b) * max_rel_diff)
		return 0.0f;

	return diff;
}

/* A line segment (p1x, p1y)-(p2x, p2y) intersects the line x = x_arg.
 * Compute the y coordinate of the intersection.
 */
static GLfloat
clip_intersect_y(GLfloat p1x, GLfloat p1y, GLfloat p2x, GLfloat p2y,
		 GLfloat x_arg)
{
	GLfloat a;
	GLfloat diff = float_difference(p1x, p2x);

	/* Practically vertical line segment, yet the end points have already
	 * been determined to be on different sides of the line. Therefore
	 * the line segment is part of the line and intersects everywhere.
	 * Return the end point, so we use the whole line segment.
	 */
	if (diff == 0.0f)
		return p2y;

	a = (x_arg - p2x) / diff;
	return p2y + (p1y - p2y) * a;
}

/* A line segment (p1x, p1y)-(p2x, p2y) intersects the line y = y_arg.
 * Compute the x coordinate of the intersection.
 */
static GLfloat
clip_intersect_x(GLfloat p1x, GLfloat p1y, GLfloat p2x, GLfloat p2y,
		 GLfloat y_arg)
{
	GLfloat a;
	GLfloat diff = float_difference(p1y, p2y);

	/* Practically horizontal line segment, yet the end points have already
	 * been determined to be on different sides of the line. Therefore
	 * the line segment is part of the line and intersects everywhere.
	 * Return the end point, so we use the whole line segment.
	 */
	if (diff == 0.0f)
		return p2x;

	a = (y_arg - p2y) / diff;
	return p2x + (p1x - p2x) * a;
}

enum path_transition {
	PATH_TRANSITION_OUT_TO_OUT = 0,
	PATH_TRANSITION_OUT_TO_IN = 1,
	PATH_TRANSITION_IN_TO_OUT = 2,
	PATH_TRANSITION_IN_TO_IN = 3,
};

static void
clip_append_vertex(struct clip_context *ctx, GLfloat x, GLfloat y)
{
	*ctx->vertices.x++ = x;
	*ctx->vertices.y++ = y;
}

static enum path_transition
path_transition_left_edge(struct clip_context *ctx, GLfloat x, GLfloat y)
{
	return ((ctx->prev.x >= ctx->clip.x1) << 1) | (x >= ctx->clip.x1);
}

static enum path_transition
path_transition_right_edge(struct clip_context *ctx, GLfloat x, GLfloat y)
{
	return ((ctx->prev.x < ctx->clip.x2) << 1) | (x < ctx->clip.x2);
}

static enum path_transition
path_transition_top_edge(struct clip_context *ctx, GLfloat x, GLfloat y)
{
	return ((ctx->prev.y >= ctx->clip.y1) << 1) | (y >= ctx->clip.y1);
}

static enum path_transition
path_transition_bottom_edge(struct clip_context *ctx, GLfloat x, GLfloat y)
{
	return ((ctx->prev.y < ctx->clip.y2) << 1) | (y < ctx->clip.y2);
}

static void
clip_polygon_leftright(struct clip_context *ctx,
		       enum path_transition transition,
		       GLfloat x, GLfloat y, GLfloat clip_x)
{
	GLfloat yi;

	switch (transition) {
	case PATH_TRANSITION_IN_TO_IN:
		clip_append_vertex(ctx, x, y);
		break;
	case PATH_TRANSITION_IN_TO_OUT:
		yi = clip_intersect_y(ctx->prev.x, ctx->prev.y, x, y, clip_x);
		clip_append_vertex(ctx, clip_x, yi);
		break;
	case PATH_TRANSITION_OUT_TO_IN:
		yi = clip_intersect_y(ctx->prev.x, ctx->prev.y, x, y, clip_x);
		clip_append_vertex(ctx, clip_x, yi);
		clip_append_vertex(ctx, x, y);
		break;
	case PATH_TRANSITION_OUT_TO_OUT:
		/* nothing */
		break;
	default:
		assert(0 && "bad enum path_transition");
	}

	ctx->prev.x = x;
	ctx->prev.y = y;
}

static void
clip_polygon_topbottom(struct clip_context *ctx,
		       enum path_transition transition,
		       GLfloat x, GLfloat y, GLfloat clip_y)
{
	GLfloat xi;

	switch (transition) {
	case PATH_TRANSITION_IN_TO_IN:
		clip_append_vertex(ctx, x, y);
		break;
	case PATH_TRANSITION_IN_TO_OUT:
		xi = clip_intersect_x(ctx->prev.x, ctx->prev.y, x, y, clip_y);
		clip_append_vertex(ctx, xi, clip_y);
		break;
	case PATH_TRANSITION_OUT_TO_IN:
		xi = clip_intersect_x(ctx->prev.x, ctx->prev.y, x, y, clip_y);
		clip_append_vertex(ctx, xi, clip_y);
		clip_append_vertex(ctx, x, y);
		break;
	case PATH_TRANSITION_OUT_TO_OUT:
		/* nothing */
		break;
	default:
		assert(0 && "bad enum path_transition");
	}

