Setting uniform values into the fragment shader (OpenGL)

Question

I'm studying OpenGL with GLUT and GLEW library. But I have a question about setting uniform values into the program's fragment shader.

Here's a part of the fragment shader code.

#version 110 
uniform vec4 cameraPosition;
uniform vec4 lightPosition;

varying vec4 fragColor;
varying vec4 fragNormal;
varying vec4 fragPosition;

void main() {
    vec4 L = lightPosition - fragPosition;
    vec4 V = cameraPosition - fragPosition;

What I want to do is set 'cameraPosition' and 'lightPosition' uniform variables. Also, in the main program, I put some code to get these variables ID and pass some values into the variables like below.

GLuint cameraPositionID = glGetUniformLocation(programID, "cameraPosition");
GLuint lightPositionID = glGetUniformLocation(programID, "lightPosition");
GLfloat camPos[4] = { 1, 1, -3, 1 };
GLfloat lightPos[4] = { 1, 0, 0, 1 };
glUniform4fv(cameraPositionID, 4, &cameraPos[0]);
glUniform4fv(lightPositionID, 4, &lightPos[0]);

So, I try to pass camPos into cameraPosition, and lightPos into lightPosition. is it correct or incorrect? If you think incorrect, Do you have any idea about this?

derhass · Accepted Answer · 2017-05-27 12:04:22Z

2

The count parameter of glUniform4fv denotes how many elements of an uniform array you are going to set - in this case, how many vec4s. You don't have an array, so count must be one. According to the spec, your command should result in a GL_INVALID_OPERATION error, and not in setting any uniform values. (Apart from that, using count 4 would also read 16 floats from the memory, while your array is only 4 floats big).

It is also unclear if you did a glUspeProgram(programID) before you try to set these uniforms. As uniforms are per-program state, the glUniform*() commands apply only to the currently bound program.

answered May 27, 2017 at 12:04

derhass

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1 Comment

xpathz Over a year ago

okay, It's now clear after I put 1 into the count, instead of 4. Much thanks :) :D:D

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