How to Debug Blank Output Images in libavif

Generating a blank, black, or fully transparent output image is a common issue when working with the libavif library. This troubleshooting guide provides a structured, step-by-step approach for developers to diagnose and resolve rendering issues, covering API return value checks, color space conversions, pixel buffer validation, and container analysis.

1. Check libavif Function Return Codes

The first step in debugging is ensuring that every libavif function call succeeds. Most library functions return an avifResult enum. You must explicitly check that every call returns AVIF_RESULT_OK. * Pay close attention to avifImageRGBToYUV(), avifEncoderAddImage(), and avifEncoderWrite(). * If any function returns an error, use avifResultToString() to print a human-readable error message to your console or log files.

2. Verify the Input RGB/RGBA Buffer

A blank image often happens because the source buffer passed to libavif is empty or incorrectly mapped. * Verify that your source pixel buffer actually contains data before passing it to avifImageRGBToYUV(). * Check that the avifRGBImage structure is correctly initialized. Ensure that the width, height, depth (e.g., 8 or 10 bits), and format match your source data. * Ensure the pixels pointer is not null and that the rowBytes stride is calculated correctly (usually width * bytes_per_pixel).

3. Confirm RGB-to-YUV Conversion

AVIF natively encodes images in YUV format. If you are starting with RGB data, you must convert it to YUV before encoding. * Ensure you allocate the YUV planes using avifImageAllocatePlanes(image, AVIF_PLANES_YUV) before calling the conversion function. * Call avifImageRGBToYUV(image, &rgb) and verify it succeeds. If this step is skipped or fails, the encoder will compress empty YUV planes, resulting in a solid green, black, or gray image depending on the color configuration.

4. Inspect Alpha Channel Configuration

If your output image is completely transparent, the alpha channel is likely misconfigured. * If your source image does not have alpha, ensure rgb.format is set to an RGB format (like AVIF_RGB_FORMAT_RGB) instead of an RGBA format. * If you are using alpha, ensure rgb.pixels contains correct alpha values (usually 255 for fully opaque). If the alpha channel is filled with zeros, the entire image will render as completely transparent (blank). * Ensure that avifImageAllocatePlanes(image, AVIF_PLANES_A) is called if you are encoding alpha.

5. Validate Color Profile (CICP) Settings

Incorrect Color Primaries, Transfer Characteristics, and Matrix Coefficients (CICP) can cause decoder software to fail to render the image. * Set standard CICP values on your avifImage structure. For standard sRGB images, use: * colorPrimaries = AVIF_COLOR_PRIMARIES_BT709 * transferCharacteristics = AVIF_TRANSFER_CHARACTERISTICS_SRGB * matrixCoefficients = AVIF_MATRIX_COEFFICIENTS_BT709 * Ensure the yuvRange is correctly set (typically AVIF_RANGE_LIMITED or AVIF_RANGE_FULL).

6. Use Command Line Tools for Verification

If your program outputs an AVIF file successfully but it renders blank in viewer applications, use the official avifdec command-line utility to inspect the file. * Run avifdec --info output.avif to display metadata, resolution, color space, and compression details. * Try converting the generated AVIF back to a PNG using avifdec output.avif test.png to see if the decoded PNG contains the expected image data. If the PNG is correct, the issue lies with the third-party viewer’s AVIF decoder compatibility.