Question

I’ll keep it simple: The code listed at the end of this post is in top-to-bottom order within my project. I have an OGLES2.0 framework in place which renders everything except textures perfectly. When rendering a single texture all I see is a black box of the correct dimensions.

Here’s what I’ve verified:

• Having output the texture bytes (it’s format GL_ALPHA) it’s plain to see there are zero and none-zero values, so the data looks correct (or at least not all black!).

• The texture ids are correct, verified by using the following in the vertex shader:
  gl_FragColor=vec4(v_texCoord.xy,0.0,1.0);
  ..and observing the expected black->green->yellow->red colour flow moving from (0,0)->(0,1)->(1,1)->(1,0) is seen.

• My texture has power of two dimensions: 256 x 64, correctly reflected in the data array.

I’d be tremendously grateful for help in determining what’s wrong, as after several hours of googling and prodding I’m stumped!

glGenTextures(1, &_textureId);
GLint savedId;
glGetIntegerv(GL_TEXTURE_BINDING_2D, &savedId);
glBindTexture(GL_TEXTURE_2D, _textureId);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
GLenum datatype = -1;
GLenum format = -1;
switch([self pixelFormat]) {
    case kGLTexturePixelFormat_RGBA8888:
        format=GL_RGBA;
        datatype=GL_UNSIGNED_BYTE;
        break;
    case kGLTexturePixelFormat_RGB565:
        format=GL_RGB;
        datatype=GL_UNSIGNED_SHORT_5_6_5;
        break;
    case kGLTexturePixelFormat_A8: // * This is current format, used for testing.
        format=GL_ALPHA;
        datatype=GL_UNSIGNED_BYTE;
        break;
    default:
        [NSException raise:NSInternalInconsistencyException format:@""];

}
glTexImage2D(GL_TEXTURE_2D, 0, format, [self pixelsWide], [self pixelsHigh], 0, format, datatype, [self data]);
glBindTexture(GL_TEXTURE_2D, savedId);
//...
GLint s_textureId = glGetUniformLocation(program, "s_texture");
//...
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, [_textureAtlas textureId]);
glUniform1i(s_textureId, 0);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

// In vertex shader:
attribute vec2 a_texCoord;
varying vec2 v_texCoord;
uniform sampler2D s_texture;
void main() {
    // ...
    v_texCoord = a_texCoord;
}

// In fragment shader:
varying vec2 v_texCoord;
uniform sampler2D s_texture;
void main() {
    // ...
    gl_FragColor = texture2D(s_texture, v_texCoord);
}

Answer 1

Your code looks fine, and I must say that I expect the output color to be black : as your texture is of GL_ALPHA format, the documentation of GL ES says :

GL_ALPHA Each element is a single alpha component. The GL converts it
to floating point and assembles it into an RGBA element by attaching 0
for red, green, and blue.

So if you want to display your alpha value as a greyscale color, you’ll need to instruct your fragment shader to do so :

gl_FragColor = texture2D(s_texture, v_texCoord).aaaa;

This will duplicate the alpha value in all red, green, red and alpha channels.

If you want your output alpha channel to remain 1.0 (opaque), then you’ll want :

gl_FragColor = vec4( texture2D(s_texture, v_texCoord).aaa, 1.0 );