What the sample should show
The 128 × 128 sample has four equal quadrants with alpha values 0, 64, 128 and 255. Each quadrant contains 4,096 pixels. The expected result is 4,096 fully transparent, 8,192 partly transparent and 4,096 opaque pixels.
Does a checkerboard mean an image is transparent?
A checkerboard can be part of the image itself. If every decoded pixel has alpha 255, no decoded alpha transparency is present, regardless of the visible pattern. If some pixels are transparent, that still does not establish that the entire background is transparent. Inspect the relevant pixels and try contrasting preview backgrounds.
Use alpha bounds to find faint edge pixels
At threshold 0, bounds include every pixel with nonzero alpha. A faint pixel far from the subject can make that rectangle unexpectedly large. Increase the threshold to see the bounds of stronger-opacity pixels. The histogram and original transparency counts remain unchanged.
Frequently asked questions
Are the counts based on the preview size?
No. Every decoded source pixel is inspected. The preview alone is scaled to fit the page. Images beyond the stated limits are rejected rather than silently reduced.
Does this recover the original alpha channel or hidden colours?
It inspects browser-decoded 8-bit RGBA, not original encoded pixel data. Higher-bit-depth input is converted by the browser. Premultiplication and colour conversion can affect RGB, especially in fully transparent pixels.
Why is JPEG opaque?
Ordinary JPEG does not store an alpha channel. Decoding it normally gives alpha 255 for every pixel.
Can it inspect animated images?
Animated PNG and WebP are rejected to avoid ambiguous frame counts. Export a still frame first. GIF and SVG are not accepted.
Does this send my image to a server?
No. Header inspection, decoding, counting and exports run in your browser.
Can this automatically remove a painted background?
No. It diagnoses the decoded alpha values. Use the background remover or an editor to change the image, then inspect the resulting file here.
Technical references: Canvas ImageData RGBA values and HTML Canvas standard.