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Optical Character Recognition (OCR) turns images of text—scans, smartphone photos, PDFs—into machine-readable strings and, increasingly, structured data. Modern OCR is a pipeline that cleans an image, finds text, reads it, and exports rich metadata so downstream systems can search, index, or extract fields. Two widely used output standards are hOCR, an HTML microformat for text and layout, and ALTO XML, a library/archives-oriented schema; both preserve positions, reading order, and other layout cues and are supported by popular engines like Tesseract.
A quick tour of the pipeline
Preprocessing. OCR quality starts with image cleanup: grayscale conversion, denoising, thresholding (binarization), and deskewing. Canonical OpenCV tutorials cover global, adaptive and Otsu thresholding—staples for documents with nonuniform lighting or bimodal histograms. When illumination varies within a page (think phone snaps), adaptive methods often outperform a single global threshold; Otsu automatically picks a threshold by analyzing the histogram. Tilt correction is equally important: Hough-based deskewing (Hough Line Transform) paired with Otsu binarization is a common and effective recipe in production preprocessing pipelines.
Detection vs. recognition. OCR is typically split into text detection (where is the text?) and text recognition (what does it say?). In natural scenes and many scans, fully convolutional detectors like EAST efficiently predict word- or line-level quadrilaterals without heavy proposal stages and are implemented in common toolkits (e.g., OpenCV’s text detection tutorial). On complex pages (newspapers, forms, books), segmentation of lines/regions and reading order inference matter:Kraken implements traditional zone/line segmentation and neural baseline segmentation, with explicit support for different scripts and directions (LTR/RTL/vertical).
Recognition models. The classic open-source workhorse Tesseract (open-sourced by Google, with roots at HP) evolved from a character classifier into an LSTM-based sequence recognizer and can emit searchable PDFs, hOCR/ALTO-friendly outputs, and more from the CLI. Modern recognizers rely on sequence modeling without pre-segmented characters. Connectionist Temporal Classification (CTC) remains foundational, learning alignments between input feature sequences and output label strings; it’s widely used in handwriting and scene-text pipelines.
In the last few years, Transformers reshaped OCR. TrOCR uses a vision Transformer encoder plus a text Transformer decoder, trained on large synthetic corpora then fine-tuned on real data, with strong performance across printed, handwritten and scene-text benchmarks (see also Hugging Face docs). In parallel, some systems sidestep OCR for downstream understanding: Donut (Document Understanding Transformer) is an OCR-free encoder-decoder that directly outputs structured answers (like key-value JSON) from document images (repo, model card), avoiding error accumulation when a separate OCR step feeds an IE system.
Engines and libraries
If you want batteries-included text reading across many scripts, EasyOCR offers a simple API with 80+ language models, returning boxes, text, and confidences—handy for prototypes and non-Latin scripts. For historical documents, Kraken shines with baseline segmentation and script-aware reading order; for flexible line-level training, Calamari builds on the Ocropy lineage (Ocropy) with (multi-)LSTM+CTC recognizers and a CLI for fine-tuning custom models.
Datasets and benchmarks
Generalization hinges on data. For handwriting, the IAM Handwriting Database provides writer-diverse English sentences for training and evaluation; it’s a long-standing reference set for line and word recognition. For scene text, COCO-Text layered extensive annotations over MS-COCO, with labels for printed/handwritten, legible/illegible, script, and full transcriptions (see also the original project page). The field also relies heavily on synthetic pretraining: SynthText in the Wild renders text into photographs with realistic geometry and lighting, providing huge volumes of data to pretrain detectors and recognizers (reference code & data).
Competitions under ICDAR’s Robust Reading umbrella keep evaluation grounded. Recent tasks emphasize end-to-end detection/reading and include linking words into phrases, with official code reporting precision/recall/F-score, intersection-over-union (IoU), and character-level edit-distance metrics—mirroring what practitioners should track.
Output formats and downstream use
OCR rarely ends at plain text. Archives and digital libraries prefer ALTO XML because it encodes the physical layout (blocks/lines/words with coordinates) alongside content, and it pairs well with METS packaging. The hOCR microformat, by contrast, embeds the same idea into HTML/CSS using classes like ocr_line and ocrx_word, making it easy to display, edit, and transform with web tooling. Tesseract exposes both—e.g., generating hOCR or searchable PDFs directly from the CLI (PDF output guide); Python wrappers like pytesseract add convenience. Converters exist to translate between hOCR and ALTO when repositories have fixed ingestion standards—see this curated list of OCR file-format tools.
