Hi, I am writing a 3D modeling application and I want to speed up rendering in OpenGL. I am currently using glBegin / glEnd, which is very slow and deprecated. I need to draw very fast flat shaded models. I generate normals for each individual frame. It is very slow. I tried using glDrawElements with indexed geometry, but there is a problem in normal generation, because the normals are indicated at the vertex not at the triangle level.
Another idea was to use GLSL to generate normals on the GPU in a geometric shader. I wrote this code for normal generation:
vec3 NormalFromTriangleVertices(vec3 triangleVertices[3])
{
// now is same as RedBook (OpenGL Programming Guide)
vec3 u = triangleVertices[0] - triangleVertices[1];
vec3 v = triangleVertices[1] - triangleVertices[2];
return cross(v, u);
}
void main()
{
// no change of position
// computes normal from input triangle and front color for that triangle
vec3 triangleVertices[3];
vec3 computedNormal;
vec3 normal, lightDir;
vec4 diffuse;
float NdotL;
vec4 finalColor;
for(int i = 0; i < gl_VerticesIn; i += 3)
{
for (int j = 0; j < 3; j++)
{
triangleVertices[j] = gl_PositionIn[i + j].xyz;
}
computedNormal = NormalFromTriangleVertices(triangleVertices);
normal = normalize(gl_NormalMatrix * computedNormal);
// hardcoded light direction
vec4 light = gl_ModelViewMatrix * vec4(0.0, 0.0, 1.0, 0.0);
lightDir = normalize(light.xyz);
NdotL = max(dot(normal, lightDir), 0.0);
// hardcoded
diffuse = vec4(0.5, 0.5, 0.9, 1.0);
finalColor = NdotL * diffuse;
finalColor.a = 1.0; // final color ignores everything, except lighting
for (int j = 0; j < 3; j++)
{
gl_FrontColor = finalColor;
gl_Position = gl_PositionIn[i + j];
EmitVertex();
}
}
EndPrimitive();
}
, . , . GLSL , , .
MacBook Geforce 9400M.
, , :
- (void)drawAsCommandsWithScale:(Vector3D)scale
{
float frontDiffuse[4] = { 0.4, 0.4, 0.4, 1 };
CGFloat components[4];
[color getComponents:components];
float backDiffuse[4];
float selectedDiffuse[4] = { 1.0f, 0.0f, 0.0f, 1 };
for (uint i = 0; i < 4; i++)
backDiffuse[i] = components[i];
glMaterialfv(GL_BACK, GL_DIFFUSE, backDiffuse);
glMaterialfv(GL_FRONT, GL_DIFFUSE, frontDiffuse);
Vector3D triangleVertices[3];
float *lastDiffuse = frontDiffuse;
BOOL flip = scale.x < 0.0f || scale.y < 0.0f || scale.z < 0.0f;
glBegin(GL_TRIANGLES);
for (uint i = 0; i < triangles->size(); i++)
{
if (selectionMode == MeshSelectionModeTriangles)
{
if (selected->at(i))
{
if (lastDiffuse == frontDiffuse)
{
glMaterialfv(GL_FRONT_AND_BACK, GL_DIFFUSE, selectedDiffuse);
lastDiffuse = selectedDiffuse;
}
}
else if (lastDiffuse == selectedDiffuse)
{
glMaterialfv(GL_BACK, GL_DIFFUSE, backDiffuse);
glMaterialfv(GL_FRONT, GL_DIFFUSE, frontDiffuse);
lastDiffuse = frontDiffuse;
}
}
Triangle currentTriangle = [self triangleAtIndex:i];
if (flip)
currentTriangle = FlipTriangle(currentTriangle);
[self getTriangleVertices:triangleVertices fromTriangle:currentTriangle];
for (uint j = 0; j < 3; j++)
{
for (uint k = 0; k < 3; k++)
{
triangleVertices[j][k] *= scale[k];
}
}
Vector3D n = NormalFromTriangleVertices(triangleVertices);
n.Normalize();
for (uint j = 0; j < 3; j++)
{
glNormal3f(n.x, n.y, n.z);
glVertex3f(triangleVertices[j].x, triangleVertices[j].y, triangleVertices[j].z);
}
}
glEnd();
}
, , . triangles - vertices.
, ( ).
glEnableClientState(GL_VERTEX_ARRAY);
uint *trianglePtr = (uint *)(&(*triangles)[0]);
float *vertexPtr = (float *)(&(*vertices)[0]);
glVertexPointer(3, GL_FLOAT, 0, vertexPtr);
glDrawElements(GL_TRIANGLES, triangles->size() * 3, GL_UNSIGNED_INT, trianglePtr);
glDisableClientState(GL_VERTEX_ARRAY);
, , , ?