StencilOperations.cpp

//
// Copyright 2014 The ANGLE Project Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
//

//            Based on Stencil_Test.c from
// Book:      OpenGL(R) ES 2.0 Programming Guide
// Authors:   Aaftab Munshi, Dan Ginsburg, Dave Shreiner
// ISBN-10:   0321502795
// ISBN-13:   9780321502797
// Publisher: Addison-Wesley Professional
// URLs:      http://safari.informit.com/9780321563835
//            http://www.opengles-book.com

#include "SampleApplication.h"

#include "util/shader_utils.h"

class StencilOperationsSample : public SampleApplication
{
  public:
    StencilOperationsSample(int argc, char **argv)
        : SampleApplication("StencilOperations", argc, argv)
    {}

    bool initialize() override
    {
        constexpr char kVS[] = R"(attribute vec4 a_position;
void main()
{
    gl_Position = a_position;
})";

        constexpr char kFS[] = R"(precision mediump float;
uniform vec4 u_color;
void main()
{
    gl_FragColor = u_color;
})";

        mProgram = CompileProgram(kVS, kFS);
        if (!mProgram)
        {
            return false;
        }

        // Get the attribute locations
        mPositionLoc = glGetAttribLocation(mProgram, "a_position");

        // Get the sampler location
        mColorLoc = glGetUniformLocation(mProgram, "u_color");

        // Set the clear color
        glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

        // Set the stencil clear value
        glClearStencil(0x01);

        // Set the depth clear value
        glClearDepthf(0.75f);

        // Enable the depth and stencil tests
        glEnable(GL_DEPTH_TEST);
        glEnable(GL_STENCIL_TEST);

        return true;
    }

    void destroy() override { glDeleteProgram(mProgram); }

    void draw() override
    {
        GLfloat vertices[] = {
            -0.75f, 0.25f,  0.50f,  // Quad #0
            -0.25f, 0.25f,  0.50f, -0.25f, 0.75f,  0.50f, -0.75f, 0.75f,  0.50f,
            0.25f,  0.25f,  0.90f,  // Quad #1
            0.75f,  0.25f,  0.90f, 0.75f,  0.75f,  0.90f, 0.25f,  0.75f,  0.90f,
            -0.75f, -0.75f, 0.50f,  // Quad #2
            -0.25f, -0.75f, 0.50f, -0.25f, -0.25f, 0.50f, -0.75f, -0.25f, 0.50f,
            0.25f,  -0.75f, 0.50f,  // Quad #3
            0.75f,  -0.75f, 0.50f, 0.75f,  -0.25f, 0.50f, 0.25f,  -0.25f, 0.50f,
            -1.00f, -1.00f, 0.00f,  // Big Quad
            1.00f,  -1.00f, 0.00f, 1.00f,  1.00f,  0.00f, -1.00f, 1.00f,  0.00f,
        };

        GLubyte indices[][6] = {
            {0, 1, 2, 0, 2, 3},        // Quad #0
            {4, 5, 6, 4, 6, 7},        // Quad #1
            {8, 9, 10, 8, 10, 11},     // Quad #2
            {12, 13, 14, 12, 14, 15},  // Quad #3
            {16, 17, 18, 16, 18, 19},  // Big Quad
        };

        static const size_t testCount = 4;
        GLfloat colors[testCount][4]  = {
            {1.0f, 0.0f, 0.0f, 1.0f},
            {0.0f, 1.0f, 0.0f, 1.0f},
            {0.0f, 0.0f, 1.0f, 1.0f},
            {1.0f, 1.0f, 0.0f, 0.0f},
        };

        GLuint stencilValues[testCount] = {
            0x7,  // Result of test 0
            0x0,  // Result of test 1
            0x2,  // Result of test 2
            0xff  // Result of test 3.  We need to fill this value in at run-time
        };

        // Set the viewport
        glViewport(0, 0, getWindow()->getWidth(), getWindow()->getHeight());

