GLSL <> operators on a vec4

问题

I'm looking at some newer GLSL code that doesn't compile to my current version of OpenGL and I'm wondering what the short form of the following means:

vec4 base;

if (base < 0.5) {
    result = (2.0 * base * blend);
}

Is this equivalent to:

if (base.r < 0.5 && base.g < 0.5 && base.b < 0.5 && base.a < 0.5) {
    result.r = 2.0 * base.r * blend.r;
    result.g = 2.0 * base.g * blend.g;
    result.b = 2.0 * base.b * blend.b;
    result.a = 2.0 * base.a * blend.a;
}

Edit:

Error:
Fragment shader failed to compile with the following errors:
Wrong operand types no operation '<' exists that takes a left-hand operand of type 'highp 3-component vector of float' and a right operand of type 'const float' (or there is no acceptable conversion)

I've also tried:

(base.rgb < vec3(0.5))
... Wrong operand types no operation '<' exists that takes a left-hand operand of type 'highp 3-component vector of float' and a right operand of type 'const highp 3-component vector of float'

I'm assuming this is because I'm using GLSL 1.2. ATI Radeon 3450

回答1:

From the spec, section 5.9 (top of page 38):

The relational operators greater than (>), less than (<), greater than or equal (>=), and less than or equal (<=) operate only on scalar integer and scalar floating-point expressions. The result is scalar Boolean. Either the operands’ types must match, or the conversions from Section 4.1.10 “Implicit Conversions” will be applied to the integer operand, after which the types must match. To do component-wise relational comparisons on vectors, use the built-in functions lessThan, lessThanEqual, greaterThan, and greaterThanEqual.

Looks like you want the lessThan function. Check section 8.6 (page 62).

• lessThan() - http://www.opengl.org/ (see also: all())

来源：https://stackoverflow.com/questions/5860033/glsl-operators-on-a-vec4