Understanding Black Borders when Overlaying a Transparent Texture Over Another in Fragment Shader

When working with transparent textures and blending them with solid colors in a fragment shader, it’s common to encounter black borders or dark lines around the edges of the blended area. In this article, we’ll delve into the reasons behind these artifacts and explore ways to mitigate them.

Premultiplied Alpha in PNG Images

One key factor contributing to black borders is premultiplied alpha in PNG images. When a pixel has an alpha value of 0, its RGB values are also set to 0. However, this behavior can lead to unexpected results when blending with other colors.

In the context of OpenGL and fragment shaders, premultiplied alpha means that the alpha channel value of each pixel is multiplied by the final color value (i.e., alpha * red becomes the final color). This process can cause issues when overlaying a transparent texture over a solid color, resulting in black borders around the edges.

Interpolation and Alpha Blending

Another factor to consider is interpolation. When blending two colors with alpha values, OpenGL uses a weighted average of the source and destination colors to produce the final result. However, this process can lead to aliasing artifacts, especially when working with transparent textures.

In our example code, we’re using the mix function in the fragment shader to blend the camera texture with the viewfinder texture:

lowp vec4 result = mix(camera, viewfinder, viewfinder.a);

This function takes three arguments: the source color (camera), the destination color (viewfinder), and the alpha value of the source color (viewfinder.a). The resulting blended color is then used for rendering.

Fixing Premultiplied Alpha and Interpolation Issues

To address premultiplied alpha issues, we can modify our blending mode in the fragment shader. One common approach is to use a non-premultiplied alpha blend function, such as GL_ONE with GL_ONE_MINUS_SRC_ALPHA. This tells OpenGL to blend the source color with the destination color using only the alpha value of the source color:

glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);

In this modified version, we’re no longer premultipling the alpha values. By doing so, we avoid multiplying the RGB values by the alpha value, which should reduce or eliminate black borders around the edges.

For interpolation-related issues, we can try adjusting the blending function used in our fragment shader. For example, using GL_SRC_ALPHA with GL_ONE_MINUS SrcAlpha (as opposed to GL_ONE) may help minimize aliasing artifacts.

Additional Fixes and Considerations

When working with transparent textures and blended colors, there are several additional factors to consider:

• Make sure that your texture data is properly normalized, as misaligned or non-normalized data can lead to rendering issues.
• Be cautious when adjusting blending modes, as incorrect settings may cause unintended effects on your final image.
• If you’re still encountering issues after modifying the blending mode and premultiplied alpha handling, consider adding a small offset to the blended color to reduce any potential aliasing artifacts.

Conclusion

By understanding the factors contributing to black borders when overlaying transparent textures over solid colors in fragment shaders, we can take steps to mitigate these issues. By adjusting blending modes, handling premultiplied alpha, and taking care with texture data normalization and interpolation, you should be able to achieve cleaner, more visually appealing results.

Here’s a complete version of the code snippet with the modifications discussed:

// Fragment shader (stripped-down version)
void main(void) {
    lowp vec4 camera = texture2D(texture0, destinationTexCoord);
    lowp vec4 viewfinder = texture2D(texture1, destinationTexCoord);

    // Fix premultiplied alpha issues and add some offset for aliasing reduction
    float alpha = viewfinder.a;
    vec4 result = (vec4(0.999, 0.998, 0.997, alpha) * camera) + (vec4(0.001, 0.002, 0.003, alpha));

    gl_FragColor = result;
}

Last modified on 2023-11-05