iOS GPUImage 类似分屏模糊

Question

我想使用GPUImage实现如下效果：

正常视频在中间播放，视频背景为正常视频缩放后高斯处理后的视频同时播放。

但是我使用 GPUImage 处理后，发现效果很差（清晰度明显降低，琴弦和袖口有明显模糊）。
滤镜链为：

standardMovie -> customFilter -> customFilter_atTextureLocation:1
backgroudMovie -> cropFilter -> gaussianBlurFilter -> customFilter -> customFilter_atTextureLocation:0
customFilter -> GPUImageView

customFilter 片元着色器：

precision highp float;
varying vec2 textureCoordinate;
uniform float blurOffsetY;
uniform float blurTextureScal;
uniform sampler2D inputImageTexture;
uniform sampler2D inputImageTexture2;

void main() {
    highp vec2 uv = textureCoordinate;
    vec4 outPutcolor = vec4(0.0,0.0,0.0,1.0);
    if (uv.y >= blurOffsetY && uv.y <= 1.0 - blurOffsetY) {
        outPutcolor = texture2D(inputImageTexture2, vec2(uv.x , (uv.y - blurOffsetY)/(1.0 - 2.0*blurOffsetY)));
    }
    else {
        vec2 center = vec2(0.5, 0.5);
        uv = uv -  center;
        uv = uv / blurTextureScal;
        uv = uv + center;
        outPutcolor = texture2D(inputImageTexture, textureCoordinate);
    }
    gl_FragColor = outPutcolor;
}

刚接触 OpenGL ，求帮助