How to change background color of sprites to transparent by changing alpha after loading it to a SDL_Surface. Are there any functions in SDL which use a floodfill kind of algorithm and change all pixel with a given color to transparent on the outside. I don't want it to happen inside the border of the sprite if the same color is used.
Sample Image:
I would like to make the background blue here transparent before I blit it on the window surface using SDL_BlitSurface.
Only a color key (SDL_SetColorKey()) or a full alpha channel are going to help you here.
Note, that you can provide an alpha channel in your source graphics if you use a format such as PNG. If you only sometimes need/want an alpha, then provide the alpha channel in your source graphics and use SDL_SetSurfaceBlendMode() with SDL_BLENDMODE_NONE to blend without the alpha and SDL_BLENDMODE_BLEND to blend with it.
Both SDL_Surface and SDL_Texture support SDL_BlendMode.
Even if SDL provided another method, such as the fill you mentioned. You wouldn't want to use that. It is more difficult, expensive, and unnecessary overhead. You should stick with best practices here.
You may want to look into SDL_Texture and SDL_Renderer and switch to using a "texture atlas" instead of individual surfaces/textures for each image to maximize performance.
I am trying to capture PNG image with transparent background. I have set GL_RGBA as format in glReadPixels. But the output PNG image looks a little blackish or with saturated color. If backgrouund is not transparent that is if I use GL_RGB format in glReadPixels expected image is captured.
Note: In both cases, I am capturing translucent(partially transparent) shaded cube. If cube is completely opaque, RGBA format works fine.
Any ideas as to why this is happening for transparent background?
Blackish image with RGBA format
Image with RGB format
The cube looks darker because it's semitransparent, and whatever you use to view the image blends the semitransparent parts of it with black background.
You might argue that the cube in the picture shouldn't be semitransparent since it's drawn on top of a completely opaque background, but the problem is that the widely-used
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
blending mode (which you seem to use as well) is known to produce incorrect alpha values when used to draw semitransparent objects.
Usually you can't see it because alpha is discarded when drawing to the default framebuffer, but it becomes prominent when inspecting outputs of glReadPixels.
As you noticed, to solve it you can simply discard the alpha component.
But if you for some reason need to have a proper blending without those problems, you need
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
Note that for this funciton to work, both source and destination images have to be in premultiplied alpha format. That is, you need to do color.rgb *= color.a
as a last step in your shaders.
The inverse operation (color.rgb /= color.a) can be used to convert an image back to the regular format, but if your images are completely opaque, this step is unnecessary.
I need to display image in openGL window.
Image changes every timer tick.
I've checked on google how, and as I can see it can be done using or glBitmap or glTexImage2D functions.
What is the difference between them?
The difference? These two functions have nothing in common.
glBitmap is a function for drawing binary images. That's not a .BMP file or an image you load (usually). The function's name doesn't refer to the colloquial term "bitmap". It refers to exact that: a map of bits. Each bit in the bitmap represents a pixel. If the bit is 1, then the current raster color will be written to the framebuffer. If the bit is 0, then the pixel in the framebuffer will not be altered.
glTexImage2D is for allocating textures and optionally uploading pixel data to them. You can later draw triangles that have that texture mapped to them. But glTexImage2D by itself does not draw anything.
What you are probably looking for is glDrawPixels, which draws an image directly into the framebuffer. If you use glTexImage2D, you have to first update the texture with the new image, then draw a shape with that texture (say, a fullscreen quad) to actually render the image.
That said, you'll be better off with glTexImage2D if...
You're using a library like JOGL that makes binding textures from images an easy operation, or
You want to scale the image or display it in perspective
Unfortunately, taking a screenshot does no replicate the problem, so I'll have to explain.
My character is a QUAD with a texture bound to it. When I move this character in any direction, the 'back end' of the pixels have a green and red 'after-glow' or strip of pixels. Very hard to explain, but I am assuming it is a problem with the double buffering. Is there a known issue associated with moving sprites and trailing pixels?
My only guess at this point is that you are only using a subset of the texture (i.e. your UVs are not just 0 and 1), and you have some colored pixels outside the rect you're drawing, and due to bilinear filtering, you catch a glimpse of them.
When creating textures with alpha, be sure to create an outline around the visible part of the texture with the same color (i.e. if your texture is a brown wooden fence, make sure that all transparent pixels near the fence are brown too).
NOTE that some texture compression algorithms will remove the color value from a pixel if it is entirely transparent, so if necessary, write a test pixel shader that ignores alpha to make sure that your texture made it through the pipeline intact.
I am rendering to a texture through a framebuffer object, and when I draw transparent primitives, the primitives are blended properly with other primitives drawn in that single draw step, but they are not blended properly with the previous contents of the framebuffer.
Is there a way to properly blend the contents of the texture with the new data coming in?
EDIT: More information requsted, I will attempt to explain more clearly;
The blendmode I am using is GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA. (I believe that is typically the standard blendmode)
I am creating an application that tracks mouse movement. It draws lines connecting the previous mouse position to the current mouse position, and as I do not want to draw the lines over again each frame, I figured I would draw to a texture, never clear the texture and then just draw a rectangle with that texture on it to display it.
This all works fine, except that when I draw shapes with alpha less than 1 onto the texture, it does not blend properly with the texture's previous contents. Let's say I have some black lines with alpha = .6 drawn onto the texture. A couple draw cycles later, I then draw a black circle with alpha = .4 over those lines. The lines "underneath" the circle are completely overwritten. Although the circle is not flat black (It blends properly with the white background) there are no "darker lines" underneath the circle as you would expect.