        // Clear the color, depth, and stencil buffers.  At this point, the stencil
        // buffer will be 0x1 for all pixels
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

        // Use the program object
        glUseProgram(mProgram);

        // Load the vertex data
        glVertexAttribPointer(mPositionLoc, 3, GL_FLOAT, GL_FALSE, 0, vertices);
        glEnableVertexAttribArray(mPositionLoc);

        // Test 0:
        //
        // Initialize upper-left region.  In this case, the stencil-buffer values will
        // be replaced because the stencil test for the rendered pixels will fail the
        // stencil test, which is
        //
        //      ref   mask   stencil  mask
        //    ( 0x7 & 0x3 ) < ( 0x1 & 0x7 )
        //
        // The value in the stencil buffer for these pixels will be 0x7.
        glStencilFunc(GL_LESS, 0x7, 0x3);
        glStencilOp(GL_REPLACE, GL_DECR, GL_DECR);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_BYTE, indices[0]);

        // Test 1:
        //
        // Initialize the upper-right region. Here, we'll decrement the stencil-buffer
        // values where the stencil test passes but the depth test fails. The stencil test is
        //
        //      ref  mask    stencil  mask
        //    ( 0x3 & 0x3 ) > ( 0x1 & 0x3 )
        //
        // but where the geometry fails the depth test. The stencil values for these pixels
        // will be 0x0.
        glStencilFunc(GL_GREATER, 0x3, 0x3);
        glStencilOp(GL_KEEP, GL_DECR, GL_KEEP);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_BYTE, indices[1]);

        // Test 2:
        //
        // Initialize the lower-left region.  Here we'll increment (with saturation) the
        // stencil value where both the stencil and depth tests pass.  The stencil test for
        // these pixels will be
        //
        //      ref  mask     stencil  mask
        //    ( 0x1 & 0x3 ) == ( 0x1 & 0x3 )
        //
        // The stencil values for these pixels will be 0x2.
        glStencilFunc(GL_EQUAL, 0x1, 0x3);
        glStencilOp(GL_KEEP, GL_INCR, GL_INCR);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_BYTE, indices[2]);

        // Test 3:
        //
        // Finally, initialize the lower-right region.  We'll invert the stencil value
        // where the stencil tests fails. The stencil test for these pixels will be
        //
        //      ref   mask    stencil  mask
        //    ( 0x2 & 0x1 ) == ( 0x1 & 0x1 )
        //
        // The stencil value here will be set to ~((2^s-1) & 0x1), (with the 0x1 being
        // from the stencil clear value), where 's' is the number of bits in the stencil
        // buffer
        glStencilFunc(GL_EQUAL, 0x2, 0x1);
        glStencilOp(GL_INVERT, GL_KEEP, GL_KEEP);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_BYTE, indices[3]);

        // Since we don't know at compile time how many stencil bits are present, we'll
        // query, and update the value correct value in the  stencilValues arrays for the
        // fourth tests. We'll use this value later in rendering.
        GLint stencilBitCount = 0;
        glGetIntegerv(GL_STENCIL_BITS, &stencilBitCount);
        stencilValues[3] = ~(((1 << stencilBitCount) - 1) & 0x1) & 0xff;

        // Use the stencil buffer for controlling where rendering will occur.  We disable
        // writing to the stencil buffer so we can test against them without modifying
        // the values we generated.
        glStencilMask(0x0);

        for (size_t i = 0; i < testCount; ++i)
        {
            glStencilFunc(GL_EQUAL, stencilValues[i], 0xff);
            glUniform4fv(mColorLoc, 1, colors[i]);
            glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_BYTE, indices[4]);
        }

        // Reset the stencil mask
        glStencilMask(0xFF);
    }

  private:
    // Handle to a program object
    GLuint mProgram;

    // Attribute locations
    GLint mPositionLoc;

    // Uniform locations
    GLint mColorLoc;
};

int main(int argc, char **argv)
{
    StencilOperationsSample app(argc, argv);
    return app.run();
}