上代码

原理很简单，将图片的透明度数据偏移一周来达到描边度效果，我想，通过mat应该能更快得到描边效果，另外由于此图的图片边缘紧挨靠边界，导致透明度出错，这个可以在前期图片上预留透明空间来避免这个问题，这里我缩小了原图，但效果不太好

shader_type canvas_item;

uniform int SAMPLES = 6;
uniform float WIDTH = 4.0;
uniform vec4 OUTLINE_COLOR = vec4(0.,0.,0.,1.);

uniform vec2 TEXTURE_SCALE = vec2(0.94);

void fragment(){
    vec2 uv_offset = (1.-TEXTURE_SCALE)*.5;
    vec2 uv = UV/TEXTURE_SCALE-uv_offset;
    
    vec4 cc = texture(TEXTURE,uv);
    if(UV.x<uv_offset.x || UV.x>1.-uv_offset.x){
        cc.a = 0.;
    }
    if(UV.y<uv_offset.y || UV.y>1.-uv_offset.y){
        cc.a = 0.;
    }
    //OUTLINE
    float outlineAlpha = 0.0;
    float angle = 0.0;
    float unit_angle = 1.0/float(SAMPLES) * 3.1415*2.0;
    for( int i=0; i<SAMPLES; i++ ){
        angle += unit_angle;
        vec2 testPoint = vec2( (WIDTH*TEXTURE_PIXEL_SIZE.x)*cos(angle), (WIDTH*TEXTURE_PIXEL_SIZE.y)*sin(angle) );
        testPoint = clamp( testPoint+uv, vec2(0.), vec2(1.));
        float sampledAlpha = texture( TEXTURE,  testPoint ).a;
        outlineAlpha = max( outlineAlpha, sampledAlpha );
    }
    COLOR = mix( vec4(0.0), OUTLINE_COLOR, outlineAlpha );
    COLOR = mix( COLOR, cc, cc.a );
}

参考：

单色描边
https://www.shadertoy.com/view/XdV3Dc
发光描边
https://www.shadertoy.com/view/XlsXRB