GLSL角色描边Shader出现异常红线问题求助

我正在制作一款GLSL着色器，目标是为角色渲染红色描边。该着色器可正常运行，但在部分特定角色上会出现不明原因的红色线条，这些线条附近并无实际颜色，我无法解决此问题。

片段着色器代码 #

uniform mat4 u_Color;
varying vec2 v_TexCoord;
varying vec2 v_TexCoord2;
uniform sampler2D u_Tex0;

const float ALPHA_TOLERANCE = 0.01;

void main()
{
    vec4 baseColor = texture2D(u_Tex0, v_TexCoord);
    vec4 texcolor = texture2D(u_Tex0, v_TexCoord2);
    if(texcolor.r > 0.9) {
        baseColor *= texcolor.g > 0.9 ? u_Color[0] : u_Color[1];
    } else if(texcolor.g > 0.9) {
        baseColor *= u_Color[2];
    } else if(texcolor.b > 0.9) {
        baseColor *= u_Color[3];
    }

    vec4 pixel1 = texture2D(u_Tex0, vec2(v_TexCoord.x + 0.001, v_TexCoord.y));
    vec4 pixel2 = texture2D(u_Tex0, vec2(v_TexCoord.x - 0.001, v_TexCoord.y));
    vec4 pixel3 = texture2D(u_Tex0, vec2(v_TexCoord.x, v_TexCoord.y + 0.001));
    vec4 pixel4 = texture2D(u_Tex0, vec2(v_TexCoord.x, v_TexCoord.y - 0.001));

    bool neighbourColor = pixel4.a > ALPHA_TOLERANCE || pixel3.a > ALPHA_TOLERANCE || pixel2.a > ALPHA_TOLERANCE || pixel1.a > ALPHA_TOLERANCE;

    if (baseColor.a < ALPHA_TOLERANCE && neighbourColor) {
        baseColor.rgb = vec3(1.0, 0.0, 0.0);
        baseColor.a = 0.7;
    }
    
    gl_FragColor = baseColor;
    if(gl_FragColor.a < 0.01) discard;
}

顶点着色器代码 #

attribute vec2 a_Vertex;
attribute vec2 a_TexCoord;
uniform mat3 u_TextureMatrix;
varying vec2 v_TexCoord;
varying vec2 v_TexCoord2;
uniform mat3 u_TransformMatrix;
uniform mat3 u_ProjectionMatrix;
uniform vec2 u_Offset;

void main()
{
    gl_Position = vec4((u_ProjectionMatrix * u_TransformMatrix * vec3(a_Vertex.xy, 1.0)).xy, 1.0, 1.0);
    v_TexCoord = (u_TextureMatrix * vec3(a_TexCoord,1.0)).xy;
    v_TexCoord2 = (u_TextureMatrix * vec3(a_TexCoord + u_Offset,1.0)).xy;
}

着色器工作逻辑：检测当前像素是否透明，若透明则检查相邻像素是否有实际颜色，若是则将当前像素替换为红色。但部分角色上出现了本不该存在的红色线条，这些线条附近并无实际颜色。

内容的提问来源于stack exchange，提问作者Lunia