llvmpipe interpolation problem

precision highp float;
uniform highp sampler2D colorTex_; //16x16 8bit texture with values in [0,3] (lowest two bits). Using linear filtering.
in vec2 frag_texcoord; //in [0,1] for each dimension, generated from full screen quad vertices
out vec4 FragData0; // 500x500 output

void main() {
    vec4 v = vec4(texture2D(colorTex_, frag_texcoord).x, vec2(0.0), 1.0);

    float umax = 255;
    float uthreshold = 1;
    if(v > 1.0/255) {
        FragData0 = vec4(1.0, 1.0, 1.0, 1.0);
    } else {
        FragData0 = vec4(0.0, 0.0, 0.0, 1.0);
    }
}