opengl es2.0渲染圖并局部重新整理

最近在鼓搗OpenGL渲染，這東西确實給顯示帶來很大便利，網上資料也比較多，不過因為這些接口都高度封裝，出問題可不太好查。我在網上看了一些資料，有很多渲染圖的例子，不過貌似渲染圖并重新整理的比較少見，是以把我的一些經驗分享一下，畢竟像我一樣臨時現學現搞的菜鳥應該還是比較多的。期間遇到一些坑，還好有大佬的幫忙。。。

1. 着色器腳本

這算是opengl的基本概念，就不做詳細解釋了

char *vertex_shader_source =
"attribute vec2 vertexCoord;"
"attribute vec2 textureCoord;"
"varying vec2 sampleCoord;"
"void main()"
"{"
"   sampleCoord = textureCoord;"
"   gl_Position = vec4(vertexCoord, 0.0, 1.0);"
"}";

char *fragment_shader_source =
"precision mediump float;"
"varying vec2 sampleCoord;"
"uniform sampler2D sampleTexture;"
"void main()"
"{"
"   gl_FragColor = texture2D(sampleTexture, sampleCoord);"
"}";

2.建構着色器程式

這個也是基本操作，八股文一樣，也不再細說了

static void check_gl_error(char *gl_operation)
{
    for (GLint error = glGetError(); error; error = glGetError())
    {
        g_info("[GL] Operation %s() glError (0x%x)", gl_operation, error);
    }
}

static GLuint load_shader(GLenum shader_type, char *shader_source)
{
    GLuint shader = glCreateShader(shader_type);
    if (shader) {
        glShaderSource(shader, 1, &shader_source, NULL);
        glCompileShader(shader);
        GLint compiled = 0;
        glGetShaderiv(shader, GL_COMPILE_STATUS, &compiled);
        if (!compiled) {
            GLint infoLen = 0;
            glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &infoLen);
            if (infoLen) {
                char* buf = (char*)malloc(infoLen);
                if (buf) {
                    glGetShaderInfoLog(shader, infoLen, NULL, buf);
                    g_info("[GL] Could not compile shader %d:%s", shader_type, buf);
                    free(buf);
                }
                glDeleteShader(shader);
                shader = 0;
            }
        }
    }
    return shader;
}

static GLuint gl_render_create_program(GlRender *render, char *vertex_shader_source, char *fragment_shader_source)
{
    g_return_val_if_fail(IS_GL_RENDER(render), 0);
    GlRenderPrivate *p = render->priv;

    p->vertex_shader = load_shader(GL_VERTEX_SHADER, vertex_shader_source);
    if (!p->vertex_shader) {
        return 0;
    }
    
    p->fragment_shader = load_shader(GL_FRAGMENT_SHADER, fragment_shader_source);
    if (!p->fragment_shader) {
        return 0;
    }
    
    GLuint program = glCreateProgram();
    if (program) {
        glAttachShader(program, p->vertex_shader);
        check_gl_error("glAttachShader");
        glAttachShader(program, p->fragment_shader);
        check_gl_error("glAttachShader");
        glLinkProgram(program);
        GLint linkStatus = GL_FALSE;
        glGetProgramiv(program, GL_LINK_STATUS, &linkStatus);
        if (linkStatus != GL_TRUE) {
            GLint bufLength = 0;
            glGetProgramiv(program, GL_INFO_LOG_LENGTH, &bufLength);
            if (bufLength) {
                char* buf = (char*)malloc(bufLength);
                if (buf) {
                    glGetProgramInfoLog(program, bufLength, NULL, buf);
                    g_info("[GL] Could not link program:%s", buf);
                    free(buf);
                }
            }
            glDeleteProgram(program);
            program = 0;
        }
    }
    return program;
}

3.坐标系初始化

定點坐标及紋理坐标初始化後可以直接使用了，不需要再對每一幅圖作坐标映射，因為圖像是局部重新整理，每次顯示還是全圖

static void gl_render_get_handles(GlRender *render)
{
    g_return_if_fail(IS_GL_RENDER(render));
    GlRenderPrivate *p = render->priv;

    glUseProgram(p->program);

    p->vertex_coord_handle = glGetAttribLocation(p->program, "vertexCoord");
    p->texture_coord_handle = glGetAttribLocation(p->program, "textureCoord");
    p->sample_texture_handle = glGetUniformLocation(p->program, "sampleTexture");

    g_info("[GL] vertex_coord_handle = %d", p->vertex_coord_handle);
    g_info("[GL] texture_coord_handle = %d", p->texture_coord_handle);
    g_info("[GL] sample_texture_handle = %d", p->sample_texture_handle);

    glEnableVertexAttribArray(p->vertex_coord_handle);
    glEnableVertexAttribArray(p->texture_coord_handle);
}

void gl_render_open(GlRender *render)
{
    g_return_if_fail(IS_GL_RENDER(render));
    GlRenderPrivate *p = render->priv;

