OpenGL ES texture degrades in quality - ios

Question

I am trying to draw a Core Graphics image generated (at screen resolution) into OpenGL. However, the image is rendering more aliased than the CG output (antialiasing is disabled in CG). The text is the texture (the blue background is respectively drawn in Core Graphics for the first image and OpenGL for the second).
CG Output:
OpenGL Render (in simulator):
Framebuffer setup:
context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
[EAGLContext setCurrentContext:context];
glGenRenderbuffers(1, &onscrRenderBuffer);
glBindRenderbuffer(GL_RENDERBUFFER, onscrRenderBuffer);
[context renderbufferStorage:GL_RENDERBUFFER fromDrawable:self.layer];
glGenFramebuffers(1, &onscrFramebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, onscrFramebuffer);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, onscrRenderBuffer);
Texture Loading Code:
-(GLuint) loadTextureFromImage:(UIImage*)image {
CGImageRef textureImage = image.CGImage;
size_t width = CGImageGetWidth(textureImage);
size_t height = CGImageGetHeight(textureImage);
GLubyte* spriteData = (GLubyte*) malloc(width*height*4);
CGColorSpaceRef cs = CGImageGetColorSpace(textureImage);
CGContextRef c = CGBitmapContextCreate(spriteData, width, height, 8, width*4, cs, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(cs);
CGContextScaleCTM(c, 1, -1);
CGContextTranslateCTM(c, 0, -CGContextGetClipBoundingBox(c).size.height);
CGContextDrawImage(c, (CGRect){CGPointZero, {width, height}}, textureImage);
CGContextRelease(c);
GLuint glTex;
glGenTextures(1, &glTex);
glBindTexture(GL_TEXTURE_2D, glTex);
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);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, (GLsizei)width, (GLsizei)height, 0, GL_RGBA, GL_UNSIGNED_BYTE, spriteData);
glBindTexture(GL_TEXTURE_2D, 0);
free(spriteData);
return glTex;
}
Vertices:
struct vertex {
float position[3];
float color[4];
float texCoord[2];
};
typedef struct vertex vertex;
const vertex bgVertices[] = {
{{1, -1, 0}, {0, 167.0/255.0, 253.0/255.0, 1}, {1, 0}}, // BR (0)
{{1, 1, 0}, {0, 222.0/255.0, 1.0, 1}, {1, 1}}, // TR (1)
{{-1, 1, 0}, {0, 222.0/255.0, 1.0, 1}, {0, 1}}, // TL (2)
{{-1, -1, 0}, {0, 167.0/255.0, 253.0/255.0, 1}, {0, 0}} // BL (3)
};
const vertex textureVertices[] = {
{{1, -1, 0}, {0, 0, 0, 0}, {1, 0}}, // BR (0)
{{1, 1, 0}, {0, 0, 0, 0}, {1, 1}}, // TR (1)
{{-1, 1, 0}, {0, 0, 0, 0}, {0, 1}}, // TL (2)
{{-1, -1, 0}, {0, 0, 0, 0}, {0, 0}} // BL (3)
};
const GLubyte indicies[] = {
3, 2, 0, 1
};
Render Code:
glClear(GL_COLOR_BUFFER_BIT);
GLsizei width, height;
glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &width);
glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, &height);
glViewport(0, 0, width, height);
glBindBuffer(GL_ARRAY_BUFFER, bgVertexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer);
glVertexAttribPointer(positionSlot, 3, GL_FLOAT, GL_FALSE, sizeof(vertex), 0);
glVertexAttribPointer(colorSlot, 4, GL_FLOAT, GL_FALSE, sizeof(vertex), (GLvoid*)(sizeof(float)*3));
glVertexAttribPointer(textureCoordSlot, 2, GL_FLOAT, GL_FALSE, sizeof(vertex), (GLvoid*)(sizeof(float)*7));
glDrawElements(GL_TRIANGLE_STRIP, sizeof(indicies)/sizeof(indicies[0]), GL_UNSIGNED_BYTE, 0);
glBindBuffer(GL_ARRAY_BUFFER, textureVertexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture);
glUniform1i(textureUniform, 0);
glVertexAttribPointer(positionSlot, 3, GL_FLOAT, GL_FALSE, sizeof(vertex), 0);
glVertexAttribPointer(colorSlot, 4, GL_FLOAT, GL_FALSE, sizeof(vertex), (GLvoid*)(sizeof(float)*3));
glVertexAttribPointer(textureCoordSlot, 2, GL_FLOAT, GL_FALSE, sizeof(vertex), (GLvoid*)(sizeof(float)*7));
glDrawElements(GL_TRIANGLE_STRIP, sizeof(indicies)/sizeof(indicies[0]), GL_UNSIGNED_BYTE, 0);
glBindTexture(GL_TEXTURE_2D, 0);
I am using the blend function glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) in case that has anything to do with it.
Any ideas where the problem is?

Answer 1

Your GL-rendered output looks all pixelated because it has fewer pixels. Per the Drawing and Printing Guide for iOS, the default scale factor for a CAEAGLLayer is 1.0, so when you set up your GL render buffers, you get one pixel in the buffer per point. (Remember, a point is a unit of UI layout, which on modern devices with Retina displays works out to several hardware pixels.) When you render that buffer full-screen, everything gets scaled up (by about 2.61x on an iPhone 6(s) Plus).
To render at the native screen resolution, you need to increase the contentScaleFactor of your view. (Preferably, you should do this early on, before, setting up renderbuffers, so that they get the new scale factor from the view's layer.)
Watch out, though: you want to use the UIScreen property nativeScale, not scale. The scale property reflects UI rendering, where, on iPhone 6(s) Plus, everything gets done at 3x and then scaled down slightly to the native resolution of the display. The nativeScale property reflects the number of actual device pixels per point — if you're doing GPU rendering, you want to target that so you don't sap performance by drawing more pixels than you need to. (On current devices other than the "Plus" iPhones, scale and nativeScale are the same. But using the latter is probably a good insurance policy.)
You can avoid a lot of these kinds of issues (and others) by letting GLKView do renderbuffer setup for you. Even if you're writing cross-platform GL, that part of your code is going to have to be pretty platform- and device-specific anyway, so you might as well reduce the amount of it that you have to write and maintain.
(Addressing previous edits of the question, for posterity's sake: this has nothing to do with multisampling or the quality of the GL texture data. Multisampling has to do with rasterization of polygon edges — points in the interior of a polygon get one fragment per pixel, but points on the edges get multiple fragments whose colors are blended in the resolve stage. And if you bind the texture to an FBO and glReadPixels from it, you'll find the image is pretty much the same one you put in.)