lt bits to store into dst_buf
var result: u32 = 0u;
// Storing snorm8 texel values
// later called by pack4x8xnorm to convert to u32.
var v: vec4<f32>;

// dstBuf value is used for starting part.
var mask: u32 = 0xffffffffu;
if (!readDstBufAtStart) {
    // coordS is used
    mask &= 0xffff0000u;
    let texel0 = textureLoadGeneral(src_tex, coordS, params.mipLevel).rg;
    v[0] = texel0.r;
    v[1] = texel0.g;
}

if (coordE.x < srcBoundary.x) {
    // coordE is used
    mask &= 0x0000ffffu;
    let texel1 = textureLoadGeneral(src_tex, coordE, params.mipLevel).rg;
    v[2] = texel1.r;
    v[3] = texel1.g;
}

if (readDstBufAtStart || readDstBufAtEnd) {
    let original: u32 = dst_buf[dstOffset];
    result = (original & mask) | (encodeVectorInU32General(v) & ~mask);
} else {
    result = encodeVectorInU32General(v);
}