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ResizeImageFileToStream

Function ResizeImageFileToStream(
  const aOriginalImageFile: string;
  out aNewImageStream: TStream;
  const NewWidth, NewHeight, aQuality: integer): boolean

Example​

procedure ScriptEvent(var Value: variant);
var
  Output: TMemoryStream;
  Succeeded: Boolean;
begin
  Output := TMemoryStream.Create;
  try
    Succeeded := ResizeImageFileToStream(
      'C:\VeloxData\Examples\source.png', Output,
      1200, 1200, 70);
    if Succeeded then
      Output.Position := 0;
    Value := Succeeded;
  finally
    Output.Free;
  end;
end;

Usage​

ResizeImageFileToStream fits a JPEG or PNG file proportionally inside a requested box and writes the source format to a caller-owned stream.

Parameters

NameTypeDescription
aOriginalImageFilestring, constExisting JPEG/PNG selected by .jpg, .jpeg or .png suffix.
aNewImageStreamTStream, outExisting writable destination; never created, assigned, cleared or freed by Velox.
NewWidthinteger, constPositive caller-validated bounding width.
NewHeightinteger, constPositive caller-validated bounding height.
aQualityinteger, constDirect JPEG quality or defective mapped PNG level; unrelated to resampling quality.

Returns

False when the source file is absent, destination is nil or source extension is unsupported. True after SaveToStream. Invalid dimensions, malformed content, encoding and stream write failures raise.

Format behavior

Stream output format matches source extension because no destination extension exists. JPEG stays JPEG; PNG stays PNG. Use a file-output resize function or another governed conversion path when the output format must change.

Errors

Decoder, dimension, GDI, quality, encoder, stream and memory errors propagate. No rollback of destination bytes.

Usage notes

Use a new empty destination stream where possible. Validate source suffix/content, positive bounded dimensions and format-specific quality. Do not expect out allocation or metadata/alpha preservation.

Additional Technical Info​

ResizeImageFileToStream loads JPEG/PNG selected by source extension, proportionally fits it within a bounding box, and writes the same source format into an already existing destination stream. It uses a 32-bit bitmap and GDI HALFTONE StretchDraw and can upscale.

The out TStream parameter is misleading: the caller must construct and own it. The example is fictional/source-reviewed only; no image function was executed.

Implementation trace

  1. Reject absent source/nil destination.
  2. Allocate source/target bitmaps and initialize graphic pointers nil.
  3. Create JPEG/PNG from source extension and load it.
  4. Convert to pf32bit DIB source bitmap (DIBNeeded first for JPEG).
  5. Calculate Aspect := Min(NewWidth / sourceWidth, NewHeight / sourceHeight) and rounded proportional target size.
  6. Stretch with GDI HALFTONE over the whole target bitmap.
  7. Create a fresh encoder of the source format. JPEG is non-progressive; PNG receives MapTo10(aQuality).
  8. Save at destination's current position, return true, free only temporaries.

out stream defect and ownership

The body reads and uses the existing aNewImageStream; it never returns a new object through the out slot. Modified PascalScript passes the current variable by address, so the caller must instantiate it first. Treat it as var and retain responsibility for freeing it on false, success or exception.

Destination position and stale bytes

Velox does not set Position := 0 or Size := 0:

  • writes begin at current position;
  • existing prefix bytes remain;
  • a shorter result can leave a stale tail from prior content;
  • success leaves position after encoded bytes;
  • exceptions can leave partial bytes;
  • callers reusing a memory/file stream should explicitly rewind/truncate before entry and rewind after success.

Dimensions and scaling

Scale is the smaller of width/source-width and height/source-height. Output fits inside the box while retaining aspect ratio; it is not padded/cropped to exact box dimensions. Both larger limits cause upscaling. Integer Round decides final sides.

There is no check for zero, negative, overflow-prone or resource-exhausting values. A tiny scale can round a side to zero. Validate dimensions and maximum target pixels before calling.

Quality and PNG defect

JPEG receives direct quality 1..100 and is forced non-progressive; current range-checked builds raise ERangeError outside that range. PNG MapTo10 maps every input 90 or greater to invalid level 10 for the installed 0..9 property, so current builds raise ERangeError; level 0 is unreachable. Do not treat the shared parameter as equivalent visual quality between formats.

Fidelity and metadata

Fresh bitmap/encoder creation discards the original encoded representation and provides no contract to preserve EXIF/IPTC/XMP, orientation, color profiles or PNG ancillary chunks. GDI bitmap stretch has no explicit alpha-aware interpolation; PNG transparency can be lost or altered. Validate rendered output in the future image test bed before relying on transparency fidelity.

Side effects

Reads/decodes the source file and mutates destination stream at its current position. Caller stream remains open/owned by caller.

Performance, limits and concurrency

Multiple full decoded/bitmap representations coexist; compressed source size is not a safe memory estimate. No pixel/dimension limits or VCL/GDI synchronization are provided.

Related entries

External references

Created 2026-07-15