Monochrome two-dimensional barcodes are rapidly becoming a de-facto standard for distributing digital data through printed medium because of their small cost and portability. Increasing the data density of these barcodes improves the flexibility and effectiveness of existing applications of barcodes, and has the potential to create novel means for data transmission and conservation. However, printing and scanning equipment introduce uncontrollable effects on image at very refined level, and the effects are beyond the scope of the error control mechanisms of existing barcode schemes. To realize high-density barcodes, it is essential to develop novel symbology and error control mechanisms which can manage these kinds of effects and provide practical reliability. To tackle this problem, this paper studies the communication channel defined by high-density barcodes, and proposes several error control techniques to increase the robustness of the barcode scheme. Some of these techniques convert the peculiar behavior of printing equipment to the well-studied model of additive white Gaussian (AWGN) channel. The use of low-density parity codes is also investigated, as they perform much better than conventional Reed-Solomon codes especially for AWGN channels. Through experimental evaluation, it is shown that the proposed error control techniques can be essential components in realizing high-density barcodes.