Practical guidance
- Start with data & cleanliness. If your images are phone photos or mixed-quality scans, invest in thresholding (adaptive & Otsu) and deskew (Hough) before any model tuning. You’ll often gain more from a robust preprocessing recipe than from swapping recognizers.
- Choose the right detector. For scanned pages with regular columns, a page segmenter (zones → lines) may suffice; for natural images, single-shot detectors like EAST are strong baselines and plug into many toolkits (OpenCV example).
- Pick a recognizer that matches your text. For printed Latin, Tesseract (LSTM/OEM) is sturdy and fast; for multi-script or quick prototypes, EasyOCR is productive; for handwriting or historical typefaces, consider Kraken or Calamari and plan to fine-tune. If you need tight coupling to document understanding (key-value extraction, VQA), evaluate TrOCR (OCR) versus Donut (OCR-free) on your schema—Donut may remove a whole integration step.
- Measure what matters. For end-to-end systems, report detection F-score and recognition CER/WER (both based on Levenshtein edit distance; see CTC); for layout-heavy tasks, track IoU/tightness and character-level normalized edit distance as in ICDAR RRC evaluation kits.
- Export rich outputs. Prefer hOCR /ALTO (or both) so you keep coordinates and reading order—vital for search hit highlighting, table/field extraction, and provenance. Tesseract’s CLI and pytesseract make this a one-liner.
Looking ahead
The strongest trend is convergence: detection, recognition, language modeling, and even task-specific decoding are merging into unified Transformer stacks. Pretraining on large synthetic corpora remains a force multiplier. OCR-free models will compete aggressively wherever the target is structured outputs rather than verbatim transcripts. Expect hybrid deployments too: a lightweight detector plus a TrOCR-style recognizer for long-form text, and a Donut-style model for forms and receipts.
Further reading & tools
Tesseract (GitHub) · Tesseract docs · hOCR spec · ALTO background · EAST detector · OpenCV text detection · TrOCR · Donut · COCO-Text · SynthText · Kraken · Calamari OCR · ICDAR RRC · pytesseract · IAM handwriting · OCR file-format tools · EasyOCR
Frequently Asked Questions
What is OCR?
Optical Character Recognition (OCR) is a technology used to convert different types of documents, such as scanned paper documents, PDF files or images captured by a digital camera, into editable and searchable data.
How does OCR work?
OCR works by scanning an input image or document, segmenting the image into individual characters, and comparing each character with a database of character shapes using pattern recognition or feature recognition.
What are some practical applications of OCR?
OCR is used in a variety of sectors and applications, including digitizing printed documents, enabling text-to-speech services, automating data entry processes, and assisting visually impaired users to better interact with text.
Is OCR always 100% accurate?
While great advancements have been made in OCR technology, it isn't infallible. Accuracy can vary depending upon the quality of the original document and the specifics of the OCR software being used.
Can OCR recognize handwriting?
Although OCR is primarily designed for printed text, some advanced OCR systems are also able to recognize clear, consistent handwriting. However, typically handwriting recognition is less accurate because of the wide variation in individual writing styles.
Can OCR handle multiple languages?
Yes, many OCR software systems can recognize multiple languages. However, it's important to ensure that the specific language is supported by the software you're using.
What's the difference between OCR and ICR?
OCR stands for Optical Character Recognition and is used for recognizing printed text, while ICR, or Intelligent Character Recognition, is more advanced and is used for recognizing hand-written text.
Does OCR work with any font and text size?
OCR works best with clear, easy-to-read fonts and standard text sizes. While it can work with various fonts and sizes, accuracy tends to decrease when dealing with unusual fonts or very small text sizes.
What are the limitations of OCR technology?
OCR can struggle with low-resolution documents, complex fonts, poorly printed texts, handwriting, and documents with backgrounds that interfere with the text. Also, while it can work with many languages, it may not cover every language perfectly.
Can OCR scan colored text or colored backgrounds?
Yes, OCR can scan colored text and backgrounds, although it's generally more effective with high-contrast color combinations, such as black text on a white background. The accuracy might decrease when text and background colors lack sufficient contrast.
What is the EPI format?
Adobe Encapsulated PostScript Interchange format
The Extended Postscript Image (EPI) format is a specialized file format designed for representing images in environments where PostScript printing and display are prevalent. This format is a derivative of the more commonly known EPS (Encapsulated PostScript) format, yet it incorporates additional features aimed at enhancing color management, compression, and overall flexibility. The use of EPI format is particularly significant in industries where high-quality printing and accurate color reproduction are essential, such as in graphic design, publishing, and digital arts.
