Is there any way to get a subimage indicating the position a

Question

Is there any way to get a subimage indicating the position and dimension from a png file to display in a Composable Image

Dima Avdeev · Answer

Yes ```Image painter = BitmapPainter image = TODO your image srcOffset = IntOffset 100 100 srcSize = IntSize 200 200 contentDescription = null ```

Dima Avdeev · Answer

In this case full png file will be loaded

Pedro Pereira · Answer

OK Thanks And can we only load the part of the image

Dima Avdeev · Answer

I think it depends on image format But I don t know how to do it with png It is not Compose related issues You need to read part of the file on Kotlin JVM level or event lower with native calls Convert part of the file to ImageBitmap It may be hard Or maybe it is not possible with png

Pedro Pereira · Answer

Thanks

ephemient · Answer

I don t believe it is possible to decode only part of a PNG the format puts the compressed pixels in a single compressed stream except in the rare case of interlacing and that doesn t help you and the filtering means each pixel depends on the pixels that came before it

ephemient · Answer

doable for JPEG though

Pedro Pereira · Answer

OK It makes sense Thanks

Pedro Pereira · Answer

I m developing a game and I have a png file with the sprites I want to display It should be possible to load the entire image into an ImageBitmap and then extract each sprite from the image to display Will I need to use specific JVM operations to get each part of the ImageBitmap Or are there desktop compose functions to do this

saket · Answer

gt I don t believe it is possible to decode only part of a PNG are you sure about this android s `BitmapRegionDecoder` is capable of decoding regions of pngs

ephemient · Answer

that works by decoding the whole bitmap

saket · Answer

do you mean by loading the whole bitmap in memory

ephemient · Answer

note how `SkCodec onDimensionsSupported` defaults to `false` and doesn t override it whereas does

ephemient · Answer

yes

saket · Answer

hmmm I don t think I understand wouldn t loading whole images in memory trigger OOMs for large images

ephemient · Answer

yes

ephemient · Answer

JPEG is block based so it is possible to decode only specific blocks but PNG is not

saket · Answer

I think I m missing something here because that sounds incorrect considering I am able to load regions of a gigantic PNG using android s `BitmapRegionDecoder` without triggering OOMs

saket · Answer

I m writing a compose first library for displaying zoomable images so I d like to learn more about this blob smile

ephemient · Answer

try loading the whole bitmap and see if you OOM

saket · Answer

no I meant I m able to load certain regions of PNGs without decoding the entire image I was responding to this message gt I don t believe it is possible to decode only part of a PNG

ephemient · Answer

yes the Android PNG decoder decodes the entire image then returns a subset of it

ephemient · Answer

you can follow the code yourself

saket · Answer

does the term decode here mean something other than loading the bitmap in memory

ephemient · Answer

no

ephemient · Answer

although it s all in native off heap memory

saket · Answer

I see So that doesn t count against an app s memory

ephemient · Answer

it does count against an app s memory usage but is invisible to GC