片段着色器中的条件语句

Question

I'm just beginner in shaders matter, so I'm probably missing something obvious here. I'm writing an android app using OpenGL ES 2.0. What I'm trying to do is to manipulate fragment's color depending on it's actual r value. Simple exercise but it made me to ask my very first question here. The basic code looks like this:

void main() {
    vec4 tex = texture2D(u_TextureUnit, v_TextureCoordinates);
    float r = tex.r;
    float g = tex.g;
    float b = tex.b;

    if (tex.r > 0.5f) {
        r = 1;
    } else {
        r = 0;
    }

    gl_FragColor = vec4(r, g, b, tex.a);
}

The problem is the if-else block. When it's there, I'm getting nothing but black screen. After removing it, objects are rendered properly, without any color modification of course.

When I remove if-else block and change the last line to for example:

gl_FragColor = vec4(1, g, b, tex.a);

it's working fine too, so I'm guessing that the conditional statement makes the problem itself.

How can I get the fragment's r value modified instead of constantly getting whole screen black?

Answer 1

You should check the success of your shader compilations. The code to do this will be something like this:

int[] statusVal = new int[1];
GLES20.glGetShaderiv(shaderId, GLES20.GL_COMPILE_STATUS, statusVal, 0);
if (statusVal[0] == GLES20.GL_FALSE) {
    String statusStr = GLES20.glGetShaderInfoLog(shaderId);
    // Look at statusStr to see error messages.
}

In your if statement:

if (tex.r > 0.5f) {
    r = 1;
} else {
    r = 0;
}

there are two issues:

1. There are no automatic type conversions in the GLSL version in ES 2.0. 1 and 0 are of type int , and you assign them to a float variable. While that's just a bad habit (IMHO) in languages like C and C++, it is not supported in ES 2.0 shaders.

2. 0.5f is not a valid float constant. The f postfix is not supported in ES 2.0 shaders. Some vendors let you get away with it without reporting a compiler error, others will correctly fail the shader compilation.

With these two issues fixed, the code will look like this:

if (tex.r > 0.5) {
    r = 1.0;
} else {
    r = 0.0;
}