I'm looking for a most efficient way of drawing a 2-dimensional background in metal. This requires rendering a textured rectangle.
The basic geometry example shows an example on how to draw a triangle. Is there an easy and non-bloated way to draw a rectangle (a polygon with 4 corners)?
The Basic Texturing sample draws a textured rectangle
Related
I am wondering if there is a way to draw a polygon on top of a MKMapView where each vertex in the polygon could potentially have a different color. The triangles of the polygon would correctly blend the colors together when rendered.
I know I can do this with OpenGL, but I would like to try to use CoreGraphics or a drawing function of MapKit first, as I don't know how well OpenGL works on top of a MKMapView.
I know there is support to fill a polyline with a gradient. Is there a similar way to fill a polygon/triangle with multiple gradients to achieve the desired effect?
Is it possible in OpenGL ES 2.0 to use GL_LINE_STRIP to draw a shape and then apply a texture to that shape ?
E.g if I draw a triangle can I then apply a triangle texture ?
GL_LINE_STRIP draws only lines. If you want polygons filled in (whether with color, lighting, or texturing), you need one of the solid polygon modes: GL_TRIANGLES, GL_TRIANGLE_STRIP, or GL_TRIANGLE_FAN.
If you want to both fill and stroke your polygons, you'll need two draw calls, one with each mode. And if you're using depth testing, you'll probably want to look into glPolygonOffset to avoid z-fighting.
I'm trying to load an image into a texture, then draw into the texture.
Am I right in thinking that to display a texture you always need an array of vertices and texture coordinates? So, to draw a square texture I would need to draw 2 triangles to make a square and attach the texture to them?
Yes, that's right. For a square texture, the alternative is to use point sprites with GL_POINTS.
I am rendering point sprites (using OpenGL ES 2.0 on iOS) as a user's drawing strokes. I am storing these points in vertex buffer objects such that I need to perform depth testing in order for the sprites to appear in the correct order when they're submitted for drawing.
I'm seeing an odd effect when rendering these drawing strokes, as shown by the following screenshot:
Note the background-coloured 'border' around the edge of the blue stroke, where it is drawn over the green. The user drew the blue stroke after the green stroke, but when the VBOs are redrawn the blue stroke gets drawn first. When it comes to draw the green stroke, depth testing kicks in and sees that it should be behind the blue stroke, and so does this, with some success. It appears to me to be some kind of blending issue, or to do with incorrectly calculating the colour in the fragment shader? The edges of all strokes should be transparent, however it appears that the fragment shader combines it with the background texture when processing those fragments.
In my app I have created a depth renderbuffer and called glEnable(GL_DEPTH_TEST) using glDepthFunc(GL_LEQUAL). I have experimented with glDepthMask() to no avail. Blending is set to glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA), and the point sprite colour uses premultiplied alpha values. The drawing routine is very simple:
Bind render-to-texture FBO.
Draw background texture.
Draw point sprites (from a number of VBOs).
Draw this FBO's texture to the main framebuffer.
Present the main framebuffer.
EDIT
Here is some code from the drawing routine.
Setup state prior to drawing:
glDisable(GL_DITHER);
glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LEQUAL);
Drawing routine:
[drawingView setFramebuffer:drawingView.scratchFramebuffer andClear:YES];
glUseProgram(programs[PROGRAM_TEXTURE]);
[self drawTexture:[self textureForBackgroundType:self.backgroundType]];
glUseProgram(programs[PROGRAM_POINT_SPRITE]);
// ...
// Draw all VBOs containing point sprite data
// ...
[drawingView setFramebuffer:drawingView.defaultFramebuffer andClear:YES];
glUseProgram(programs[PROGRAM_TEXTURE]);
[self drawTexture:drawingView.scratchTexture];
[drawingView presentFramebuffer:drawingView.defaultFramebuffer];
Thanks for any help.
If you want to draw non opaque geometries you have to z-sort them from back to front. This has been the only way to get a proper blending for many years. These days there are some algorithms for order independent transparency like Dual Depth Peeling but they are not applicable to iOS.
I'm having trouble drawing to a CCRenderTexture with a texture that includes transparency due to the blending that I've used.
The sprite that I am drawing with has a blending function of:
glBlendFunc(GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA).
The render texture that it is drawing to has the blending function:
glBlendFuncSeparate(GL_DST_COLOR, GL_ONE_MINUS_SRC_ALPHA, GL_SRC_COLOR, GL_ONE_MINUS_SRC_ALPHA).
The render texture is set up with it being mostly transparent with some completely opaque areas on it. The above blend functions allow me to draw with a completely solid sprite on the solid areas while it not drawing on the transparent areas.
Trouble arises when I try to change the sprite that I draw with to include some transparency. It tends to "erase" the opaque areas on the render texture. Ideally I would like the transparent areas on the sprite that I draw with to have no effect. I was wondering if an openGL blending guru was able to give me some guidance?
To clarify, the final effect that I am trying to achieve is to be able draw a sprite that includes some transparent areas to my render texture. The transparent areas of the sprite should not draw to my render texture. The opaque areas of the sprite should only draw to the render texture if they are over an opaque area on the render texture.