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I have placed a greyscale TIF on a spread. When I place another PNG image on the same spread, the brightness of the previously placed graphic appears to change. The graphic appears more bright/less dark (see below). Pay attention to the graphic on the right and watch how it brightens up once the PNG is placed.
My questions are: Does anyone know why this is happening / what is happening? Is this only effecting the way things appear within InDesign or is it going effect the print?
InDesign shows a low-quality preview on screen, to speed up performance. It's not intended to show a pixel-perfect print preview.
Use 'View → Display Performance → High Quality Display' if you want to view a high quality preview.
It comes from how the transparency is flattened. Have a single pixel of transparency (as an effec or photo) on the spread or the pasteboard, the flattening is needed. Grayscale TIFs get lighter if transparency flattener's colorspace is CMYK. I assume due the PNGs that you compose for onscreen. Select the transparency flattener to work in RGB (Edit > Transparency Blend Space > Document RGB)
If you compose for printing, then selecting RGB surely causes something unwanted. I do not know what and why. Hopefully someone that has true underhood knowledge about the internal color mixing and the handling of black and transparency, makes a better answer.
It also happens in my copy of ID, so it must be the way it behaves when placing different formats. Probably a preview issue. Never had any issues with PDF exports however.
