📊 Excel to PNG Converter

Frequently Asked Technical Questions

Question 1: What library parses the XLS/XLSX file, and why not parse it entirely from scratch?

Answer: This tool uses SheetJS, an open-source, MIT-licensed, purely client-side JavaScript library that decodes both the legacy binary BIFF8 (.xls) format and the ZIP/XML-based (.xlsx) format. Writing a full parser for either format from scratch would mean reimplementing a compressed ZIP reader, an XML parser, and Microsoft's binary record structures — none of which is unique or beneficial to reinvent. This mirrors how the X3F to PNG converter relies on the browser's native JPEG decoder rather than hand-writing one: complex, well-solved container parsing is delegated to a proven engine, while the actual rendering and PNG-encoding pipeline is fully custom-built for this tool.

Question 2: How does the automatic column-width algorithm work?

Answer: If the workbook stores an explicit column width (in pixels or "characters" units) in its !cols metadata, that value is used directly (converted to pixels and clamped between 40–400px). Otherwise, the tool measures the rendered pixel width of every visible cell's text in that column using Canvas's measureText() API at the selected font size, takes the maximum width found, adds padding, and clamps the result between 50–320px to avoid pathologically narrow or wide columns.

Question 3: How are merged cells detected and rendered?

Answer: The workbook's !merges array (a list of start/end row-column ranges) is normalized relative to the sheet's used range, then converted into a lookup map: the top-left cell of each merge is flagged with its row-span and column-span, while every other cell inside the merged region is flagged to be skipped during drawing. When rendering, the anchor cell's background and border are drawn across its full spanned width and height, and its text is centered within that combined area — accurately reproducing the visual merge from Excel.

Question 4: Why doesn't the tool re-evaluate spreadsheet formulas?

Answer: Excel stores both a formula string (e.g., =SUM(A1:A10)) and the last computed numeric/text result for every formula cell. This tool reads only that cached, already-computed result — reimplementing Excel's full formula engine (hundreds of functions, circular reference handling, cross-sheet references) is out of scope for a browser-based image converter, and using the cached value is both simpler and matches what a user visually sees when they last saved the file in Excel.

Question 5: How is the PNG pHYs chunk constructed, and is it the same technique used in your other converters?

Answer: Yes — this reuses the same genuine binary technique used across this tool family: a 9-byte payload (4-byte big-endian pixels-per-meter for X, 4-byte pixels-per-meter for Y, 1-byte unit specifier) is computed from the chosen DPI value (DPI ÷ 0.0254 = pixels per meter), wrapped in a standard chunk structure (length + type + data + CRC-32), and spliced into the PNG binary immediately after the mandatory IHDR chunk, exactly as required by the PNG specification's chunk-ordering rules.

Question 6: How does CRC-32 checksum computation work here?

Answer: Every PNG chunk (except the file signature) ends with a 4-byte CRC-32 checksum computed over its type and data bytes combined. This tool implements the standard IEEE 802.3 CRC-32 algorithm from scratch in JavaScript using a precomputed 256-entry lookup table and the polynomial 0xEDB88320 — the exact algorithm mandated by the PNG specification (RFC 2083 / ISO 15948).

Question 7: Why combine sheets into one tall stacked PNG instead of an Animated PNG slideshow?

Answer: While a related converter in this family bundles multiple images into an Animated PNG (APNG) for slideshow-style viewing, that model doesn't fit spreadsheets well — users typically want to see all sheets of a report at once, scroll through them like a continuous document, and paste the whole thing into another document or print it. A single tall image achieves that far more usefully than a frame-by-frame animation would, so this tool renders each selected sheet to its own canvas (with a labeled header bar), then concatenates those canvases vertically into one continuous PNG.

Question 8: Does PNG support a selectable byte order like TIFF does?

Answer: No. The PNG specification mandates that all multi-byte integer values (chunk lengths, CRCs, pHYs values, etc.) are always stored in big-endian ("network") byte order. Unlike TIFF, which allows either byte order via its header flag, PNG offers no such choice — so this tool has no byte-order setting, because none exists in the format itself.

Question 9: How does the "Output Scale" setting actually increase image resolution?

Answer: The entire canvas is created at dimensions multiplied by the chosen scale factor (e.g., 2× for 200%), and the drawing context is scaled via ctx.scale(factor, factor) before any content is drawn. This means every rectangle, border, and piece of text is rendered at genuinely higher pixel density — true supersampling — rather than simply stretching a smaller finished image, which would produce blurry results.

Question 10: Why does the tool cap rendering at a hard maximum of rows and columns?

Answer: Browsers impose practical canvas dimension limits (commonly 16,384–32,767 pixels per axis, and total pixel-area limits that vary by browser), and extremely large canvases also consume significant memory and CPU during rendering and PNG encoding. This tool applies a hard safety cap (5,000 rows, 300 columns) regardless of user settings, to guarantee the conversion completes reliably across all supported browsers.

Question 11: How does the tool distinguish numbers from text for right-alignment?

Answer: Each cell object from the parsed workbook includes a type flag (t): 'n' for numeric cells (including dates, which Excel stores internally as serial numbers), 's' for strings, and 'b' for booleans. This tool right-aligns cells with type 'n' and left-aligns everything else, matching Excel's own default alignment conventions.

Question 12: Can the generated PNG be opened and further edited in image software?

Answer: Yes. The output is a fully standard, baseline PNG file readable by any image viewer or editor (Photoshop, GIMP, Preview, browsers). Since it's a flattened raster image, individual cell "text" cannot be re-edited as spreadsheet data — for that, the original XLS/XLSX file should be edited and re-converted.