	ctx->prev.x = x;
	ctx->prev.y = y;
}

static void
clip_context_prepare(struct clip_context *ctx, const struct polygon8 *src,
		      GLfloat *dst_x, GLfloat *dst_y)
{
	ctx->prev.x = src->x[src->n - 1];
	ctx->prev.y = src->y[src->n - 1];
	ctx->vertices.x = dst_x;
	ctx->vertices.y = dst_y;
}

static int
clip_polygon_left(struct clip_context *ctx, const struct polygon8 *src,
		  GLfloat *dst_x, GLfloat *dst_y)
{
	enum path_transition trans;
	int i;

	clip_context_prepare(ctx, src, dst_x, dst_y);
	for (i = 0; i < src->n; i++) {
		trans = path_transition_left_edge(ctx, src->x[i], src->y[i]);
		clip_polygon_leftright(ctx, trans, src->x[i], src->y[i],
				       ctx->clip.x1);
	}
	return ctx->vertices.x - dst_x;
}

static int
clip_polygon_right(struct clip_context *ctx, const struct polygon8 *src,
		   GLfloat *dst_x, GLfloat *dst_y)
{
	enum path_transition trans;
	int i;

	clip_context_prepare(ctx, src, dst_x, dst_y);
	for (i = 0; i < src->n; i++) {
		trans = path_transition_right_edge(ctx, src->x[i], src->y[i]);
		clip_polygon_leftright(ctx, trans, src->x[i], src->y[i],
				       ctx->clip.x2);
	}
	return ctx->vertices.x - dst_x;
}

static int
clip_polygon_top(struct clip_context *ctx, const struct polygon8 *src,
		 GLfloat *dst_x, GLfloat *dst_y)
{
	enum path_transition trans;
	int i;

	clip_context_prepare(ctx, src, dst_x, dst_y);
	for (i = 0; i < src->n; i++) {
		trans = path_transition_top_edge(ctx, src->x[i], src->y[i]);
		clip_polygon_topbottom(ctx, trans, src->x[i], src->y[i],
				       ctx->clip.y1);
	}
	return ctx->vertices.x - dst_x;
}

static int
clip_polygon_bottom(struct clip_context *ctx, const struct polygon8 *src,
		    GLfloat *dst_x, GLfloat *dst_y)
{
	enum path_transition trans;
	int i;

	clip_context_prepare(ctx, src, dst_x, dst_y);
	for (i = 0; i < src->n; i++) {
		trans = path_transition_bottom_edge(ctx, src->x[i], src->y[i]);
		clip_polygon_topbottom(ctx, trans, src->x[i], src->y[i],
				       ctx->clip.y2);
	}
	return ctx->vertices.x - dst_x;
}

#define max(a, b) (((a) > (b)) ? (a) : (b))
#define min(a, b) (((a) > (b)) ? (b) : (a))
#define clip(x, a, b)  min(max(x, a), b)

int
clip_simple(struct clip_context *ctx,
	    struct polygon8 *surf,
	    GLfloat *ex,
	    GLfloat *ey)
{
	int i;
	for (i = 0; i < surf->n; i++) {
		ex[i] = clip(surf->x[i], ctx->clip.x1, ctx->clip.x2);
		ey[i] = clip(surf->y[i], ctx->clip.y1, ctx->clip.y2);
	}
	return surf->n;
}

int
clip_transformed(struct clip_context *ctx,
		 struct polygon8 *surf,
		 GLfloat *ex,
		 GLfloat *ey)
{
	struct polygon8 polygon;
	int i, n;

	polygon.n = clip_polygon_left(ctx, surf, polygon.x, polygon.y);
	surf->n = clip_polygon_right(ctx, &polygon, surf->x, surf->y);
	polygon.n = clip_polygon_top(ctx, surf, polygon.x, polygon.y);
	surf->n = clip_polygon_bottom(ctx, &polygon, surf->x, surf->y);

	/* Get rid of duplicate vertices */
	ex[0] = surf->x[0];
	ey[0] = surf->y[0];
	n = 1;
	for (i = 1; i < surf->n; i++) {
		if (float_difference(ex[n - 1], surf->x[i]) == 0.0f &&
		    float_difference(ey[n - 1], surf->y[i]) == 0.0f)
			continue;
		ex[n] = surf->x[i];
		ey[n] = surf->y[i];
		n++;
	}
	if (float_difference(ex[n - 1], surf->x[0]) == 0.0f &&
	    float_difference(ey[n - 1], surf->y[0]) == 0.0f)
		n--;

	return n;
}