概念
- MiterLimit：尖角长度 / 线宽 的比值上限，超出则自动退化为 Bevel。
- 默认值 4.0：SVG 标准默认值，适合大多数场景。
- NonScaling：启用后描边宽度保持像素恒定，不随 Shape 缩放变换改变。
API 速查
| 函数 | 说明 |
|---|---|
int xgeShapeExStrokeMiterLimit(xge_shape_ex pShape, float fLimit) | 设置斜接限制 |
int xgeShapeExStrokeMiterLimitGet(xge_shape_ex pShape, float* pLimit) | 获取斜接限制 |
int xgeShapeExStrokeNonScaling(xge_shape_ex pShape, int bNonScaling) | 设置非缩放描边 |
int xgeShapeExStrokeNonScalingGet(xge_shape_ex pShape, int* pNonScaling) | 获取非缩放状态 |
编译本章示例
gcc -O2 -I. -o ch48.exe ch48_main.c build\xge.lib -lm -lws2_32 -liphlpapi -lgdi32 -luser32 -lshell32 -lopengl32 -lole32 -lwinmm -lavrt
.\ch48.exe示例 1：MiterLimit 对比
// 锐角三角形 — 观察不同 MiterLimit 效果
float limits[] = { 1.0f, 4.0f, 20.0f };
for (int i = 0; i < 3; i++) {
xge_shape_ex s = NULL;
xgeShapeExCreate(&s);
// 尖锐三角形
xgeShapeExMoveTo(s, 100+i*250, 400);
xgeShapeExLineTo(s, 200+i*250, 100);
xgeShapeExLineTo(s, 300+i*250, 400);
xgeShapeExClose(s);
xgeShapeExStrokeColor(s, 0xFFB454FF);
xgeShapeExStrokeWidth(s, 8.0f);
xgeShapeExStrokeJoin(s, XGE_SHAPE_EX_JOIN_MITER);
xgeShapeExStrokeMiterLimit(s, limits[i]);
xgeShapeExDraw(s, 0.25f);
xgeShapeExDestroy(s);
}示例 2：NonScaling 描边
xge_shape_ex shape = NULL;
xgeShapeExCreate(&shape);
xgeShapeExAppendRect(shape, -50,-50, 100,100, 0,0, 1);
xgeShapeExStrokeColor(shape, 0x4FD8C2FF);
xgeShapeExStrokeWidth(shape, 2.0f);
// 启用非缩放 → 放大 3 倍后描边仍为 2px
xgeShapeExStrokeNonScaling(shape, 1);
xgeShapeExTransformScale(shape, 3.0f, 3.0f);
xgeShapeExTransformTranslate(shape, 400, 300);
xgeShapeExDraw(shape, 0.25f);完整代码
将以下代码保存为 ch48_main.c，编译后运行即可看到 MiterLimit 与 NonScaling 描边效果：
#include "xge.h"
static int onFrame(void* pUser) {
(void)pUser;
xgeClear(0x081114FF);
// MiterLimit 对比：三个尖锐三角形
float limits[] = { 1.0f, 4.0f, 20.0f };
for (int i = 0; i < 3; i++) {
xge_shape_ex s = NULL;
xgeShapeExCreate(&s);
xgeShapeExMoveTo(s, 60+i*250, 250);
xgeShapeExLineTo(s, 160+i*250, 50);
xgeShapeExLineTo(s, 260+i*250, 250);
xgeShapeExClose(s);
xgeShapeExStrokeColor(s, 0xFFB454FF);
xgeShapeExStrokeWidth(s, 8.0f);
xgeShapeExStrokeJoin(s, XGE_SHAPE_EX_JOIN_MITER);
xgeShapeExStrokeMiterLimit(s, limits[i]);
xgeShapeExDraw(s, 0.25f);
xgeShapeExDestroy(s);
}
// NonScaling 描边：放大 3 倍后描边仍为 2px
xge_shape_ex ns = NULL;
xgeShapeExCreate(&ns);
xgeShapeExAppendRect(ns, -50,-50, 100,100, 0,0, 1);
xgeShapeExStrokeColor(ns, 0x4FD8C2FF);
xgeShapeExStrokeWidth(ns, 2.0f);
xgeShapeExStrokeNonScaling(ns, 1);
xgeShapeExTransformScale(ns, 3.0f, 3.0f);
xgeShapeExTransformTranslate(ns, 400, 430);
xgeShapeExDraw(ns, 0.25f);
xgeShapeExDestroy(ns);
return XGE_OK;
}
int main(void) {
xge_desc_t d = {0};
d.iWidth = 800;
d.iHeight = 600;
d.sTitle = "ch48 - MiterLimit & NonScaling";
d.iFlags = XGE_INIT_VSYNC;
xgeInit(&d);
xgeRun(onFrame, NULL);
xgeUnit();
return 0;
}代码讲解
- MiterLimit：当尖角长度/宽度比超过 Limit 时，自动退化为 Bevel，防止尖刺。
- Limit=1.0：几乎所有尖角都退化；Limit=20 则保留尖锐效果。
- NonScaling：启用后描边宽度不受图形缩放影响，始终为设定像素值。
- 典型场景：地图缩放时保持边界线宽度恒定。
要点与陷阱
- 仅 Miter Join 有效：Round/Bevel 连接不受 MiterLimit 影响。
- Limit = 1.0：几乎所有尖角都退化为 Bevel。
- NonScaling 典型场景：地图缩放时保持边界线宽度恒定。
- 性能：NonScaling 需要额外计算逆变换，大量图形时注意开销。
进阶练习
- 绘制星形，逐步降低 MiterLimit 观察尖角退化过程。
- 实现可缩放画布：NonScaling 描边 + 动态 Scale 变换。
