In batch conversion -> advanced, when you select both decreasing the color depth and changing brightness/contrast/gamma, the images will be converted to lower color depth earlier than the brightness/contrast/gamma changes are applied. Certainly needless to say, but this causes needless image quality loss, even a lot if the brightness/contrast/gamma changes are strong.
The same kind of issue may exists with other conversion options as well; I didn't tried it. The a rule of thumb is that if you will decrease the color depth then do it as the very last step, but if you will increase color depth then do it as the very first step.
(Yes, a workaround exists: doing the conversion in two steps. However most users will not realize from the look of the result that something went suboptimally, not mention realizing what exactly causes the poor quality.)
The same kind of issue may exists with other conversion options as well; I didn't tried it. The a rule of thumb is that if you will decrease the color depth then do it as the very last step, but if you will increase color depth then do it as the very first step.
(Yes, a workaround exists: doing the conversion in two steps. However most users will not realize from the look of the result that something went suboptimally, not mention realizing what exactly causes the poor quality.)
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