An EPI file essentially contains a description of an image or a drawing in the PostScript language, which is a programming language optimized for printing. PostScript is a dynamically typed, concatenative programming language and was created by Adobe Systems in 1982. It is unique because it can describe, with high precision, both text and graphic information in a single file. In the context of EPI, this capability is leveraged to encapsulate complex graphic designs, including sharp text and detailed illustrations, in a format that can be reliably printed on PostScript-compatible printers.
One of the primary features that distinguish the EPI format from its predecessors is its improved support for color management. Color management is a crucial aspect of digital image processing, as it ensures that colors are represented consistently across different devices. EPI files incorporate color profiles based on the International Color Consortium (ICC) standards, which define how colors should be reproduced on various devices. This means that an image saved in the EPI format can retain its intended color accuracy whether it is viewed on a computer monitor, printed on paper, or reproduced in any other medium.
Compression is another area where the EPI format excels. High-quality images are often large in size, which can be a limitation when transferring files or saving storage space. EPI supports several compression algorithms, including both lossy and lossless methods. Lossy compression, like JPEG, reduces file size by slightly lowering image quality, which might be acceptable for certain applications. Lossless compression, such as ZIP or LZW used in TIFF files, retains the original image quality but might not reduce the file size as significantly. The choice of compression can be customized based on the specific needs of the user, balancing between image quality and file size.
Additionally, the EPI format is designed to enhance scalability and resolution independence. Images stored in this format can be scaled up or down without loss of detail, which is particularly useful for printing applications where different sizes might be required. This is achieved through the use of vector graphics for illustrations and text, alongside bitmap images for photographic content. Vector graphics are based on mathematical equations to draw shapes and lines, allowing them to be resized infinitely without pixelation. This feature makes EPI an ideal choice for creating logos, banners, and other marketing materials that need to be reproduced at various sizes.
EPI also features advanced embedding capabilities that allow it to contain a complete subset of the PostScript language. This allows for the inclusion of functions, variables, and control structures within an EPI file, providing a powerful tool for creating dynamic and interactive images. For example, an EPI file can include code that adjusts the colors of an image based on the output device, whether it's a high-resolution printer or a standard computer monitor. This flexibility opens up new possibilities for cross-media publishing and ensures that images can adapt to different contexts without requiring manual adjustments.
The standardization of the EPI format plays a significant role in its adoption and interoperability. By adhering to well-established PostScript conventions and incorporating modern features such as ICC color profiles and various compression methods, EPI files can be seamlessly integrated into existing workflows. Additionally, the widespread support of PostScript across different operating systems and software applications ensures that EPI files are accessible and usable by a broad audience. This compatibility removes barriers to collaboration and allows for the efficient exchange of high-quality images between designers, printers, and publishers.
Creating and manipulating EPI files requires specialized software that understands the PostScript language and supports the features specific to the EPI format. Adobe Illustrator and Photoshop are examples of such software, offering extensive tools for designing and exporting images in EPI format. These applications not only provide a rich set of drawing and editing capabilities but also include features for color management, allowing designers to work with precise color specifications and to preview how their images will look across various output devices.
In terms of file structure, an EPI file is composed of a header, a body, and a trailer. The header includes metadata about the file, such as the creator, creation date, and the bounding box which defines the physical dimensions of the image. The body contains the actual PostScript code describing the image, and may include embedded ICC profiles, font definitions, and other resources required for rendering the image. The trailer marks the end of the file and can include additional information such as thumbnails or preview images. This structured approach ensures that EPI files are both flexible and self-contained, making them easy to manage and exchange.
Despite its many advantages, the EPI format is not without challenges. The complexity of the PostScript language can make generating and editing EPI files somewhat daunting for those not familiar with programming. Furthermore, because EPI files can contain executable code, they must be handled with care to avoid security vulnerabilities. This necessitates the use of trusted software and cautious handling of files from unknown sources.
In conclusion, the Extended Postscript Image (EPI) format represents a powerful and versatile tool for digital image processing, particularly in fields requiring high-quality printing and accurate color reproduction. Its support for advanced color management, compression, scalability, and embedding capabilities make it an ideal choice for professionals in graphic design, publishing, and related industries. While it requires specialized software and knowledge to fully exploit its potential, the benefits of using the EPI format in terms of flexibility, quality, and efficiency are substantial. As digital imaging and printing technology continue to evolve, the EPI format stands as a testament to the enduring value of combining technical precision with creative flexibility.
