OpenGL Float texture 實驗

最近在寫一個智慧剪圖的 app Cutto, 裡面因為需要執行大量的影像處理，所以有非常多的浮點運算，需要用 OpenGL 來維持使用的流暢度。在使用 OpenGL 來做計算的時候，input 常常是是以一個圖像（或稱 texture）的方式來輸入。對 Cutto 來說，input 必須要是浮點數，因此驗證 iPhone 是否志願 float textures 就變得很重要了。

下面的實驗結果證實在 iPhone 4S 以上的裝置上，GL_FLOAT 或是 GL_HALF_FLOAT_ES 的 texture 都是支援的。這個結果真是欣慰！
不過順帶一提的是，假如我們的 output 也要是浮點數怎麼辦？答案是在 OpenGL 裏，所有浮點 texture 都不是 color-renderable OpenGL ES 3.0 spec Table 3.12，也就是不能當作 output render to FrameBuffer object。唯一的辦法是自己做 floating point pack in glsl。看起來 OpenCL 真的是有他的需求在，希望 apple 趕快在 iOS 上支援啊。

下面是實驗的 code:

	void floatTextureExperiment
	{
	GLenum err = 0;
	GLuint texture1;
	glGenTextures(1, &texture1);
	glActiveTexture(GL_TEXTURE1);
	glBindTexture(GL_TEXTURE_2D, texture1);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
	GLuint texture2;
	glGenTextures(1, &texture2);
	glActiveTexture(GL_TEXTURE2);
	glBindTexture(GL_TEXTURE_2D, texture2);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
	GLuint frameBuffer;
	glGenFramebuffers(1, &frameBuffer);
	glBindFramebuffer(GL_FRAMEBUFFER, frameBuffer);
	char* bgVShaderStr =
	"attribute vec4 vPosition;\n"
	"attribute vec2 in_texCoord;\n"
	"varying vec2 texCoord;\n"
	"void main(){\n"
	" texCoord = in_texCoord;\n"
	" gl_Position = vPosition;\n"
	"}\n";
	char* bgFShaderStr =
	"uniform sampler2D tex;\n"
	"varying highp vec2 texCoord;\n"
	"void main(){\n"
	" highp vec4 color = texture2D(tex, texCoord);\n"
	" //gl_FragColor = vec4(color.r+1.0/256.0, 1.0, 0.0, 1.0);\n"
	" gl_FragColor = vec4(color.r/2.0, 1.0, 0.0, 1.0);\n"
	"}\n";
	GLuint program = esLoadProgram(bgVShaderStr, bgFShaderStr);
	int width = 2;
	int height = 2;
	// uint8_t buf[] = {20, 0, 0, 255,
	// 100, 0, 0, 255,
	// 180, 0, 0, 255,
	// 255, 0, 0, 255};
	// glActiveTexture(GL_TEXTURE1);
	// glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, buf);
	float buf[] = {0.2, 0.0, 0.0, 1.0,
	0.8, 0.0, 0.0, 1.0,
	1.6, 0.0, 0.0, 1.0,
	1.9, 0.0, 0.0, 1.0};
	hfloat* hfbuf = (hfloat*)malloc(16 * sizeof(hfloat));
	for (int i = 0; i < 16; i++) {
	hfbuf[i] = convertFloatToHFloat(buf+i);
	}
	glActiveTexture(GL_TEXTURE1);
	glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_HALF_FLOAT_OES, hfbuf);
	free(hfbuf);
	glUseProgram(program);
	static const GLfloat imageVertices[] = {
	-1.0f, -1.0f,
	1.0f, -1.0f,
	-1.0f, 1.0f,
	1.0f, 1.0f,
	};
	GLint attribLoc = glGetAttribLocation(program, "vPosition");
	glVertexAttribPointer(attribLoc, 2, GL_FLOAT, GL_FALSE, 0, imageVertices);
	static const GLfloat noRotationTextureCoordinates[] = {
	0.0f, 0.0f,
	1.0f, 0.0f,
	0.0f, 1.0f,
	1.0f, 1.0f,
	};
	attribLoc = glGetAttribLocation(program, "in_texCoord");
	glVertexAttribPointer(attribLoc, 2, GL_FLOAT, GL_FALSE, 0, noRotationTextureCoordinates);
	glViewport(0, 0, width, height);
	glActiveTexture(GL_TEXTURE2);
	glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
	glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture2, 0);
	GLenum frameBufferStatus = glCheckFramebufferStatus(GL_FRAMEBUFFER);
	if (frameBufferStatus != GL_FRAMEBUFFER_COMPLETE) {
	printf("Incomplete framebuffer: %d\n", frameBufferStatus - GL_FRAMEBUFFER_COMPLETE);
	printFrameBufferStatus();
	}
	int texLoc = glGetUniformLocation(program, "tex");
	glUniform1i(texLoc, GL_TEXTURE1 - GL_TEXTURE0);
	glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
	uint8_t* m = (uint8_t*)malloc(4 * 4 * sizeof(uint8_t));
	memset(m, 0, 4 * 4 * sizeof(uint8_t));
	glReadPixels(0, 0, 2, 2, GL_RGBA, GL_UNSIGNED_BYTE, m);
	for (int i = 0; i < 4; i++) {
	for (int j = 0; j < 4; j++) {
	printf("%d, ", m[4*i+j]);
	}
	printf("\n");
	}
	free(m);
	}
	void printFrameBufferStatus()
	{
	if (frameBufferStatus == GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT) {
	printf("GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT\n");
	} else if (frameBufferStatus == GL_FRAMEBUFFER_INCOMPLETE_DIMENSIONS) {
	printf("GL_FRAMEBUFFER_INCOMPLETE_DIMENSIONS\n");
	} else if (frameBufferStatus == GL_FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT) {
	printf("GL_FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT\n");
	} else if (frameBufferStatus == GL_FRAMEBUFFER_UNSUPPORTED) {
	printf("GL_FRAMEBUFFER_UNSUPPORTED\n");
	} else {
	printf("Unknown status!\n");
	}
	}

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