OpenGL 2.0 rendering multiple objects with the same shader - ios

Question

I would like to know how to use the same shader for multiple objects but allow them objects to have a different colour
I have many cubes on the screen which all currently load the same shader, the only difference is when it is draw, I change the cubes colour. If I set the same _program for all of them, they are become all the same colour.
- (void)draw:(float)eyeOffset
{
// Calculate the per-eye model view matrix:
GLKMatrix4 temp = GLKMatrix4MakeTranslation(eyeOffset, 0.0f, 0.0f);
GLKMatrix4 eyeBaseModelViewMatrix = GLKMatrix4Multiply(temp, self.baseModelViewMatrix);
if (self.isTransparant)
{
glEnable (GL_BLEND);
glDisable(GL_CULL_FACE);
//glDisable(GL_DEPTH_TEST);
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
}
if (self.textureInfo)
{
glBindTexture(self.textureInfo.target, self.textureInfo.name);
}
glBindVertexArrayOES(_vertexArray);
//See if we are sharing a program shader
if (self.tprogram)
{
glUseProgram(self.tprogram);
}
else
{
glUseProgram(_program);
}
self.modelViewMatrix = GLKMatrix4MakeTranslation(self.position.x,self.position.y, self.position.z );//(float)x, (float)y, -1.5f)
self.modelViewMatrix = GLKMatrix4Scale(self.modelViewMatrix, self.scale.x, self.scale.y, self.scale.z);
//rotation +=0.01;
self.modelViewMatrix = GLKMatrix4Rotate(self.modelViewMatrix,self.spinRotation, 0.0 ,0.0 ,1.0);
self.modelViewMatrix = GLKMatrix4Multiply(eyeBaseModelViewMatrix, self.modelViewMatrix);
GLKMatrix3 normalMatrix = GLKMatrix3InvertAndTranspose(GLKMatrix4GetMatrix3(self.modelViewMatrix), NULL);
GLKMatrix4 modelViewProjectionMatrix = GLKMatrix4Multiply(self.projectionMatrix, self.modelViewMatrix);
glUniformMatrix4fv(uniforms[UNIFORM_MODELVIEWPROJECTION_MATRIX], 1, 0, modelViewProjectionMatrix.m);
glUniformMatrix3fv(uniforms[UNIFORM_NORMAL_MATRIX], 1, 0, normalMatrix.m);
_colorSlot = glGetUniformLocation(_program, "color");
GLfloat color[] = {
self.color.x, self.color.y, self.color.z, self.color.a};
glUniform4fv(_colorSlot, 1, color);
glDrawArrays(GL_TRIANGLES, 0, 36);
if (self.isTransparant)
{
glEnable(GL_CULL_FACE);
//glEnable(GL_DEPTH_TEST);
glDisable(GL_BLEND);
}
}
//setup for each cube
- (void)setup;
{
glGenVertexArraysOES(1, &_vertexArray);
glBindVertexArrayOES(_vertexArray);
glGenBuffers(1, &_vertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, _vertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(gCubeVertexData), gCubeVertexData, GL_STATIC_DRAW);
glEnableVertexAttribArray(GLKVertexAttribPosition);
glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, 32, BUFFER_OFFSET(0));
glEnableVertexAttribArray(GLKVertexAttribNormal);
glVertexAttribPointer(GLKVertexAttribNormal, 3, GL_FLOAT, GL_FALSE, 32, BUFFER_OFFSET(12));
glEnableVertexAttribArray(GLKVertexAttribTexCoord0);
glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, 32, BUFFER_OFFSET(24));
glBindVertexArrayOES(0);
}
Shader
attribute vec4 position;
attribute vec3 normal;
uniform vec4 color;
varying lowp vec4 colorVarying;
uniform mat4 modelViewProjectionMatrix;
uniform mat3 normalMatrix;
void main()
{
//vec4 diffuseColor = color;
vec3 eyeNormal = normalize(normalMatrix * normal);
vec3 lightPosition = vec3(0.0, 0.0, 1.0);
//diffuseColor = vec4(0.4, 0.4, 1.0, 1.0);
float nDotVP = max(0.7, dot(eyeNormal, normalize(lightPosition))); // 0.0
colorVarying = color * nDotVP;
gl_Position = modelViewProjectionMatrix * position;
}
I thought uniform vec4 color; allowed me to change the colour at anytime and if every object has a shader, it works fine, I can change object colours on the fly

Answer 1

How about sending a different uniform for each cube (say uniform vec4 cubeColor and use it in your fragment shader) before calling glDrawArrays() on it ?
Alternatively, you could consider uploading, for each cube, both vertices and vertex colors during the setup then, when drawing, bind the appropriate vertex buffers (e.g. attribute vec3 a_vertex) and vertex-color buffers (e.g. attribute vec4 a_vertexColor, which you assign, in your vertex shader, to varying vec4 v_vertexColor and use in your fragment shader as varying vec4 v_vertexColor).
Also, as a side note, if you're planning to use the same program, you can call glUseProgram() once, during the setup (OpenGL is based on a state machine, which means that it recalls certain parameters (aka. states, such as the current program) as long as you don't change them). This might enhance the performance of your program a little bit ;-)
Good luck.