Supported formats
AAI.aai
AAI Dune image
AI.ai
Adobe Illustrator CS2
AVIF.avif
AV1 Image File Format
BAYER.bayer
Raw Bayer Image
BMP.bmp
Microsoft Windows bitmap image
CIN.cin
Cineon Image File
CLIP.clip
Image Clip Mask
CMYK.cmyk
Raw cyan, magenta, yellow, and black samples
CUR.cur
Microsoft icon
DCX.dcx
ZSoft IBM PC multi-page Paintbrush
DDS.dds
Microsoft DirectDraw Surface
DPX.dpx
SMTPE 268M-2003 (DPX 2.0) image
DXT1.dxt1
Microsoft DirectDraw Surface
EPDF.epdf
Encapsulated Portable Document Format
EPI.epi
Adobe Encapsulated PostScript Interchange format
EPS.eps
Adobe Encapsulated PostScript
EPSF.epsf
Adobe Encapsulated PostScript
EPSI.epsi
Adobe Encapsulated PostScript Interchange format
EPT.ept
Encapsulated PostScript with TIFF preview
EPT2.ept2
Encapsulated PostScript Level II with TIFF preview
EXR.exr
High dynamic-range (HDR) image
FF.ff
Farbfeld
FITS.fits
Flexible Image Transport System
GIF.gif
CompuServe graphics interchange format
HDR.hdr
High Dynamic Range image
HEIC.heic
High Efficiency Image Container
HRZ.hrz
Slow Scan TeleVision
ICO.ico
Microsoft icon
ICON.icon
Microsoft icon
J2C.j2c
JPEG-2000 codestream
J2K.j2k
JPEG-2000 codestream
JNG.jng
JPEG Network Graphics
JP2.jp2
JPEG-2000 File Format Syntax
JPE.jpe
Joint Photographic Experts Group JFIF format
JPEG.jpeg
Joint Photographic Experts Group JFIF format
JPG.jpg
Joint Photographic Experts Group JFIF format
JPM.jpm
JPEG-2000 File Format Syntax
JPS.jps
Joint Photographic Experts Group JPS format
JPT.jpt
JPEG-2000 File Format Syntax
JXL.jxl
JPEG XL image
MAP.map
Multi-resolution Seamless Image Database (MrSID)
MAT.mat
MATLAB level 5 image format
PAL.pal
Palm pixmap
PALM.palm
Palm pixmap
PAM.pam
Common 2-dimensional bitmap format
PBM.pbm
Portable bitmap format (black and white)
PCD.pcd
Photo CD
PCT.pct
Apple Macintosh QuickDraw/PICT
PCX.pcx
ZSoft IBM PC Paintbrush
PDB.pdb
Palm Database ImageViewer Format
PDF.pdf
Portable Document Format
PDFA.pdfa
Portable Document Archive Format
PFM.pfm
Portable float format
PGM.pgm
Portable graymap format (gray scale)
PGX.pgx
JPEG 2000 uncompressed format
PICT.pict
Apple Macintosh QuickDraw/PICT
PJPEG.pjpeg
Joint Photographic Experts Group JFIF format
PNG.png
Portable Network Graphics
PNG00.png00
PNG inheriting bit-depth, color-type from original image
PNG24.png24
Opaque or binary transparent 24-bit RGB (zlib 1.2.11)
PNG32.png32
Opaque or binary transparent 32-bit RGBA
PNG48.png48
Opaque or binary transparent 48-bit RGB
PNG64.png64
Opaque or binary transparent 64-bit RGBA
PNG8.png8
Opaque or binary transparent 8-bit indexed
PNM.pnm
Portable anymap
PPM.ppm
Portable pixmap format (color)
PS.ps
Adobe PostScript file
PSB.psb
Adobe Large Document Format
PSD.psd
Adobe Photoshop bitmap
RGB.rgb
Raw red, green, and blue samples
RGBA.rgba
Raw red, green, blue, and alpha samples
RGBO.rgbo
Raw red, green, blue, and opacity samples
SIX.six
DEC SIXEL Graphics Format
SUN.sun
Sun Rasterfile
SVG.svg
Scalable Vector Graphics
TIFF.tiff
Tagged Image File Format
VDA.vda
Truevision Targa image
VIPS.vips
VIPS image
WBMP.wbmp
Wireless Bitmap (level 0) image
WEBP.webp
WebP Image Format
YUV.yuv
CCIR 601 4:1:1 or 4:2:2
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