THotPDF.DecodeLoadedImageRegionAtResolution Method

Decodes a source-coordinate rectangle while allowing the decoder to select a lower resolution level that still satisfies the requested output size

Declaration

function DecodeLoadedImageRegionAtResolution(
  AImageStream: THPDFStreamObject;
  RegionLeft, RegionTop, RegionRight, RegionBottom,
  TargetPixelWidth, TargetPixelHeight: Integer): TBitmap;

Unit

HPDFDoc

Parameters

Name	Purpose
`AImageStream`	Loaded image XObject stream to decode
`RegionLeft`	Inclusive left source coordinate
`RegionTop`	Inclusive top source coordinate
`RegionRight`	Exclusive right source coordinate
`RegionBottom`	Exclusive bottom source coordinate
`TargetPixelWidth`	Minimum useful decoded width before final rendering scale
`TargetPixelHeight`	Minimum useful decoded height before final rendering scale

Return value

Returns a newly allocated bitmap for the clamped non-empty region, or nil when the image or requested geometry cannot be decoded

Resolution selection

JPEG 2000 selects the greatest power-of-two reduction whose decoded width and height both remain at least the requested pixel size
The selection never under-resolves either dimension and therefore avoids an unnecessary upscale before drawing
Native region selection and resolution selection are combined before wavelet reconstruction and pixel expansion
Crash-isolated JPEG 2000 workers receive the same reduction request and use a correspondingly smaller output budget
When native fallback handles a region that intersects multiple codestream tiles, independent tiles decode concurrently within cancellation, worker-count, output, and peak-memory limits
Parallel results are composed in stable row-major order and insufficient working budget transparently selects the sequential region path
A JPEG image in DeviceRGB or DeviceGray goes through a reduced-size DCT decoder when the region or the requested size differs from the full image: it decodes at a reduced DCT scale that still covers the request and returns the region resampled to exactly the requested size
Other supported image formats, and JPEG images in other color spaces, preserve exact region decoding and leave any final scaling to the caller
The caller owns a non-nil returned bitmap and must free it

Example

Preview := PDF.DecodeLoadedImageRegionAtResolution(
  ImageStream, 0, 0, SourceWidth, SourceHeight, 320, 200);
try
  Canvas.StretchDraw(TargetRect, Preview);
finally
  Preview.Free;
end;