Question

I am using OpenGL ES 2.0 (on Android) to draw simple 2D scene has few images. I have background image and some others which have alpha channel.

I would like to draw outline around non-transparent pixels in texture using only shader programs. After somewhat extensive search I failed to find example code. It looks like GLES 2.0 is still not that popular.

Can you provide some sample code or point me in right direction where I can find more information on how to do this?

Answer 1

To get the pixel shader drawing something, there needs to be geometry.
As far as I understand, you want to draw a border around these images,
but the outermost fragments generated would be image pixels in a basic implementation,
so you’d overdraw them with any border.

If you want a ‘line border’, you cannot do anything else than drawing the image triangles/quads (GL_TRIANGLES,GL_QUADS), and in an additional call the outline (using GL_LINES), where you may share the vertices of a single quad.
Consider, that lines can’t be drawn efficiently by many GPU’s)

Otherwise, see below solutions:

Solution 1:

• Draw the rectangle as big as the image + border will be and adjust texture coords for the image, so that it will be placed within the rectangle appropriately.
  This way, no extra geometry or draw calls are required.
• Set the texture border property (single 4 component color), there will be no need to do extra fragment shader calculations, the texture unit/sampler does all the work.

Texture properties:

glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP_TO_BORDER)
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_CLAMP_TO_BORDER)
glTexParameterfv(GL_TEXTURE_2D,GL_TEXTURE_BORDER_COLOR,borderColor4f)

I’ve never used a color border for a single channel texture, so this approach needs to be verified.

Solution 2:

Similar to 1, but with calculations in the fragment shader to check, whether the texture coords are within the border area, instead of the texture border. Without modification, the scalars of a texture coord range from 0.0 to 1.0.

Texture properties may be:

glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP)
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_CLAMP)

The fragment color could be determined by any of these methods:

• an additional border color attribute for the rectangle, where either the texel or that border color is selected then (could be a vertex attribute, but more likely an uniform or constant).

• combination of the alpha texture with a second texture as background for the whole rectangle (like a picture frame) and here too, either texel is choosen.

• some other math function

Of course, the color values could be mixed for image/border gradients.

EDIT:

As the number, length and position of such outline segments will vary and can even form concave shapes, you’d need to do this with a geometry shader, which is not available in ES 2.0 core. The best thing you can do is to precompute a line loop for each image on the CPU. Doing such tests in a shader is rather inefficient and even overkill, depending on image size, the hardware you actually run it on etc. If you’d draw a fixed amount of line segments and transform them using the vertex shader, you can not properly cover all cases, at least not without immense effort and GPU workload.

Should you intend to change the color values of corresponding texels, your fragment shader would need to fetch a massive and varying amount of texels for each neighbour pixel towards the texture edges as in all other implementations. Such brute force techniques are usually a replacement for recursive and iterative algos, for which the CPU is a better choice. So I suggest that you do it there by either modifying the texture or generate a second one for combination in the fragment shader.

Basically, you need to implement a path finding algo, which tries to ‘get around’ opaque pixels towards any edge.