    p->texture_id = 0;
    p->texture_width = 0;
    p->texture_height = 0;
    p->surface_width = 0;
    p->surface_height = 0;

    p->vertex_coord[0] = -1.0f;
    p->vertex_coord[1] = -1.0f;
    p->vertex_coord[2] = 1.0f;
    p->vertex_coord[3] = -1.0f;
    p->vertex_coord[4] = -1.0f;
    p->vertex_coord[5] = 1.0f;
    p->vertex_coord[6] = 1.0f;
    p->vertex_coord[7] = 1.0f;

    p->texture_coord[0] = 0.0f;
    p->texture_coord[1] = 1.0f;
    p->texture_coord[2] = 1.0f;
    p->texture_coord[3] = 1.0f;
    p->texture_coord[4] = 0.0f;
    p->texture_coord[5] = 0.0f;
    p->texture_coord[6] = 1.0f;
    p->texture_coord[7] = 0.0f;

    p->program = gl_render_create_program(render, vertex_shader_source, fragment_shader_source);
    if (!p->program)
    {
        g_info("[GL] init render failed");
    }

    gl_render_get_handles(render);
}

4.加載紋理到GPU

考慮切換分辨率的情況，當view的尺寸或紋理的尺寸出現變化時，通過設定glTexImage2D最後一個參數為NULL配置設定一個全圖的緩存，尺寸不變的時候通過glTexSubImage2D往緩沖區寫入局部的圖像資料，達到局部重新整理的效果

GLuint gl_render_load_texture(GlRender *render, GlContext *context,
                                void* data, int x, int y, int width, int height, int max_width, int max_height)
{
    g_return_val_if_fail(IS_GL_RENDER(render), 0);
    GlRenderPrivate *p = render->priv;

    int surface_width = 0;
    int surface_height = 0;
    gl_context_get_width_height(context, &surface_width, &surface_height);

    if (p->texture_width != max_width || p->texture_height != max_height
        || p->surface_width != surface_width || p->surface_height != surface_height)
    {
        p->texture_width = max_width;
        p->texture_height = max_height;
        p->surface_width = surface_width;
        p->surface_height = surface_height;

        glViewport(0, 0, surface_width, surface_height);
        g_info("[GL] set opengl view width %d, height %d, texture width %d, texture height %d",
                surface_width, surface_height, max_width, max_height);

        if (p->texture_id)
        {
            glDeleteTextures(1, &p->texture_id);
            p->texture_id = 0;
        }

        glGenTextures(1, &p->texture_id);
        check_gl_error("glGenTextures");

        glBindTexture(GL_TEXTURE_2D, p->texture_id);
        check_gl_error("glBindTexture");

        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

        glTexImage2D(GL_TEXTURE_2D, 0, GL_BGRA_IMG, max_width, max_height, 0, GL_BGRA_IMG, GL_UNSIGNED_BYTE, NULL);
        check_gl_error("glTexImage2D");
    }

    glBindTexture(GL_TEXTURE_2D, p->texture_id);
    glTexSubImage2D(GL_TEXTURE_2D, 0, x, y, width, height, GL_BGRA_IMG, GL_UNSIGNED_BYTE, data);
    check_gl_error("glTexSubImage2D");

    glBindTexture(GL_TEXTURE_2D, 0);

    return p->texture_id;
}

5.繪制

void gl_render_draw(GlRender *render, GlContext *context)
{
    g_return_if_fail(IS_GL_RENDER(render));
    GlRenderPrivate *p = render->priv;

    glUseProgram(p->program);

    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, p->texture_id);

    glUniform1i(p->sample_texture_handle, 0);

    glVertexAttribPointer(p->vertex_coord_handle, 2, GL_FLOAT, GL_FALSE, 0, p->vertex_coord);
    glVertexAttribPointer(p->texture_coord_handle, 2, GL_FLOAT, GL_FALSE, 0, p->texture_coord);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
    check_gl_error("glDrawArrays");

    gl_context_swap_buffers(context);
}

6.用完後記得釋放資源

if (p->texture_id) {
    glDeleteTextures(1, &p->texture_id);
}
if (p->fragment_shader) {
    glDeleteShader(p->fragment_shader);
    glDetachShader(p->program, p->fragment_shader);
}
if (p->vertex_shader) {
    glDeleteShader(p->vertex_shader);
    glDetachShader(p->program, p->vertex_shader);
}
if (p->program) {
    glDeleteProgram(p->program);
}

7.小結

用法：先調用gl_render_load_texture加載紋理（第一次調用最好傳一幅全圖），然後調用gl_render_draw繪制，不斷循環這個過程。關鍵點是頂點坐标及紋理坐标，以及圖像紋理的加載方式。本人用的是gobiect，跟c++的class是類似的東西，這裡的GlContext其實是另一個抽象類，裡面有egl的實作，主要是建立surface還有egl的swapbuffers，這個實作網上很多，這裡就不再細述了

opengl es2.0渲染圖并局部重新整理

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