jetpack compose blendMode or PorterDuff.Mode DST_IN

Question

I have a BLURRED background Bitmap and a horizontal scrollable row. What I want to achieve is that the background image must be visible only in the row item area...

I've tried to apply DST_IN with blendMode but no success. Any ideas how to do it in jetpack compose?

What I'm trying to achieve is this

I want to have a blurred version of the background Image only visible though the row items, so that it will seem like a gloss mirror background on row items when scrolled

Answer 1

You either need to set alpha something less than 1f

    Canvas(
        modifier = Modifier
            .fillMaxSize()
            // Provide a slight opacity to for compositing into an
            // offscreen buffer to ensure blend modes are applied to empty pixel information
            // By default any alpha != 1.0f will use a compositing layer by default
            .graphicsLayer(alpha = 0.99f)
    ) {


        val dimension = (size.height.coerceAtMost(size.width) / 2f).toInt()

        drawImage(
            image = imageBitmapDst,
            dstSize = IntSize(dimension, dimension)
        )
        drawImage(
            image = imageBitmapSrc,
            dstSize = IntSize(dimension, dimension),
            blendMode = BlendMode.DstIn
        )
    }
}

or use a layer inside canvas

 with(drawContext.canvas.nativeCanvas) {

    val checkPoint = saveLayer(null, null)

// Destination
    drawImage(
        image = dstImage,
        srcSize = IntSize(canvasWidth / 2, canvasHeight / 2),
        dstSize = IntSize(canvasWidth, canvasHeight),
    )

    // Source
    drawImage(
        image = srcImage,
        srcSize = IntSize(canvasWidth / 2, canvasHeight / 2),
        dstSize = IntSize(canvasWidth, canvasHeight),
        blendMode = BlendMode.DstIn

    )
    restoreToCount(checkPoint)
}

You can check this tutorial for more about Blend modes.

Setting alpha less than 1f might look like a hack but Image's Painter source code uses it too to decide whether to create layer or not

private fun configureAlpha(alpha: Float) {
    if (this.alpha != alpha) {
        val consumed = applyAlpha(alpha)
        if (!consumed) {
            if (alpha == DefaultAlpha) {
                // Only update the paint parameter if we had it allocated before
                layerPaint?.alpha = alpha
                useLayer = false
            } else {
                obtainPaint().alpha = alpha
                useLayer = true
            }
        }
        this.alpha = alpha
    }
}

And creates a layer as for Image

    fun DrawScope.draw(
        size: Size,
        alpha: Float = DefaultAlpha,
        colorFilter: ColorFilter? = null
    ) {
        configureAlpha(alpha)
        configureColorFilter(colorFilter)
        configureLayoutDirection(layoutDirection)

        // b/156512437 to expose saveLayer on DrawScope
        inset(
            left = 0.0f,
            top = 0.0f,
            right = this.size.width - size.width,
            bottom = this.size.height - size.height
        ) {

            if (alpha > 0.0f && size.width > 0 && size.height > 0) {
                if (useLayer) {
                    val layerRect = Rect(Offset.Zero, Size(size.width, size.height))
                    // TODO (b/154550724) njawad replace with RenderNode/Layer API usage
                    drawIntoCanvas { canvas ->
                        canvas.withSaveLayer(layerRect, obtainPaint()) {
                            onDraw()
                        }
                    }
                } else {
                    onDraw()
                }
            }
        }
    }
}