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f04c457a51
Counter-intuitively, defining the PNG_DEBUG macro as 1 (i.e. setting the debug verbosity level to 1) does *not* cause any of the existing debug traces to show up. This setting would have worked if we had trace statements like `png_debug(0, message)`, but we don't have any. Debug traces do show up for verbosity levels from 2 to 5. The libpng manual describes in detail how they are meant to be used. According to the history of the CMake file, as well as the history of other project files, PNG_DEBUG was never set to a numeric value larger than 1. In other words, these debug settings inside these build files never produced any output in any (unmodified) libpng version. (As for the configure build, PNG_DEBUG has no special treatment there.) Considering the plethora of alternative methods to pass C preprocessor options through our build files, scripts and projects onto libpng, we'd rather discontinue PNG_DEBUG as a dedicated build option.