If I draw the lines and the circle in the same frame however, they blend properly. My guess is that the texture just does not blend with it's previous contents. It's like it's only blending with the glclearcolor. (Which, in this case is <1.0f, 1.0f, 1.0f, 1.0f>)
I think there are two possible problems here.
Remember that all of the overlay lines are blended twice here. Once when they are blended into the FBO texture, and again when the FBO texture is blended over the scene.
So the first possibility is that you don't have blending enabled when drawing one line over another in the FBO overlay. When you draw into an RGBA surface with blending off, the current alpha is simply written directly into the FBO overlay's alpha channel. Then later when you blend the whole FBO texture over the scene, that alpha makes your lines translucent. So if you have blending against "the world" but not between overlay elements, it is possible that no blending is happening.
Another related problem: when you blend one line over another in "standard" blend mode (src alpha, 1 - src alpha) in the FBO, the alpha channel of the "blended" part is going to contain a blend of the alphas of the two overlay elements. This is probably not what you want.
For example, if you draw two 50% alpha lines over each other in the overlay, to get the equivalent effect when you blit the FBO, you need the FBO's alpha to be...75%. (That is, 1 - (1-.5) * (1-0.5), which is what would happen if you just drew two 50% alpha lines over your scene. But when you draw the two 50% lines, you'll get 50% alpha in the FBO (a blend of 50% with...50%.
This brings up the final issue: by pre-mixing the lines with each other before you blend them over the world, you are changing the draw order. Whereas you might have had:
blend(blend(blend(background color, model), first line), second line);
now you will have
blend(blend(first line, second line), blend(background color, model)).
In other words, pre-mixing the overlay lines into an FBO changes the order of blending and thus changes the final look in a way you may not want.
First, the simple way to get around this: don't use an FBO. I realize this is a "go redesign your app" kind of answer, but using an FBO is not the cheapest thing, and modern GL cards are very good at drawing lines. So one option would be: instead of blending lines into an FBO, write the line geometry into a vertex buffer object (VBO). Simply extend the VBO a little bit each time. If you are drawing less than, say, 40,000 lines at a time, this will almost certainly be as fast as what you were doing before.
(One tip if you go this route: use glBufferSubData to write the lines in, not glMapBuffer - mapping can be expensive and doesn't work on sub-ranges on many drivers...better to just let the driver copy the few new vertices.)
If that isn't an option (for example, if you draw a mix of shape types or use a mix of GL state, such that "remembering" what you did is a lot more complex than just accumulating vertices) then you may want to change how you draw into the VBO.
Basically what you'll need to do is enable separate blending; initialize the overlay to black + 0% alpha (0,0,0,0) and blend by "standard blending" the RGB but additive blending the alpha channels. This still isn't quite correct for the alpha channel but it's generally a lot closer - without this, over-drawn areas will be too transparent.
Then, when drawing the FBO, use "pre-multiplied" alpha, that is, (one, one-minus-src-alph).
Here's why that last step is needed: when you draw into the FBO, you have already multiplied every draw call by its alpha channel (if blending is on). Since you are drawing over black, a green (0,1,0,0.5) line is now dark green (0,0.5,0,0.5). If alpha is on and you blend normally again, the alpha is reapplied and you'l have 0,0.25,0,0.5.). By simply using the FBO color as is, you avoid the second alpha multiplication.
This is sometimes called "pre-multiplied" alpha because the alpha has already been multiplied into the RGB color. In this case you want it to get correct results, but in other cases, programmers use it for speed. (By pre-multiplying, it removes a mult per pixel when the blend op is performed.)
Hope that helps! Getting blending right when the layers are not mixed in order gets really tricky, and separate blend isn't available on old hardware, so simply drawing the lines every time may be the path of least misery.
Clear the FBO with transparent black (0, 0, 0, 0), draw into it back-to-front with
glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
and draw the FBO with
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
to get the exact result.
As Ben Supnik wrote, the FBO contains colour already multiplied with the alpha channel, so instead of doing that again with GL_SRC_ALPHA, it is drawn with GL_ONE. The destination colour is attenuated normally with GL_ONE_MINUS_SRC_ALPHA.
The reason for blending the alpha channel in the buffer this way is different:
The formula to combine transparency is
resultTr = sTr * dTr
(I use s and d because of the parallel to OpenGL's source and destination, but as you can see the order doesn't matter.)
Written with opacities (alpha values) this becomes
1 - rA = (1 - sA) * (1 - dA)
<=> rA = 1 - (1 - sA) * (1 - dA)
= 1 - 1 + sA + dA - sA * dA
= sA + (1 - sA) * dA
which is the same as the blend function (source and destination factors) (GL_ONE, GL_ONE_MINUS_SRC_ALPHA) with the default blend equation GL_FUNC_ADD.
As an aside:
The above answers the specific problem from the question, but if you can easily choose the draw order it may in theory be better to draw premultiplied colour into the buffer front-to-back with
glBlendFunc(GL_ONE_MINUS_DST_ALPHA, GL_ONE);
and otherwise use the same method.
My reasoning behind this is that the graphics card may be able to skip shader execution for regions that are already solid. I haven't tested this though, so it may make no difference in practice.
As Ben Supnik said, the best way to do this is rendering the entire scene with separate blend functions for color and alpha. If you are using the classic non premultiplied blend function try glBlendFuncSeparateOES(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE) to render your scene to FBO. and glBlendFuncSeparateOES(GL_ONE, GL_ONE_MINUS_SRC_ALPHA) to render the FBO to screen.
It is not 100% accurate, but in most of the cases that will create no unexpected transparency.
Keep in mind that old Hardware and some mobile devices (mostly OpenGL ES 1.x devices, like the original iPhone and 3G) does not support separated blend functions. :(