Greyscale png with transparency

[Chops211]

I have an image I create that is normalized to 8-bit (0->255) stored as quint8*. I pass this to QImage and it works well. Is there anyway to set the black pixels to transparent without using libpng to set the tRNS?

[Santosh Reddy]

Convert the QImage to one of the formats which support alpha channel (example QImage::Format_ARGB32) , then set the black color pixels with alpha color.

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QImage::convertToFormat();
QImage:: pixelColor();
QImage::setPixelColor();

[Chops211]

So after I make the QImage in greyscale, convert to ARGB, set the pixels to transparent, and then back to greyscale? There isn't a more efficient method?

I have that working properly but only when the image is saved as ARGB. I want to keep it in Format_Grayscale8 when creating the PNG. No reason to use 4x the memory.

Added after 8 minutes:

If anyone wants the solution, none of this is necessary. If you want a value to be transparent in an 8-bit grayscale QImage that will be saved to a PNG, there is not anyway to do that inheritely in Qt it seems. You need to manually add the tRNS header and value to the PNG header.