Downsampling PNG with SkCodec (or anything else)

[Matthew Leibowitz]

I just wanted to confirm whether or not downsampling a PNG during the decode is supported. I tried a jpg and webp, and those work. I also tested on the native Android, and PNG doesn't downsample.

This most probably is a limitation of the codec, since that is what is loading the bitmap.

If this is not supported, what would be the recommended way to load a huge, but downsampled, PNG into memory? I am thinking of a mobile device that just downloaded a huge image. If I want to generate a thumbnail, what would I do? Sure I can load it into memory and then resize, but there is the problem of limited memory...

Waiting on the experts,

Matthew

[Matt Sarett]

SkCodec supports scaled jpeg and webp decodes because the native decoding libraries (libjpeg-turbo, libwebp) support downscaling.  Ex: the jpeg library can perform a faster decode when downscaling because it will drop some of the DCT coefficients.  There's no similar option for png.

However, SkAndroidCodec (wraps SkCodec) allows for decoding any format with a fSampleSize specified in AndroidOptions.  fSampleSize=N instructs the codec to sample every Nth pixel as it decodes.  This should do what you want.

https://github.com/google/skia/blob/master/include/codec/SkAndroidCodec.h

Best,

Matt

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