Bug: fix for bitmap too large crash

app/design-system/design-system-hedvig/src/androidMain/kotlin/com/hedvig/android/design/system/hedvig/pdfrenderer/PdfDecoder.kt

@@ -1,6 +1,6 @@
 package com.hedvig.android.design.system.hedvig.pdfrenderer
 
-import android.graphics.Bitmap
+import android.graphics.Color
 import android.graphics.pdf.PdfRenderer
 import android.os.ParcelFileDescriptor
 import androidx.core.graphics.createBitmap
@@ -12,30 +12,52 @@ import coil3.decode.Decoder
 import coil3.decode.ImageSource
 import coil3.fetch.SourceFetchResult
 import coil3.request.Options
+import kotlin.math.roundToInt
+import kotlin.math.sqrt
 
 class PdfDecoder(
   private val source: ImageSource,
   private val options: Options,
 ) : Decoder {
   override suspend fun decode(): DecodeResult {
-    val context = options.context
-    val pdfRenderer = PdfRenderer(
-      ParcelFileDescriptor.open(
-        source.file().toFile(),
-        ParcelFileDescriptor.MODE_READ_ONLY,
-      ),
+    val fileDescriptor = ParcelFileDescriptor.open(
+      source.file().toFile(),
+      ParcelFileDescriptor.MODE_READ_ONLY,
     )
-    val page = pdfRenderer.openPage(0)
-
-    val bitmap = createBitmap(page.width * 2, page.height * 2)
-    page.render(bitmap, null, null, PdfRenderer.Page.RENDER_MODE_FOR_DISPLAY)
-    page.close()
-    pdfRenderer.close()
+    return fileDescriptor.use { descriptor ->
+      PdfRenderer(descriptor).use { renderer ->
+        renderer.openPage(0).use { page ->
+          val scale = page.renderScale()
+          val bitmap = createBitmap(
+            width = (page.width * scale).roundToInt().coerceAtLeast(1),
+            height = (page.height * scale).roundToInt().coerceAtLeast(1),
+          )
+          // `render` blends onto the destination, so unpainted and transparent regions of the page keep the bitmap's
+          // initial fully transparent pixels unless the background is filled in first.
+          bitmap.eraseColor(Color.WHITE)
+          page.render(bitmap, null, null, PdfRenderer.Page.RENDER_MODE_FOR_DISPLAY)
+          // Rendering blends onto the opaque white fill, leaving every pixel opaque. Declaring that lets consumers
+          // skip compositing the result onto a background of their own.
+          bitmap.setHasAlpha(false)
+          DecodeResult(
+            image = bitmap.toDrawable(options.context.resources).asImage(),
+            isSampled = scale < RENDER_SCALE,
+          )
+        }
+      }
+    }
+  }
 
-    return DecodeResult(
-      image = bitmap.toDrawable(context.resources).asImage(),
-      isSampled = false,
-    )
+  /**
+   * The factor to scale the page's native point size by when rendering it into a bitmap.
+   *
+   * [RENDER_SCALE] normally, lowered for pages whose box is large enough that rendering at that scale would exceed
+   * [MAX_BITMAP_BYTES]. A bitmap above the hardware canvas' 100 MB limit throws from `Canvas#drawBitmap` during the
+   * draw pass, which no amount of error handling around the image request can recover from.
+   */
+  private fun PdfRenderer.Page.renderScale(): Double {
+    val bytesAtNativeSize = width.toDouble() * height * BYTES_PER_PIXEL
+    return RENDER_SCALE.coerceAtMost(sqrt(MAX_BITMAP_BYTES / bytesAtNativeSize))
   }
 
   class Factory : Decoder.Factory {
@@ -49,5 +71,8 @@ class PdfDecoder(
 
   companion object {
     private const val MIME_TYPE_PDF = "application/pdf"
+    private const val RENDER_SCALE = 2.0
+    private const val BYTES_PER_PIXEL = 4
+    private const val MAX_BITMAP_BYTES = 32L * 1024 * 1024
   }
 }

app/feature/feature-image-viewer/src/main/kotlin/com/hedvig/android/feature/imageviewer/ImageViewerDestination.kt

@@ -135,6 +135,9 @@ class WhiteBackgroundTransformation : Transformation() {
   override val cacheKey = "white_background"
 
   override suspend fun transform(input: Bitmap, size: Size): Bitmap {
+    // Without an alpha channel there is no transparency for the white background to show through, so compositing
+    // would allocate a second full-size copy of a visually identical image.
+    if (!input.hasAlpha()) return input
     val output = createBitmap(input.width, input.height)
     val canvas = AndroidCanvas(output)
     canvas.drawColor(AndroidColor.WHITE)