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ResizeImageStreamToFile

Function ResizeImageStreamToFile(
  const aOriginalImageStream: TStream;
  const aNewImageFile: string;
  const NewWidth, NewHeight, aQuality: integer): boolean

Example​

procedure ScriptEvent(var Value: variant);
var
  Source: TMemoryStream;
begin
  Source := TMemoryStream.Create;
  try
    Source.LoadFromFile('C:\VeloxData\Examples\source.png');
    Source.Position := 0;
    Value := ResizeImageStreamToFile(
      Source, 'C:\VeloxData\Examples\resized.jpg',
      1200, 1200, 80);
  finally
    Source.Free;
  end;
end;

Usage​

ResizeImageStreamToFile detects JPEG or PNG in a stream, proportionally fits it inside a box and writes the format selected by destination extension.

Parameters

NameTypeDescription
aOriginalImageStreamTStream, constNon-nil, nonempty, readable/seekable source; detection/loading starts at current position. Caller owns it.
aNewImageFilestring, const.jpg/.jpeg or .png destination selecting encoder; directories may be created and existing file deleted.
NewWidthinteger, constPositive, bounded width of the fit box; not validated.
NewHeightinteger, constPositive, bounded height of the fit box; not validated.
aQualityinteger, constDirect JPEG quality or defective mapped PNG compression level.

Returns

False for nil/zero-size source, failed CheckFolder, unrecognized source signature or unrecognized destination extension. True after saving. Other failures raise.

Errors

Seek/decode/dimension/GDI/quality/encode/path/write/memory errors propagate. False covers recognized guards only.

Usage notes

Set source position explicitly, validate remaining bytes/dimensions/output suffix/quality, enforce pixel limits, normalize orientation/transparency where required and save to a temporary path for robust replacement.

Additional Technical Info​

ResizeImageStreamToFile detects JPEG/PNG from source bytes at current position, fits decoded pixels proportionally within a bounding box, and writes JPEG/PNG selected by destination filename extension. It supports JPEG-to-PNG and PNG-to-JPEG conversion through a 32-bit bitmap/GDI stretch.

The source is consumed and existing destination is replaced non-atomically. The example is fictional/source-reviewed only; no image function was executed.

Implementation trace

  1. Reject nil source or total Size = 0; verify/create destination folder.
  2. Allocate source/target bitmaps and initialize graphic pointers nil.
  3. Probe JPEG then PNG at current source position; probes restore position.
  4. Decode from that position, force JPEG DIB if needed, assign to pf32bit DIB bitmap.
  5. Calculate minimum width/height scale and rounded proportional target size.
  6. GDI HALFTONE StretchDraw source bitmap to target bitmap.
  7. Select fresh JPEG/PNG from destination extension, apply compression/non-progressive JPEG, assign target bitmap.
  8. Delete existing destination, save, return true; free temporaries and leave source caller-owned.

Source stream contract

  • Must support Size, reading, seeking and Position assignment.
  • Total size zero is rejected; remaining bytes are not checked. A stream at its end can pass the guard and then be unrecognized.
  • Detection starts at current position, not automatically at zero.
  • Probes restore position, but LoadFromStream consumes and advances it; Velox does not restore after success/error.
  • JPEG's probe uses a limited initial-marker heuristic; PNG requires exact eight-byte signature.
  • Caller must restore position if later logic needs the same bytes.

Unlike the Compress stream functions, unrecognized content is handled safely here because SourceGraphic is initialized nil and guarded with Assigned; the function returns false after any directory side effect.

Dimensions and output size

Aspect := Min(NewWidth/sourceWidth, NewHeight/sourceHeight). The output fits within the box with no crop/pad and can be upscaled. Both sides are rounded independently. Velox does not reject zero/negative dimensions, zero-rounded sides, extreme allocation requests or target pixel counts.

Output format and transparency

Destination extension selects encoding:

  • .jpg/.jpeg: new non-progressive JPEG at supplied quality;
  • .png: new PNG at mapped compression level;
  • other suffix: false after decoding/resizing, with no output save.

JPEG-to-PNG changes encoding but cannot recreate metadata lost during bitmap conversion. PNG-to-JPEG cannot represent alpha and has no configurable background/matte. The GDI bitmap stretch is not explicitly alpha-aware; even PNG-to-PNG transparency is not guaranteed to survive correctly.

Quality mapping defect

JPEG direct quality outside 1..100 raises ERangeError in current range-checked builds. PNG MapTo10 maps every input 90 or greater to level 10, which raises ERangeError against installed range 0..9; level 0 cannot be requested. PNG setting changes lossless work, not visual quality.

Metadata and orientation

The function creates new bitmap/encoder objects and has no copy policy for EXIF/IPTC/XMP, color profiles, JPEG markers, PNG ancillary chunks or EXIF orientation. Treat output as newly rendered pixels, not a metadata-preserving transformation.

Destination effects and atomicity

  • CheckFolder may create directories before source is fully validated.
  • Bare filename causes installed ForceDirectories('') to raise.
  • Existing file is deleted before save; deletion result is ignored.
  • No temporary file, atomic rename, rollback or backup.
  • Save failure after deletion loses prior destination.

Side effects

Reads/advances source, may create directories, allocates GDI resources, may delete destination and writes a file. Caller retains source ownership.

Performance, limits and concurrency

Multiple full pixel representations coexist, with no compressed-size, dimension or pixel-count limit. Untrusted images can exhaust memory/CPU/GDI handles. The wrapper supplies no VCL/GDI concurrency synchronization.

Related entries

External references

Created 2026-07-15